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- Define exotoxin and list three types of exotoxins.
- State the major way the body defends itself against exotoxins.
Exotoxins (def) are toxins, often proteins in nature, secreted from a living bacterium but also released upon bacterial lysis. In addition, some bacteria use various secretion systems such as the type 3 secretion system to inject toxins directly into human cells. (As learned earlier, the lipopolysaccharide or LPS portion of the Gram-negative bacterial cell wall is known as endotoxin (def), a PAMP that can initiate an excessive inflammatory response in the host. It was originally called endotoxin because it was located within the Gram-negative cell wall as opposed to being secreted from bacteria as in the case of exotoxins.)
Not all exotoxins are necessarily produced to harm humans. Some may be designed to play a role in bacterial physiology, such as resisting bacteriophages, regulating cellular function, or quorum sensing. Other toxins may be produced primarily to target protozoa, insects, and smaller animals and harming human cells becomes an accidental side effect.
There are three main types of exotoxins:
- superantigens (Type I toxins);
- exotoxins that damage host cell membranes (Type II toxins); and
- A-B toxins and other toxin that interfere with host cell function (Type III toxins).
The body's major defense against exotoxins is the production of antitoxin antibodies. Once the antibody binds to the exotoxin, the toxin can no longer bind to the receptors on the host cell membrane.
We will now look at each of these three types of exotoxins.
- Exotoxins are toxins, often protein in nature, secreted from a living bacterium.
- Some bacteria use various secretion systems to inject toxins directly into human cells.
- There are three main types of exotoxins: superantigens (type I toxins); exotoxins that damage host cell membranes (type II toxins); and A-B toxins and other toxin that interfere with host cell function (type III toxins).
- The body's major defense against exotoxins is the production of antitoxin antibodies. Once the antibody binds to the exotoxin, the toxin can no longer bind to the receptors on the host cell membrane.
Study the material in this section and then write out the answers to these questions. Do not just click on the answers and write them out. This will not test your understanding of this tutorial.
- List three types of exotoxins.
- Define exotoxin. (ans)
- The body's major defense against exotoxins is _______________________________________________. (ans)
What We Know About the Chernobyl Animal Mutations
The 1986 Chernobyl accident resulted in one of the highest unintentional releases of radioactivity in history. The graphite moderator of reactor 4 was exposed to air and ignited, shooting plumes of radioactive fallout across what is now Belarus, Ukraine, Russia, and Europe. While few people live near Chernobyl now, animals living in the vicinity of the accident allow us to study the effects of radiation and gauge recovery from the disaster.
Most domestic animals have moved away from the accident, and those deformed farm animals that were born did not reproduce. After the first few years following the accident, scientists focused on studies of wild animals and pets that had been left behind, in order to learn about Chernobyl's impact.
Although the Chernobyl accident can't be compared to effects from a nuclear bomb because the isotopes released by the reactor differ from those produced by a nuclear weapon, both accidents and bombs cause mutations and cancer.
It's crucial to study the effects of the disaster to help people understand the serious and long-lasting consequences of nuclear releases. Moreover, understanding the effects of Chernobyl may help humanity react to other nuclear power plant accidents.
Bacteria generate toxins which can be classified as either exotoxins or endotoxins. Exotoxins are generated and actively secreted endotoxins remain part of the bacteria. Usually, an endotoxin is part of the bacterial outer membrane, and it is not released until the bacterium is killed by the immune system. The body's response to an endotoxin can involve severe inflammation. In general, the inflammation process is usually considered beneficial to the infected host, but if the reaction is severe enough, it can lead to sepsis.
Some bacterial toxins can be used in the treatment of tumors. 
Toxinosis is pathogenesis caused by the bacterial toxin alone, not necessarily involving bacterial infection (e.g. when the bacteria have died, but have already produced toxin, which are ingested). It can be caused by Staphylococcus aureus toxins, for example. 
Cyanobacteria are an important autotrophic bacteria in the water food web. Explosions of cyanobacteria known as algal blooms can produce toxins harmful to both the ecosystem and human health. Detection of the extent of an algal bloom begins by taking samples of water at various depths and locations in the bloom. 
Solid-phase adsorption toxin tracking (SPATT) Edit
SPATT is a useful tool in tracking algal blooms as it is reliable, sensitive, and inexpensive. One of the downsides is that it does not give very good results for water soluble toxins as compared to hydrophobic compounds. This tool is mainly used to determine intercellular concentrations of toxins but the cyanobacteria can also be lysed to determine the total toxin amount in a sample. 
Polymerase chain reaction (PCR) Edit
PCR is a molecular tool that allows for analysis of genetic information. PCR is used to amplify the amount of certain DNA within a sample which are usually specific genes within a sample. Genetic targets for cyanobacteria in PCR include the 16S ribosomal RNA gene, phycocyanin operon, internal transcribed spacer region, and the RNA polymerase β subunit gene. PCR is effective when the gene of a known enzyme for producing the microbial toxin or the microbial toxin itself is known. 
Enzyme inhibition Edit
There are many diverse ways of monitoring enzyme levels through the use of enzyme inhibition. The general principle in many of these is the use the knowledge that many enzymes are driven by phosphate-releasing compounds such as adenosine triphosphate. Using radiolabelled 32 P phosphate a fluorometric analysis can be used. Or unique polymers can be used to immobilize enzymes and act in an electrochemical biosensor. Overall, the benefits include a fast response time and little sample preparation. Some of the downsides include a lack of specificity in terms of being able to get readings of very small amounts of toxin and the rigidity of the assays in apply certain procedures to different toxins. 
Immunochemical methods Edit
This detection method uses mammalian antibodies to bind to microbial toxins which can then be processed in a variety of different ways. Of the commercial ways of using immunochemical detection would be enzyme-linked immunosorbent assays (ELISA). This assay has the advantage of being able to screen for a broad range of toxins but could have issues with specificity depending on the antibody used.  A more exotic setup involves the use of CdS quantum dots which are used in an electro-chemiluminescent immunosensor.  A major aspect of immunochemical methods being tested in laboratories are uses of nanowires and other nanomaterials to detect microbial toxins. 
There are over 200 Clostridium species in the world that live in mundane places such as soil, water, dust, and even our digestive tracts. Some of these species produce harmful toxins such as botulinum toxin and tetanus toxin among others. Most clostridium species that do have toxins typically have binary toxins with the first unit involved in getting the toxin into the cell and the second unit cause cellular stress or deformation. 
Botulinum neurotoxin Edit
Botulinum neurotoxins (BoNTs) are the causative agents of the deadly food poisoning disease botulism, and could pose a major biological warfare threat due to their extreme toxicity and ease of production. They also serve as powerful tools to treat an ever expanding list of medical conditions. 
Tetanus toxin Edit
Clostridium tetani produces tetanus toxin (TeNT protein), which leads to a fatal condition known as tetanus in many vertebrates (including humans) and invertebrates.
These toxins are produced by vibrio species of bacteria and like to accumulate in marine life such as the pufferfish. These toxins are produced when vibrio bacteria are stressed by changes in temperature and salinity of environment which leads towards production of toxins. The main hazard towards humans is during consumption of contaminated seafood. Tetrodotoxin poisoning is becoming common in more northern and typically colder marine waters as higher precipitation and warmer waters from climate change triggers vibrio bacteria to produce toxins. 
Immune evasion proteins from Staphylococcus aureus have a significant conservation of protein structures and a range of activities that are all directed at the two key elements of host immunity, complement and neutrophils. These secreted virulence factors assist the bacterium in surviving immune response mechanisms. 
There is only one viral toxin that has been described so far: NSP4 from rotavirus. It inhibits the microtubule-mediated secretory pathway and alters cytoskeleton organization in polarized epithelial cells. It has been identified as the viral enterotoxin based on the observation that the protein caused diarrhea when administered intraperitoneally or intra-ileally in infant mice in an age-dependent manner.  NSP4 can induce aqueous secretion in the gastrointestinal tract of neonatal mice through activation of an age- and Ca2+-dependent plasma membrane anion permeability. 
6.2: The Ability to Produce Harmful Exotoxins: An Overview - Biology
Toxigenesis, or the ability to produce toxins, is an underlying mechanism by which many bacterial pathogens produce disease. At a chemical level, there are two main types of bacterial toxins, lipopolysaccharides, which are associated with the cell wall of Gram-negative bacteria, and proteins, which are released from bacterial cells and may act at tissue sites removed from the site of bacterial growth. The cell-associated toxins are referred to as endotoxins and the extracellular diffusible toxins are referred to as exotoxins.
Endotoxins are cell-associated substances that are structural components of bacteria. Most endotoxins are located in the cell envelope. In the context of this article, endotoxin refers specifically to the lipopolysaccharide (LPS) or lipooligosaccharide (LOS) located in the outer membrane of Gram-negative bacteria. Although structural components of cells, soluble endotoxins may be released from growing bacteria or from cells that are lysed as a result of effective host defense mechanisms or by the activities of certain antibiotics. Endotoxins generally act in the vicinity of bacterial growth or presence.
Exotoxins are usually secreted by bacteria and act at a site removed from bacterial growth. However, in some cases, exotoxins are only released by lysis of the bacterial cell. Exotoxins are usually proteins, minimally polypeptides, that act enzymatically or through direct action with host cells and stimulate a variety of host responses. Most exotoxins act at tissue sites remote from the original point of bacterial invasion or growth. However, some bacterial exotoxins act at the site of pathogen colonization and may play a role in invasion.
BACTERIAL PROTEIN TOXINS
Exotoxins are usually secreted by living bacteria during exponential growth. The production of the toxin is generally specific to a particular bacterial species that produces the disease associated with the toxin (e.g. only Clostridium tetani produces tetanus toxin only Corynebacterium diphtheriae produces the diphtheria toxin). Usually, virulent strains of the bacterium produce the toxin while nonvirulent strains do not, and the toxin is the major determinant of virulence (e.g. tetanus and diphtheria). At one time, it was thought that exotoxin production was limited mainly to Gram-positive bacteria, but clearly both Gram-positive and Gram-negative bacteria produce soluble protein toxins.
Bacterial protein toxins are the most powerful human poisons known and retain high activity at very high dilutions. The lethality of the most potent bacterial exotoxins is compared to the lethality of strychnine, snake venom, and endotoxin in Table 1 below.
TABLE 1. LETHALITY OF BACTERIAL PROTEIN TOXINS
|Toxin ||Toxic Dose (mg) ||Host ||Lethal toxicity ||compared with: |
| || || ||Strychnine||Endotoxin (LPS)||Snake Venom|
|Botulinum toxin||0.8x10 -8||Mouse||3x10 6||3x10 7||3x10 5|
|Tetanus toxin||4x10 -8||Mouse||1x10 6||1x10 7||1x10 5|
|Shiga toxin||2.3x10 -6||Rabbit||1x10 6||1x10 7||1x10 5|
|Diphtheria toxin||6x10 -5||Guinea pig||2x10 3||2x10 4||2x10 2|
Usually the site of damage caused by an exotoxin indicates the location for activity of that toxin. Terms such as enterotoxin, neurotoxin, leukocidin or hemolysin are descriptive terms that indicate the target site of some well-defined protein toxins. A few bacterial toxins that obviously bring about the death of an animal are known simply as lethal toxins, and even though the tissues affected and the target site or substrate may be known, the precise mechanism by which death occurs is not clear (e.g. anthrax LF).
Some bacterial toxins are utilized as invasins because they act locally to promote bacterial invasion. Examples are extracellular enzymes that degrade tissue matrices or fibrin, allowing the bacteria to spread. This includes collagenase, hyaluronidase and streptokinase. Other toxins, also considered invasins, degrade membrane components, such as phospholipases and lecithinases. The pore-forming toxins that insert a pore into eucaryotic membranes are considered as invasins, as well, but they will be reviewed here.
Some protein toxins have very specific cytotoxic activity (i.e., they attack specific types of cells). For example, tetanus and botulinum toxins attack only neurons. But some toxins (as produced by staphylococci, streptococci, clostridia, etc.) have fairly broad cytotoxic activity and cause nonspecific death of various types of cells or damage to tissues, eventually resulting in necrosis. Toxins that are phospholipases act in this way. This is also true of pore-forming hemolysins and leukocidins.
Bacterial protein toxins are strongly antigenic. In vivo , specific antibody neutralizes the toxicity of these bacterial exotoxins (antitoxin). However, in vitro, specific antitoxin may not fully inhibit their activity. This suggests that the antigenic determinant of the toxin may be distinct from the active portion of the protein molecule. The degree of neutralization of the active site may depend on the distance from the antigenic site on the molecule. However, since the toxin is fully neutralized in vivo , this suggests that other host factors must play a role in toxin neutralization in nature.
Protein exotoxins are inherently unstable. In time they lose their toxic properties but retain their antigenic ones. This was first discovered by Ehrlich who coined the term "toxoid" for this product. Toxoids are detoxified toxins which retain their antigenicity and their immunizing capacity. The formation of toxoids can be accelerated by treating toxins with a variety of reagents including formalin, iodine, pepsin, ascorbic acid, ketones, etc. The mixture is maintained at 37 degrees at pH range 6 to 9 for several weeks. The resulting toxoids can be used for artificial immunization against diseases caused by pathogens where the primary determinant of bacterial virulence is toxin production. Toxoids are effective immunizing agents against diphtheria and tetanus that are part of the DPT (DTP) vaccine.
Toxins with Enzymatic Activity
As proteins, many bacterial toxins resemble enzymes in a number of ways. Like enzymes, they are denatured by heat, acid and proteolytic enzymes, they act catalytically, and they exhibit specificity of action. The substrate (in the host) may be a component of tissue cells, organs or body fluid.
A plus B Subunit Arrangement
Many protein toxins, notably those that act intracellularly (with regard to host cells), consist of two components: one component (subunit A) is responsible for the enzymatic activity of the toxin the other component (subunit B) is concerned with binding to a specific receptor on the host cell membrane and transferring the enzyme across the membrane. The enzymatic component is not active until it is released from the native (A+B) toxin. Isolated A subunits are enzymatically active but lack binding and cell entry capability. Isolated B subunits may bind to target cells (and even block the binding of the native toxin), but they are nontoxic.
There are a variety of ways that toxin subunits may be synthesized and arranged: A + B indicates that the toxin is synthesized and secreted as two separate protein subunits that interact at the target cell surface A-B or A-5B indicates that the A and B subunits are synthesized separately, but associated by noncovalent bonds during secretion and binding to their target 5B indicates that the binding domain of the protein is composed of 5 identical subunits. A/B denotes a toxin synthesized as a single polypeptide, divided into A and B domains that may be separated by proteolytic cleavage.
Attachment and Entry of Toxins
There are at least two mechanisms of toxin entry into target cells.
In one mechanism called direct entry, the B subunit of the native (A+B) toxin binds to a specific receptor on the target cell and induces the formation of a pore in the membrane through which the A subunit is transferred into the cell cytoplasm.
In an alternative mechanism, the native toxin binds to the target cell and the A+B structure is taken into the cell by the process of receptor-mediated endocytosis (RME). The toxin is internalized in the cell in a membrane-enclosed vesicle called an endosome. H + ions enter the endosome lowering the internal pH which causes the A+B subunits to separate. The B subunit affects the release of the A subunit from the endosome so that it will reach its target in the cell cytoplasm. The B subunit remains in the endosome and is recycled to the cell surface.
In both cases above, a large protein molecule must insert into and cross a membrane lipid bilayer, either the cell membrane or the endosome membrane. This activity is reflected in the ability of most A+B or A/B toxins, or their B components, to insert into artificial lipid bilayers, creating ion permeable pathways. If the B subunit contains a hydrophobic region (of amino acids) that insert into the membrane (as in the case of the diphtheria toxin), it may be referred to as the T (translocation) domain of the toxin.
A few bacterial toxins (e.g. diphtheria) are known to utilize both direct entry and RME to enter into host cells, which is not surprising since both mechanisms are variations on a theme. Bacterial toxins with similar enzymatic mechanisms may enter their target cells by different mechanisms. Thus, the diphtheria toxin and Pseudomonas exotoxin A, which have identical mechanisms of enzymatic activity, enter their host cells in slightly different ways. The adenylate cyclase toxin of Bordetella pertussis (pertussis AC) and anthrax EF produced by Bacillus anthracis, act similarly to catalyze the production of cAMP from host cell intracellular ATP reserves. However, the anthrax toxin enters cells by receptor mediated endocytosis, whereas the pertussis adenylate cyclase traverses the cell membrane directly.
The specific receptors for the B subunit of toxins on target cells or tissues are usually sialogangliosides (glycoproteins) called G-proteins on the cell membrane. For example, the cholera toxin utilizes the ganglioside GM1, and tetanus toxin utilizes ganglioside GT1 and/or GD1b as receptors on host cells.
The best known and studied bacterial toxin is the diphtheria toxin, produced by Corynebacterium diphtheriae. Diphtheria toxin is a bacterial exotoxin of the A/B prototype. It is produced as single polypeptide chain with a molecular weight of 60,000 daltons. The function of the protein is distinguishable into two parts: subunit A, with a m.w. of 21,000 daltons, contains the enzymatic activity for inhibition of elongation factor-2 involved in host protein synthesis subunit B, with a m.w. of 39,000 daltons, is responsible for binding to the membrane of a susceptible host cell. The B subunit possesses a region T (translocation) domain which inserts into the endosome membrane thus securing the release of the enzymatic component into the cytoplasm.
Figure 1. Diphtheria Toxin (Dtx). A (red) is the catalytic domain B (yellow) is the binding domain which displays the receptor for cell attachment T (blue) is the hydrophobic domain responsible for insertion into the endosome membrane to secure the release of A. The protein is illustrated in its "closed" configuration.
In vitro, the native toxin is produced in an inactive form which can be activated by the proteolytic enzyme trypsin in the presence of thiol (reducing agent). The enzymatic activity of Fragment A is masked in the intact toxin. Fragment B is required to enable Fragment A to reach the cytoplasm of susceptible cells. The C terminal end of Fragment B is hydrophilic and contains determinants that interact with specific membrane receptors on sensitive cell membranes and the N-terminal end of Fragment B (called the T domain) is strongly hydrophobic. The specific membrane receptor for the B fragment has been shown to be a transmembranous heparin-binding protein on the susceptible cell's surface.
The diphtheria toxin enters its target cells by either direct entry or receptor mediated endocytosis. The first step is the irreversible binding of the C-terminal hydrophilic portion of Fragment B (AA 432-535) to the receptor. During RME, the whole toxin is then taken up in an endocytic vesicle. In the vesicle, the pH drops to about 5 which allows unfolding of the A and B chains. This exposes hydrophobic regions of both the A and B chains that can insert into the vesicle membrane. The result is exposure of the A chain to the cytoplasmic side of the membrane. There, reduction and proteolytic cleavage releases the A chain in the cytoplasm. The A fragment is released as an extended chain but regains its active (enzymatic) globular conformation in the cytoplasm. The A chain catalyzes the ADP ribosylation of elongation factor-2 (EF-2) as shown in Figure 2.
Figure 2. Entry and activity of diphtheria toxin (Dtx) in susceptible cells. The B domain of the toxin binds to a cognate receptor on a susceptible cell. The toxin is taken up in an endosome by receptor mediated encocytosis. Acidification of the endocytic vesicle allows unfolding of the A and B chains exposing the hydrophobic T domain of the toxin. The T domain inserts into the endosome membrane translocating the A fragment into the cytoplasm where it regains its enzymatic configuration . The enzymatic A component utilizes NAD as a substrate. It catalyzes the attachment of the ADP-ribose portion of NAD to elongation factor (EF-2) which inactivates its function in protein synthesis.
Table 2 describes several bacterial toxins with known enzymatic activity and the biological effects of the toxins in humans.
TABLE 2. BIOLOGICAL EFFECTS OF SOME BACTERIAL EXOTOXINS WITH ENZYMATIC ACTIVITY
|TOXIN (subunit arr)*||ENZYMATIC ACTIVITY||BIOLOGICAL EFFECTS|
|Cholera toxin (A-5B)||ADP ribosylates eucaryotic adenylate cyclase Gs regulatory protein||Activates adenylate cyclase increased level of intracellular cAMP promote secretion of fluid and electrolytes in intestinal epithelium leading to diarrhea|
|Diphtheria toxin (A/B)||ADP ribosylates elongation factor 2|| |
Inhibits protein synthesis in animal cells resulting in death of the cells
|Pertussis toxin (A-5B)||ADP ribosylates adenylate cyclase Gi regulatory protein|| |
Blocks inhibition of adenylate cyclase increased levels of cAMP affect hormone activity and reduce phagocytic activity
|E. coli heat-labile toxin LT (A-5B)||ADP ribosylates adenylate cyclase Gs regulatory protein||Similar or identical to cholera toxin|
|Shiga toxin (A/5B||Glycosidase cleavage of ribosomal RNA (cleaves a single Adenine base from the 28S rRNA)|| |
Inactivates the mammalian 60S ribosomal subunit and leads to inhibition of protein synthesis and death of the susceptible cells pathology is diarrhea, hemorrhagic colitis (HC) and/or hemolytic uremic syndrome (HUS)
|Pseudomonas Exotoxin A (A/B)||ADP ribosylates elongation factor-2 analogous to diphtheria toxin|| |
Inhibits protein synthesis in susceptible cells, resulting in death of the cells
|Botulinum toxin (A/B)||Zn ++ dependent protease acts on synaptobrevin at motor neuron ganglioside|| |
Inhibits presynaptic acetylycholine release from peripheral cholinergic neurons resulting in flaccid paralysis
|Tetanus toxin (A/B)||Zn ++ dependent protease acts on synaptobrevin in central nervous system|| |
Inhibits neurotransmitter release from inhibitory neurons in the CNS resulting in spastic paralysis
|Anthrax toxin LF (A2+B)||Metallo protease that cleaves MAPKK (mitogen-activated protein kinase kinase) enzymes || |
Combined with the B subunit (PA), LF induces cytokine release and death of target cells or experimental animals
|Bordetella pertussis AC toxin (A/B) and Bacillus anthracis EF (A1+B)|| |
Calmodulin-regulated adenylate cyclases that catalyze the formation of cyclic AMP from ATP in susceptible cells, as well as the formation of ion-permeable pores in cell membranes
|Increases cAMP in phagocytes leading to inhibition of phagocytosis by neutrophils and macrophages also causes hemolysis and leukolysis|
|Staphylococcus aureus Exfoliatin B||Cleaves desmoglein 1, a cadherin found in desmosomes in the epidermis |
(also a superantigen)
Separation of the stratum granulosum of the epidermis, between the living layers and the superficial dead layers.
* toxin subunit arrangements: A-B or A-5B indicates subunits synthesized separately and associated by noncovalent bonds A/B denotes subunit domains of a single protein that may be separated by proteolytic cleavage A+B indicates subunits synthesized and secreted as separate protein subunits that interact at the target cell surface 5B indicates that the binding domain is composed of 5 identical subunits.
Pore-forming toxins, as the name suggests, insert a transmembranous pore into a host cell membrane, thereby disrupting the selective influx and efflux of ions across the membrane. This group of toxins includes the RTX toxins of Gram-negative bacteria, streptolysin O produced by S. pyogenes, and S. aureus alpha toxin. Generally, these toxins are produced as subunits that self-assemble as a pore on the eucaryotic membrane.
S. aureus alpha-toxin is considered the model of oligomerizing pore-forming cytotoxins. The alpha-toxin is synthesized as a 319 amino acid precursor molecule that contains an N-terminal signal sequence of 26 amino acids. The secreted mature toxin, or protomer, is a hydrophilic molecule with a molecular weight of 33 kDa. Seven toxin protomers assemble to form a 232 kDa mushroom-shaped heptamer comprising three distinct domains. The cap and rim domains of the heptamer are situated at the surface of the plasma membrane, while the stem domain serves as a transmembranous ion channel through the membrane.
TABLE 3. SOME PORE-FORMING BACTERIAL TOXINS
|Toxin ||Bacterial source ||Target ||Disease |
|perfringiolysin O ||Clostridium perfringens ||cholesterol ||gas gangrene |
|hemolysin ||Escherichia coli ||cell membrane ||UTI |
|listeriolysin ||Listeria monocytogenes ||cholesterol ||systemic meningitis |
|anthrax EF ||Bacillus anthracis ||cell membrane ||anthrax (edema) |
|alpha toxin||Staphylococcus aureus ||cell membrane ||abcesses |
|pneumolysin ||Streptococcus pneumoniae ||cholesterol ||pneumonia otitis media |
|streptolysin O ||Streptococcus pyogenes ||cholesterol ||strep throat |
|leukocidin ||Staphylococcus aureus||phagocyte membrane ||pyogenic infections |
Superantigens: Toxins that Stimulate the Immune System
Several bacterial toxins can act directly on the T cells and antigen-presenting cells of the immune system. Impairment of the immunologic functions of these cells by toxin can lead to human disease. One large family of toxins in this category are the so-called pyrogenic exotoxins produced by staphylococci and streptococci, whose biological activities include potent stimulation of the immune system, pyrogenicity, and enhancement of endotoxin shock.
Pyrogenic exotoxins are secreted toxins of 22 kDa to 30 kDa, and include staphylococcal enterotoxins serotypes A-E, G, and H group A streptococcal pyrogenic exotoxins A-C staphylococcal exfoliatin toxin and staphylococcal TSST-1.
In general, the potent immunostimulatory properties of superantigens are a direct result of toxin binding to distinct regions outside the peptide binding cleft of the major histocompatibility class II molecules (MHC II), expressed on the surface of antigen-presenting cells, and to specific Vß elements on the T-cell receptor of T-lymphocytes. This results in a massive proliferation of up to 20% of peripheral T cells. Concomitant to T-cell proliferation is a massive release of cytokines from lymphocytes (e.g. interleukin-2, tumor necrosis factor ß, gamma interferon) and monocytes (e.g. IL-1, IL-6, tumor necrosis factor a). These cytokines serve as mediators of the hypotension, high fever, and diffuse erythematous rash that are characteristic of toxic-shock syndrome.
The staphylococcal enterotoxins are superantigens, but it is not known if this activity contributes to vomiting or diarrhea characteristic of staphylococcal food poisoning.
Control of Synthesis and the Release of Protein Toxins
The regulation of synthesis and secretion of many bacterial toxins is tightly controlled by regulatory elements that are sensitive to environmental signals. For example, the production of diphtheria toxin is totally repressed by the availability of adequate amounts of iron in the medium for bacterial growth. Only under conditions of limiting amounts of iron in the growth medium does toxin production become derepressed. The expression of cholera toxin and related virulence factors (adhesins) is controlled by environmental osmolarity and temperature. In B. pertussis, induction of different virulence components is staggered, such that attachment factors are produced initially to establish the infection, and toxins are synthesized and released later to counter the host defenses and promote bacterial survival.
The processes by which protein toxins are assembled and secreted by bacterial cells are also variable. Many of the classic exotoxins are synthesized with an NH terminal leader (signal) sequence consisting of a few (1-3) charged amino acids and a stretch of (14-20) hydrophobic amino acids. The signal sequence may bind and insert into the cytoplasmic membrane during translation such that the polypeptide is secreted while being synthesized. The signal peptide is cleaved as the toxin (protein) is released into the periplasm. Alternatively, the toxin may be synthesized intracytoplasmically, then bound to a leader sequence for passage across the membrane. Frequently, chaperone proteins are required to guide this process. Some multicomponent toxins, such as the cholera toxin, have their subunits synthesized and secreted separately into the periplasm where they are assembled. In Gram-negative bacteria, the outer membrane poses an additional permeability barrier that a protein toxin usually has to mediate if it is to be released in a soluble form. It has been proposed that some Gram-negative exotoxins (e.g. E. coli ST enterotoxin) might be released in membrane vesicles composed of outer membrane components. Since these vesicles possibly possess outer membrane-associated attachment factors, they could act as "smart bombs" capable of specifically interacting with and possibly entering target cells to release their contents of toxin.
The genetic ability to produce a toxin, including regulatory genes, may be found on the bacxterial chromosome, plasmids and lysogenic bacteriophages. Sometimes they occur within pathogenicity islands. In any case, the processes of genetic exchange in bacteria, notably conjugation and transduction, can mobilize genetic elements between strains and species of bacteria. Horizontal gene transfer (HGT) of genes that encode virulence is known to occur between species of bacteria. This explains how E. coli and Vibrio cholerae produce a nearly identical diarrhea-inducing toxin, as well as how E. coli O157:H7 acquired ability to produce shiga toxin form Shigella dysenteriae . The intestinal tract is probably an ideal habitat for bacteria to undergo HGT with one another.
There is conclusive evidence for the pathogenic role of diphtheria, tetanus and botulinum toxins, various enterotoxins, staphylococcal toxic shock syndrome toxin, and streptococcal pyrogenic exotoxins. And there is good evidence for the pathological involvement of pertussis toxin, anthrax toxin, shiga toxin and the necrotizing toxins of clostridia, in bacterial disease. But why certain bacteria produce such potent toxins is mysterious and is analogous to asking why an organism should produce an antibiotic. The production of a toxin may play a role in adapting a bacterium to a particular niche, but it is not essential to the viability of the organism. Most toxigenic bacteria are free-living in nature and in associations with humans in a form which is phenotypically identical to the toxigenic strain but lacking the ability to produce the toxin.
A summary of bacterial protein toxins and their activities is given in Tables 4. Details of the mechanisms of action of these toxins and their involvement in the pathogenesis of disease is discussed in chapters with the specific bacterial pathogens.
For more information and references on bacterial toxins go to this website: Bacterial Toxins: Friends or Foes?
Biological evaluation and regulation of medical devices in Japan
17.3.7 Pyrogen test
Pyrogens include endotoxins (toxins derived from gram-negative-bacteria) and non-endotoxic pyrogens (substances derived from microorganisms other than gram-negative-bacteria, or from chemical substances). Iryokiki-Shinsa No.36 mentions the pyrogen test – using a rabbit – that can detect both endotoxins and non-endotoxic pyrogenic substances, and the in vitro endotoxin test, widely used as a method of detecting microbial contamination (endotoxins). The pyrogen test using a rabbit involves administering a physiological saline solution extract of the test substance venously, and determining the existence of pyrogens from the rise in body temperature after administration. The in vitro endotoxin test includes the gelation method, which indexes the formation of a gel that occurs via endotoxins derived from gram-negative bacteria acting on a lysate test solution prepared from limulus amebocyte lysate, and the turbidimetric and colorimetric methods that index optical changes. The test method selected differs according to the purpose of the test (a limit test to check below default values, a quantitative test to check content) and the characteristics (color, existence of reaction interference factors) of the prepared extract. Endotoxins and non-endotoxic pyrogens display both behavior and body temperature raising mechanisms that are complex and indistinct and it is necessary to select extraction conditions according to the properties of the material and the product’s purpose of use.
The test method given in Iryokiki-Shinsa No.36 is fundamentally determination criteria equivalent to the pyrogen test method included in the general test methods listed by the Japanese pharmacopeia. It differs slightly from US Pharmacopeia (USP) or European Pharmacopoeia (EP) determination standards. In any case, the test is conducted in consideration of various terms and conditions, such as the content, characteristic, application, etc., of the medical device’s raw materials. The test substance or its extract, is administered into the auricular vein of a rabbit, body temperature (rectal temperature) is measured continuously over 3 h, and any difference from the reference body temperature is looked for. When the body temperature (rectal temperature) of at least 2 out of 3 animals rises 0.6°C or more within 3 h post-administration, the result is judged as being positive for pyrogens. Although it cannot be considered as a positive result in and of itself, if one animal shows a body temperature rise of 0.6°C or the sum of the maximum temperature of three animals exceeds 1.4°C, a second test must be performed. Five animals are used for this second test and, if two or more animals then show a body temperature rise of 0.6°C or more, the result is considered positive.
For medical devices that use naturally-derived materials with the possibility of contamination by endotoxins (collagen, gelatin, alginate, etc.), it is preferable for an in vitro endotoxin test to be performed as well and the quantity of endotoxins measured. This test and evaluation must be performed for medical devices in contact with circulating blood or implanted in the body.
ISO 10993 contains no parts that address the pyrogen test. This test is, however, noted in Annex F (informative) of ISO 10993-11, not as being indispensable but as requiring a determination on whether it needs to be performed based on the examination of various factors, such as whether it involves the materials and components of the medical device or a new substance. Just like guidelines in Japan, ASTM F 748 contains a selection table for the corresponding test items. Iryokiki-Shinsa No.36 recognizes that tests conducted under standards such as ISO 10993-11 and USP display a detection sensitivity that is equivalent to domestic test methods.
How Many Types of Exotoxins are there?
There are three types of exotoxins. They are:
1. A-B toxin:
A-B toxins are the polypeptides composed of part A and part B. Part A is an active enzymatic component taking part in altering the functions of host cells through inhibition of protein synthesis. Part B is a binding component that mediates the attachment of the toxin to the receptors of host cells.
Bacteria that produce exotoxins: Clostridium tetani, Clostridium diphtheria, Vibrio cholerae, S. aureus.
Toxins: Tetanospasmin, Diphtheria toxin, Cholera toxin, Exfoliatin.
Target cells: Different types of cells throughout the body.
Mechanisms of these types of exotoxins: Different toxins show different mechanisms of actions. Few of them are as below:
- Tetanospasmin inhibits the release of inhibitory neurotransmitters.
- Diphtheria toxins cause ADP ribosylation of EF-2 resulting in inhibition of protein synthesis.
- Cholera toxins cause ADP ribosylation of adenylate cyclase, leading to rising in cyclic AMP.
- Neurotoxin prevents the transmission of nerve impulses: flaccid paralysis results.
- Neurotoxin blocks nerve impulses to muscle relaxation pathways: results in uncontrollable muscle contractions.
- Cytotoxin inhibits protein synthesis, especially in nerve, heart, and kidney cells.
- Enterotoxin causes secretion of large amounts of fluids and electrolytes that result in diarrhea.
- Exfoliatin is a protease that breaks down desmoglein protein in desmosomes of skins causing scalded skin syndromes.
Diseases: Spastic paralysis Myopathy, Polyneuropathy, Profuse watery diarrhea
Botulism, Tetanus, Diphtheria, Scalded skin syndrome, Cholera, Traveler’s diarrhea, Anthrax.
2. Membrane-disrupting Toxins:
Among three types of exotoxins, membrane-disrupting toxins are the toxins that cause damage to host cells by breaking down the structure of their plasma membranes or by forming protein channels or by disrupting the phospholipids.
Bacteria that produce exotoxins: Staphylococcus aureus, Listeria monocytogenes, Clostridium perfringens, Clostridium difficile.
Toxins: Leukocidin, α-Toxin (lecithinase), Hemolysin, Listeriolysin, Perfringolysin-O, Streptolysins. Streptolysin O and Streptolysin L.
Target cells: Many cell types including erythrocytes, phagocytic leukocytes, and macrophages.
Mechanisms: Different toxins show different mechanisms of actions. Few of them are as below:
- Leukocidins and Hemolysins destroy their target cells (leukocytes and erythrocytes respectively) by forming protein channels in the plasma membrane of their target cells.
- The alpha-toxin such as lecithinase causes lysis of cell membranes by breaking down lecithin into phosphorylcholine and diglyceride.
- Phospholipase removes polar head groups from phospholipids.
- Cytotoxin destroys massive red blood cell (hemolysis) and destroys host cell cytoskeleton.
- Enterotoxin causes the release of fluids and electrolytes resulting in diarrhea.
Diseases: Tissue necrosis, Gas gangrene, Food poisoning, Antibiotic-associated diarrhea.
Superantigens are the antigens that stimulate the production of a large amount of T cells and also stimulate these T cells to produce a large amount of cytokine.
Bacteria that produce exotoxins (superantigens): Streptococcus pyogenes, Staphylococcus aureus.
Superantigens/toxins: Streptococcal pyogenic exotoxin, Toxic shock syndrome toxin, Staphylococcal enterotoxin A (SEA).
Target cells: T cells, macrophages.
Mechanisms: Different toxins/superantigens show different mechanisms of actions. Few of them are as below:
- Both streptococcal pyogenic exotoxin and toxic shock syndrome toxin cause T cell stimulation, the release of cytokines (IL-1, IL-2, TNF), possible enhancement of LPS activities.
- Released cytokines stimulate enteric nervous systems to activate the vomiting center in the brain.
- Enterotoxin causes secretion of fluids and electrolytes that results in diarrhea.
- The toxin causes secretion of fluids and electrolytes from capillaries that decreases blood volume and lowers blood pressure.
Diseases: Fever, rash, nausea, vomiting, diarrhea toxic shock-like syndrome, Toxic shock syndrome, food poisoning.
Plants are capable of producing and synthesizing diverse groups of organic compounds and are divided into two major groups: primary and secondary metabolites.  Secondary metabolites are metabolic intermediates or products which are not essential to growth and life of the producing plants but rather required for interaction of plants with their environment and produced in response to stress. Their antibiotic, antifungal and antiviral properties protect the plant from pathogens. Some secondary metabolites such as phenylpropanoids protect plants from UV damage.  The biological effects of plant secondary metabolites on humans have been known since ancient times. The herb Artemisia annua which contains Artemisinin, has been widely used in Chinese traditional medicine more than two thousand years ago. [ citation needed ] Plant secondary metabolites are classified by their chemical structure and can be divided into four major classes: terpenes, phenylpropanoids (i.e. phenolics), polyketides, and alkaloids. 
Terpenes constitute a large class of natural products which are composed of isoprene units. Terpenes are only hydrocarbons and terpenoids are oxygenated hydrocarbons. The general molecular formula of terpenes are multiples of (C5H8)n, where 'n' is number of linked isoprene units. Hence, terpenes are also termed as isoprenoid compounds. Classification is based on the number of isoprene units present in their structure. Some terpenoids (i.e. many sterols) are primary metabolites. Some terpenoids that may have originated as secondary metabolites have subsequently been recruited as plant hormones, such as gibberellins, brassinosteroids, and strigolactones.
|Number of isoprene units||Name||Carbon atoms|
|More than 8||Polyterpene|
- , present in Azadirachta indica, the (Neem tree) , present in Artemisia annua, Chinese wormwood , present in Cannabis sativa, cannabis , glycosylated triterpenes present in e.g. Chenopodium quinoa, quinoa.
Phenolic compounds Edit
Phenolics are a chemical compound characterized by the presence of aromatic ring structure bearing one or more hydroxyl groups. Phenolics are the most abundant secondary metabolites of plants ranging from simple molecules such as phenolic acid to highly polymerized substances such as tannins. Classes of phenolics have been characterized on the basis of their basic skeleton.
|No. of carbon atoms||Basic skeleton||Class|
|7||C6 - C1||Phenolic acids|
|8||C6 - C2||Acetophenone, Phenyle acetic acid|
|9||C6 - C3||Phenylepropanoids, hydroxycinnamic acid, coumarins|
|10||C6 - C4||Naphthoquinone|
|13||C6 - C1- C6||Xanthone|
|14||C6 - C2 - C6||Stilbene, anthraquinone|
|15||C6 - C3 - C6||Flavonoids, isoflavanoids|
|18||(C6 - C3 ) 2||lignans, neolignans|
|30||( C6 - C3 - C6)2||Biflavonoids|
An example of a plant phenol is:
Alkaloids are a diverse group of nitrogen-containing basic compounds. They are typically derived from plant sources and contain one or more nitrogen atoms. Chemically they are very heterogeneous. Based on chemical structures, they may be classified into two broad categories:
- Non heterocyclic or atypical alkaloids, for example hordenine or N-methyltyramine, colchicine, and taxol
- Heterocyclic or typical alkaloids, for example quinine, caffeine, and nicotine
Examples of alkaloids produced by plants are:
- , present in Datura stramonium , present in Atropa belladonna, deadly nightshade , present in Erythroxylum coca the Coca plant , present in the Solanaceae (nightshade) plant family and morphine, present in Papaver somniferum, the opium poppy and vinblastine, mitotic inhibitors found in Catharanthus roseus, the rosy periwinkle
Many alkaloids affect the central nervous system of animals by binding to neurotransmitter receptors.
Glucosinolates are secondary metabolites that include both sulfur and nitrogen atoms, and are derived from glucose, an amino acid and sulfate.
An example of a glucosinolate in plants are:
Many drugs used in modern medicine are derived from plant secondary metabolites.
The two most commonly known terpenoids are artemisinin and paclitaxel. Artemisinin was widely used in Traditional Chinese medicine and later rediscovered as a powerful antimalarial by a Chinese scientist Tu Youyou. She was later awarded the Nobel Prize for the discovery in 2015. Currently, the malaria parasite, Plasmodium falciparum, has become resistant to artemisinin alone and the World Health Organization recommends its use with other antimalarial drugs for a successful therapy. Paclitaxel the active compound found in Taxol is a chemotherapy drug used to treat many forms of cancers including ovarian cancer, breast cancer, lung cancer, Kaposi sarcoma, cervical cancer, and pancreatic cancer.  Taxol was first isolated in 1973 from barks of a coniferous tree, the Pacific Yew. 
Morphine and codeine both belong to the class of alkaloids and are derived from opium poppies. Morphine was discovered in 1804 by a German pharmacist Friedrich Sertürnert. It was the first active alkaloid extracted from the opium poppy. It is mostly known for its strong analgesic effects, however, morphine is also used to treat shortness of breath and treatment of addiction to stronger opiates such as heroin.   Despite its positive effects on humans, morphine has very strong adverse effects, such as addiction, hormone imbalance or constipation.   Due to its highly addictive nature morphine is a strictly controlled substance around the world, used only in very severe cases with some countries underusing it compared to the global average due to the social stigma around it. 
Codeine, also an alkaloid derived from the opium poppy, is considered the most widely used drug in the world according to World Health Organization. It was first isolated in 1832 by a French chemist Pierre Jean Robiquet, also known for the discovery of caffeine and a widely used red dye alizarin.  Primarily codeine is used to treat mild pain and relief coughing  although in some cases it is used to treat diarrhea and some forms of irritable bowel syndrome.  Codeine has the strength of 0.1-0.15 compared to morphine ingested orally,  hence it is much safer to use. Although codeine can be extracted from the opium poppy, the process is not feasible economically due to the low abundance of pure codeine in the plant. A chemical process of methylation of the much more abundant morphine is the main method of production. 
Atropine is an alkaloid first found in Atropa belladonna, a member of the nightshade family. While atropine was first isolated in the 19th century, its medical use dates back to at least the fourth century B.C. where it was used for wounds, gout, and sleeplessness. Currently atropine is administered intraveinously to treat bradycardia and as an antidote to organophosphate poisoning. Overdosing of atropine may lead to atropine poisoning which results in side effects such as blurred vision, nausea, lack of sweating, dry mouth and tachycardia. 
Resveratrol is a phenolic compound of the flavonoid class. It is highly abundant in grapes, blueberries, raspberries and peanuts. It is commonly taken as a dietary supplement for extending life and reducing the risk of cancer and heart disease, however no there is no strong evidence supporting its efficacy.   Nevertheless, flavonoids are in general thought to have beneficial effects for humans. [ citation needed ] Certain studies shown that flavonoids have direct antibiotic activity.  A number of in vitro and limited in vivo studies shown that flavonoids such as quercetin have synergistic activity with antibiotics and are able to suppress bacterial loads. 
Digoxin is a cardiac glycoside first derived by William Withering in 1785 from the foxglove (Digitalis) plant. It is typically used to treat heart conditions such as atrial fibrillation, atrial flutter or heart failure.  Digoxin can, however, have side effects such as nausea, bradycardia, diarrhea or even life-threatening arrhythmia.
The three main classes of fungal secondary metabolites are: polyketides, nonribosomal peptides and terpenes. Although fungal secondary metabolites are not required for growth they play an essential role in survival of fungi in their ecological niche.  The most known fungal secondary metabolite is penicillin discovered by Alexander Fleming in 1928. Later in 1945, Fleming, alongside Ernst Chain and Howard Florey, received a Nobel Prize for its discovery which was pivotal in reducing the number of deaths in World War II by over 100,000. 
Examples of other fungal secondary metabolites are:
Lovastatin was the first FDA approved secondary metabolite to lower cholesterol levels. Lovastatin occurs naturally in low concentrations in oyster mushrooms,  red yeast rice,  and Pu-erh.  Lovastatin's mode of action is competitive inhibition of HMG-CoA reductase, and a rate-limiting enzyme responsible for converting HMG-CoA to mevalonate.
Fungal secondary metabolites can also be dangerous to humans. Claviceps purpurea, a member of the ergot group of fungi typically growing on rye, results in death when ingested. The build-up of poisonous alkaloids found in C. purpurea lead to symptoms such as seizures and spasms, diarrhea, paresthesias, Itching, psychosis or gangrene. Currently, removal of ergot bodies requires putting the rye in brine solution with healthy grains sinking and infected floating. 
Bacterial production of secondary metabolites starts in the stationary phase as a consequence of lack of nutrients or in response to environmental stress. Secondary metabolite synthesis in bacteria is not essential for their growth, however, they allow them to better interact with their ecological niche. The main synthetic pathways of secondary metabolite production in bacteria are b-lactam, oligosaccharide, shikimate, polyketide and non-ribosomal pathways.  Many bacterial secondary metabolites are toxic to mammals. When secreted those poisonous compounds are known as exotoxins whereas those found in the prokaryotic cell wall are endotoxins.
An example of a bacterial secondary metabolite with a positive and negative effect on humans is botulinum toxin synthesised by Clostridium botulinum. This exotoxin often builds up in incorrectly canned foods and when ingested blocks cholinergic neurotransmission leading to muscle paralysis or death. However, botulinum toxin also has multiple medical uses such as treatment of muscle spasticity, migraine and cosmetics use.
Examples of other bacterial secondary metabolites are:
Nonribosomal peptides Edit
- , from the soil bacterium Amycolatopsis orientalis. , from Actinoplanes strain ATCC 33076. , from Actinoplanes teicomyceticus. , from Brevibacillus brevis. , from Bacillus subtilis (Tracy strain). , from Paenibacillus polymyxa.
Ribosomal peptides Edit
Selective breeding was used as one of the first biotechnological techniques used to reduce the unwanted secondary metabolites in food, such as naringin causing bitterness in grapefruit.  In some cases increasing the content of secondary metabolites in a plant is the desired outcome. Traditionally this was done using in-vitro plant tissue culture techniques which allow for: control of growth conditions, mitigate seasonality of plants or protect them from parasites and harmful-microbes. [ citation needed ] Synthesis of secondary metabolites can be further enhanced by introducing elicitors into a tissue plant culture, such as jasmonic acid, UV-B or ozone. These compounds induce stress onto a plant leading to increased production of secondary metabolites.
To further increase the yield of SMs new approaches have been developed. A novel approach used by Evolva uses recombinant yeast S. cerevisiae strains to produce secondary metabolites normally found in plants. The first successful chemical compound synthesised with Evolva was vanillin, widely used in the food beverage industry as flavouring. The process involves inserting the desired secondary metabolite gene into an artificial chromosome in the recombinant yeast leading to synthesis of vanillin. Currently Evolva produces a wide array of chemicals such as stevia, resveratrol or nootkatone.
Nagoya protocol Edit
With the development of recombinant technologies the Nagoya Protocol on Access to Genetic Resources and the Fair and Equitable Sharing of Benefits Arising from their Utilization to the Convention on Biological Diversity was signed in 2010. The protocol regulates the conservation and protection of genetic resources to prevent the exploitation of smaller and poorer countries. If genetic, protein or small molecule resources sourced from biodiverse countries become profitable a compensation scheme was put in place for the countries of origin. 
What is Exotoxin?
Exotoxins are soluble proteins which can act as enzymes. Being an enzyme, it can catalyze many biochemical reactions, and it is reusable. A small amount of exotoxins is enough to generate toxicity. They are secreted to the cell surrounding during their exponential growth or during cell lysis. Therefore, exotoxins are considered an extracellular component. Both gram-negative and gram-positive bacteria produce exotoxins.
Exotoxins are more toxic than endotoxins. Furthermore, they are specific to certain bacterial strains. They produce diseases only specific to that toxin. For example, Clostridium tetani produce tetanus toxin. Sometimes exotoxins act in very remote regions from where they originate by the growth or lysis. Exotoxins can destroy a part of host cells or inhibit their function.
Figure 02: Immune Response to Exotoxins
There are three types of exotoxins: enterotoxins, neurotoxins, and cytotoxins. Their names give an indication of the site of action. Enterotoxins act on the lining of the gastrointestinal tract while neurotoxins act on the function of neurons, and cytotoxins damage the functioning of host cells. Cholera, diphtheria, and tetanus are diseases that arise due to exotoxins. In fact, exotoxins are highly antigenic. Hence, they can stimulate the immune system. By stimulating the immune system, they produce antitoxins to neutralize the toxin.
6.2: The Ability to Produce Harmful Exotoxins: An Overview - Biology
Mechanisms of Bacterial Pathogenicity (page 1)
A pathogen is a microorganism that is able to cause disease in a plant, animal or insect. Pathogenicity is the ability to produce disease in a host organism. Microbes express their pathogenicity by means of their virulence, a term which refers to the degree of pathogenicity of the microbe. Hence, the determinants of virulence of a pathogen are any of its genetic or biochemical or structural features that enable it to produce disease in a host.
The relationship between a host and a pathogen is dynamic, since each modifies the activities and functions of the other. The outcome of such a relationship depends on the virulence of the pathogen and the relative degree of resistance or susceptibility of the host, due mainly to the effectiveness of the host defense mechanisms.
Staphylococcus aureus , arguably the most prevalent pathogen of humans, may cause up to one third of all bacterial diseases ranging from boils and pimples to food poisoning, to septicemia and toxic shock. Electron micrograph from Visuals Unlimited, with permission.
The Underlying Mechanisms of Bacterial Pathogenicity
Two broad qualities of pathogenic bacteria underlie the means by which they cause disease:
1. Invasiveness is the ability to invade tissues. It encompasses mechanisms for colonization (adherence and initial multiplication), production of extracellular substances which facilitate invasion ( invasins ) and ability to bypass or overcome host defense mechanisms.
2. Toxigenesis is the ability to produce toxins. Bacteria may produce two types of toxins called exotoxins and endotoxins. Exotoxins are released from bacterial cells and may act at tissue sites removed from the site of bacterial growth. Endotoxins are cell-associated substance. (In a classic sense, the term endotoxin refers to the lipopolysaccharide component of the outer membrane of Gram-negative bacteria). However, endotoxins may be released from growing bacterial cells and cells that are lysed as a result of effective host defense (e.g. lysozyme) or the activities of certain antibiotics (e.g. penicillins and cephalosporins). Hence, bacterial toxins, both soluble and cell-associated, may be transported by blood and lymph and cause cytotoxic effects at tissue sites remote from the original point of invasion or growth. Some bacterial toxins may also act at the site of colonization and play a role in invasion.
Acid-fast stain of Mycobacterium tuberculosis, the agent of tuberculosis (TB). The bacteria are the small pink-staining rods. More than one-third of the world population is infected. The organism has caused more human deaths than any other bacterium in the history of mankind. Although its ability to produce disease is multifactorial, it is not completely understood. American Society of Microbiology, with permission.
Molecular Biology of the Cell. 4th edition.
Our adaptive immune system saves us from certain death by infection. An infant born with a severely defective adaptive immune system will soon die unless extraordinary measures are taken to isolate it from a host of infectious agents, including bacteria, viruses, fungi, and parasites. Indeed, all multicellular organisms need to defend themselves against infection by such potentially harmful invaders, collectively called pathogens. Invertebrates use relatively simple defense strategies that rely chiefly on protective barriers, toxic molecules, and phagocytic cells that ingest and destroy invading microorganisms (microbes) and larger parasites (such as worms). Vertebrates, too, depend on such innate immune responses as a first line of defense (discussed in Chapter 25), but they can also mount much more sophisticated defenses, called adaptive immune responses. The innate responses call the adaptive immune responses into play, and both work together to eliminate the pathogens (Figure 24-1). Unlike innate immune responses, the adaptive responses are highly specific to the particular pathogen that induced them. They can also provide long-lasting protection. A person who recovers from measles, for example, is protected for life against measles by the adaptive immune system, although not against other common viruses, such as those that cause mumps or chickenpox. In this chapter, we focus mainly on adaptive immune responses, and, unless we indicate otherwise, the term immune responses refers to them. We discuss innate immune responses in detail in Chapter 25.
Innate and adaptive immune responses. Innate immune responses are activated directly by pathogens and defend all multicellular organisms against infection. In vertebrates, pathogens, together with the innate immune responses they activate, stimulate adaptive (more. )
The function of adaptive immune responses is to destroy invading pathogens and any toxic molecules they produce. Because these responses are destructive, it is crucial that they be made only in response to molecules that are foreign to the host and not to the molecules of the host itself. The ability to distinguish what is foreign from what is self in this way is a fundamental feature of the adaptive immune system. Occasionally, the system fails to make this distinction and reacts destructively against the host's own molecules. Such autoimmune diseases can be fatal.
Of course, many foreign molecules that enter the body are harmless, and it would be pointless and potentially dangerous to mount adaptive immune responses against them. Allergic conditions such as hayfever and asthma are examples of deleterious adaptive immune responses against apparently harmless foreign molecules. Such inappropriate responses are normally avoided because the innate immune system calls adaptive immune responses into play only when it recognizes molecules characteristic of invading pathogens called pathogen-associated immunostimulants (discussed in Chapter 25). Moreover, the innate immune system can distinguish between different classes of pathogens and recruit the most effective form of adaptive immune response to eliminate them.
Any substance capable of eliciting an adaptive immune response is referred to as an antigen (antibody generator). Most of what we know about such responses has come from studies in which an experimenter tricks the adaptive immune system of a laboratory animal (usually a mouse) into responding to a harmless foreign molecule, such as a foreign protein. The trick involves injecting the harmless molecule together with immunostimulants (usually microbial in origin) called adjuvants, which activate the innate immune system. This process is called immunization. If administered in this way, almost any macromolecule, as long as it is foreign to the recipient, can induce an adaptive immune response that is specific to the administered macromolecule. Remarkably, the adaptive immune system can distinguish between antigens that are very similar—such as between two proteins that differ in only a single amino acid, or between two optical isomers of the same molecule.
Adaptive immune responses are carried out by white blood cells called lymphocytes. There are two broad classes of such responses𠅊ntibody responses and cell-mediated immune responses, and they are carried out by different classes of lymphocytes, called B cells and T cells, respectively. In antibody responses, B cells are activated to secrete antibodies, which are proteins called immunoglobulins. The antibodies circulate in the bloodstream and permeate the other body fluids, where they bind specifically to the foreign antigen that stimulated their production (Figure 24-2). Binding of antibody inactivates viruses and microbial toxins (such as tetanus toxin or diphtheria toxin) by blocking their ability to bind to receptors on host cells. Antibody binding also marks invading pathogens for destruction, mainly by making it easier for phagocytic cells of the innate immune system to ingest them.
The two main classes of adaptive immune responses. Lymphocytes carry out both classes of responses. Here, the lymphocytes are responding to a viral infection. In one class of response, B cells secrete antibodies that neutralize the virus. In the other, (more. )
In cell-mediated immune responses, the second class of adaptive immune response, activated T cells react directly against a foreign antigen that is presented to them on the surface of a host cell. The T cell, for example, might kill a virus-infected host cell that has viral antigens on its surface, thereby eliminating the infected cell before the virus has had a chance to replicate (see Figure 24-2). In other cases, the T cell produces signal molecules that activate macrophages to destroy the invading microbes that they have phagocytosed.
We begin this chapter by discussing the general properties of lymphocytes. We then consider the functional and structural features of antibodies that enable them to recognize and neutralize extracellular microbes and the toxins they make. Next, we discuss how B cells can produce a virtually unlimited number of different antibody molecules. Finally, we consider the special features of T cells and the cell-mediated immune responses they are responsible for. Remarkably, T cells can detect microbes hiding inside host cells and either kill the infected cells or help other cells to eliminate the microbes.
- Lymphocytes and the Cellular Basis of Adaptive Immunity
- B Cells and Antibodies
- The Generation of Antibody Diversity
- T Cells and MHC Proteins
- Helper T Cells and Lymphocyte Activation
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