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Do slime mold expand or multiply?

Do slime mold expand or multiply?


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After reading about slime mold I was wondering, do slime mold simply expand outwards or multiply?

e.g. if I put 100 grams of slime mold in 1 location, will it expand to say, 2 locations, and have the 1st location with 80 grams left, and 2nd location with 20 grams (i.e total 100 grams), or is it that they multiply; 1st location still 100 grams, and 2nd location with 20 grams?


The answer is that they will do either or both, depending on conditions: this is one of the things that has caused complex systems scientists to be so interested in things like using slime molds in modeling optimization problems.

In good nutritional conditions, the cells will happily multiply. At the same time, they will flow and reshape themselves to spread and migrate in search of food when nutrients are not uniform.


Mycologist Explains How a Slime Mold Can Solve Mazes

Physarum polycephalum is a single-celled, brainless organism that can make “decisions,” and solve mazes. Anne Pringle, who is a mycologist at the University of Wisconsin-Madison, explains everything you need to know about what these slime molds are and how they fit into our ecosystem.

Maybe you've seen the pictures.

It's a yellow organism that looks a bit like a fern,

or maybe a coral, or maybe a moss.

But really it's something called a slime mold,

and when the Paris Zoological Park

put one on display recently, calling it the Blob,

the internet got itself in a tizzy.

It may look simple, but this brainless organism

can actually make decisions and solve mazes.

To learn more, we sat down with mycologist Anne Pringle

at the University of Wisconsin.

What is this, quote unquote, Blob?

The Blob is a species of slime mold.

People are calling Physarum polycephalum the Blob,

It's not a plant, it's not an animal.

Outside of those three kingdoms

that we're really familiar with,

there are all these other kids of organisms,

a lot of different groups,

that we're just less familiar with

but they're really diverse.

They play important roles in our environment,

and Physarum is characterized by this

part of its life cycle called the plasmodium,

which is basically a giant, enormous cell.

I like this idea of a slime mold

as completely defying human expectations,

and I guess by extension, befuddling the internet of late.

All of a sudden, the Paris Zoo put this on display

It gives lots of people lots of opportunities

to talk about a subject that, you know.

Gosh, when I go to a cocktail party,

I'm not sure a lot of people really wanna hear

about Physarum, or at least it takes a while to get into it.

and I don't know who made that decision or how they made it,

but it feels brilliant to me,

to be able to put something so different out there

I think when I teach my class in biology the fungi,

there are a few comments that I consistently get.

One comment is, I see them all the time now.

In a way that you can't see it

unless you're taught to see it.

And so if you're taught to see it,

then all of a sudden, everywhere you go,

there's a fungus, which is true.

Everywhere you go, there is a fungus.

And then a strong reaction I get is amazement.

How did I not know this before?

How come no-one told me about this before?

So I'm not surprised by that reaction,

and it's also pretty fun to watch it play out.

There are lots of species of Physarum, more than 100.

And if you went to your local woods

and it was the right time of year and a moist habitat,

youɽ absolutely find it yourself.

Most people think of a cell as

this really tiny thing you can't see,

but this is one giant cell.

Can you walk us through how it operates

as one big cell with many, many nuclei within?

We understand some answers to that question.

There are other things that we don't know.

So, we do know that there are no cross-walls.

No septa, no barriers of any kind

So fluids can flow across the body very easily,

and that's obviously really different from us.

This is a very, very tiny plasmodium,

and a drop of water just got put on the right hand side,

and now can you see this rhythmic contraction?

another drop, but that's food.

Can you see the tips swelling?

And then just watch, and you'll see waves

So, those waves are caused by peristolsis,

which is basically rhythmic contraction.

Expansion, contraction, expansion, contraction, expansion,

that moves fluids, and we think information,

How Physarum communicates across itself,

how it integrates information,

and coordinates behavior, has to do with

some kind of signaling molecule

that's propagated in a very particular way.

What that signaling molecule is, it might be calcium.

It might be something else. Can we talk about

how the organism is, in a sense,

exploring its environment in the way that

it's almost pathfinding in an environment?

And I think that's part of the buzz around it, right?

People hear about it, and they hear things

like it has so many sexes, and it forages,

and that comes across as a really remarkable behavior.

And I think that's because we tend

to associate a word like behavior

with something like a dog or an animal.

But here, clearly, there's an organism

that's moving around and it's foraging.

It's sensing in its environment,

for example, where there are resource-rich patches.

It doesn't have the kind of sensory apparatus

that we're used to, and so in some sense

its foraging has to do with sending out tendrils,

if you will, or parts of itself,

to explore the surrounding habitat.

And that's really what it's doing

in all of these experiments

that have to do with getting Physarum

to grow in particular ways, mimic the optimal network

that would connect the rail stations of Tokyo

or state capitals of the United States, or et cetera.

It's really growing across a lot of space,

and then it's reorganizing itself

into a configuration that allows it to optimally eat,

and eventually optimally reproduce.

Anne, it's hard to talk about intelligence

with an animal like this, ɼos we have so many

concepts around intelligence.

We base it so much on humans and other animals.

But this isn't like it knows it's in a maze,

It's doing what it would typically do

It absolutely is doing what it would typically do

When people think about Physarum solving a maze,

what a lot of people think Physarum is doing

is solving the maze the way you or I would solve a maze.

it finds the shortest route through the maze,

That's not what Physarum is doing.

Physarum grows through the entire maze.

It searches all that space.

Then it retracts its body just to the shortest path.

So, that's a really different way

of quote unquote solving a maze

than what we tend to think about.

And I think that's a really important distinction.

I find the world intelligence really problematic

like the word sex, just because both words

are so loaded with meaning,

and with meaning that has nothing to do

with the biology of fungi or fungal life organisms.

When we talk about sex in Physarum,

first of all, Physarum has a really complex life cycle,

or more complex than we're used to as humans,

and in a different part of the life cycle

there are single cells called swarm cells.

when mating happens, they fuse.

And they fuse to become one body.

So, I love my husband very much,

we did not fuse into a single body.

My body is here, his body is over in his office.

So it's a really different kind of process

when we're talking about mating for Physarum.

And basically, whether or not two cells

can fuse and form one body has to do with three genes.

And one of the genes there are 16 loci, at least 16.

And another there are 15, and another there are three.

So, if you multiply 16 times 15 times three,

you get 720 possible combinations,

and that's the start of this idea

that it has more than 700 sexes.

And then the giant plasmodium, when it reproduces,

the whole body turns into spores

that then disperse on the wind.

It's just a very different path.

So outside of a maze in a natural environment,

what are these organisms actually eating?

So, in the lab we feed it Quaker Oats,

and it seems to like the oats very much,

and it actually seems to eat the oats,

and digest them down into little bits

and discard bits of it out of its body.

In nature, we think that it's hoovering up

bacteria and fungal spores and other

really small creatures that are on the substrates,

the rock, or the woods, or the mulch that it's growing on,

Where exactly in the tree of life

Imagine that you were back in a time

where you were just looking at organisms.

Grouping organisms according to what they looked like.

Then you would quite reasonably

look at a Physarum, look at a slime mold,

and say, gosh, it looks a lot like a fungus.

And then something comes along called a PCR machine,

and we can start sequencing DNA,

and when you start sequencing DNA,

there's a massive rearrangement in our understanding of

how things are related to each other.

So we begin to understand that

slime molds are not the true fungi, as we call them,

even though they're fungal-like organisms,

but they're something really apart.

And so at this point I would say

it's one of the most exciting areas of biology,

is we know what plants are, more or less.

We understand what animals are, more or less.

Fungi, maybe less than more, but

we think we know what fungi are.

But then there are all these groups of organisms

that aren't any of those three,

and even more interestingly,

we don't have a sense of their biodiversity.

There're very few people who study

the biodiversity of slime molds.

It's genuinely fun to talk about this stuff.

Also because, I think, the language around

intelligence and maze-solving

is fantastic in its own way, but for me

it also just becomes a tool to talk about

some much larger issues about biodiversity

and what do we know about this earth that we're living on,

And to me, it seems like we don't know quite a lot.

And it would be fantastic to have

more people involved and thinking about

how many slime molds are there in my woods?

Gosh, is it a local Physarum?

So, it seems a real opportunity.

Thank you for being here and

bringing up all these crazy philosophical questions

about a seemingly simple organism.

My pleasure! [chime ringing]

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Slime mold absorbs substances to memorize them

Fusion of the venous network of two blobs. Credit: © David Villa / CNRS Photothèque

In 2016, CNRS scientists demonstrated that the slime mold Physarum polycephalum, a single-cell organism without a nervous system, could learn to no longer fear a harmless but aversive substance and could transmit this knowledge to a fellow slime mold. In a new study, a team from CNRS and the Université Toulouse III - Paul Sabatier has shown what might support this memory, and in fact, it could be the aversive substance itself.

Physarum polycephalum is a complex single-cell organism that has no nervous system, however it can learn and transfer its knowledge to its fellow slime moulds via fusion. How it does so was a mystery until recently. Researchers at the Centre de Recherches sur la Cognition Animale (CNRS/UT3 Paul Sabatier) have recently demonstrated that slime moulds learn to tolerate a substance by absorbing it.

This discovery stems from an observation: slime moulds only exchange information when their venous networks fuse. In that case, does knowledge circulate through these veins? Is it the substance that the slime mould gets used to that supports its memory?

First the team of scientists forced the slime moulds to cross salty environments for six days to habituate them to salt. Then they evaluated the salt concentration inside the slime moulds: they contained ten times more salt than "naive" slime moulds. The researchers then placed the habituated slime moulds in a neutral environment and observed that they excreted the salt absorbed within two days, losing the "memory." This experiment therefore seemed to show a link between the salt concentration within the organism and the "memory" of the habituation.

To further advance and confirm this hypothesis, the scientists introduced the "memory" into naive blobs by injecting a salt solution directly into the organisms. Two hours later, the slime moulds were no longer naive and behaved like slime moulds that had undergone a six-day training

When environmental conditions deteriorate, slime moulds can enter into a dormant stage. The researchers demonstrated that slime moulds habituated to salt stored the salt absorbed before entering the dormant stage and could store the knowledge for up to a month.

The results of this study prove that the aversive substance could be the support of the slime mould's memory. The researchers are now trying to establish whether the slime moulds can memorise several aversive substances at the same time and to what extent they can get used to them.


Ride the Slime Mold Express!

If you want to design a railway system, you could do worse than hire a slime mold. Researchers have shown that, when grown on a map of Japan, the gelatinous, funguslike organism connects points of interest in a pattern similar to Tokyo's train network. Engineers might be able to take a cue from the organism's approach to design more-efficient transportation systems.

The trick has to do with how slime molds eat. When Physarum polycephalum, a slime mold often found inside decaying logs, discovers bacteria or spores, it grows over them and begins to digest them through its body. To continue growing and exploring, the slime mold transforms its Byzantine pattern of thin tendrils into a simpler, more-efficient network of tubes: Those carrying a high volume of nutrients gradually expand, while those that are little used slowly contract and eventually disappear.

Researchers have harnessed this behavior to amusing effect in the past. In 2000, for example, a team led by mathematical biologist Toshiyuki Nakagaki of Hokkaido University in Japan, showed that P. polycephalum could find the shortest path through a maze to connect two food resources. (The work won an Ig Nobel prize.)

But that was a puzzle with a single correct solution. In the new work, the team wanted to know how the mold would perform in a real-world situation in which several competing objectives had to be balanced at once. Designing a railway network that connects many cities presents just such a problem. "The planning is very difficult because of the tradeoffs," says cell biologist Mark Fricker of the University of Oxford in the United Kingdom, who was also involved in the research. For example, connecting all cities by the shortest possible length of track often compels travelers to take highly indirect routes between any two points and can mean that a single failure isolates a large part of the network. Building in more redundancy makes the network more convenient and more resilient, but at a higher cost.

Because they couldn't mathematically determine a "perfect" solution, the researchers decided to task the slime mold with a problem human designers had already tackled. They placed oat flakes (a slime mold favorite) on agar plates in a pattern that mimicked the locations of cities around Tokyo and impregnated the plates with P. polycephalum at the point representing Tokyo itself. They then watched the slime mold grow for 26 hours, creating tendrils that interconnected the food supplies.

Different plates exhibited a range of solutions, but the visual similarity to the Tokyo rail system was striking in many of them, the researchers report in tomorrow's issue of Science. Where the slime mold had chosen a different solution, its alternative was just as efficient.

If researchers could construct a computer model of the slime mold's behavior, says Fricker, it might help engineers design better transportation networks. "The idea would be that, if one put it into a new context, a system using these rules would build a network that ought to have respectable properties."

The work is "a very interesting example of how biology can inspire new methods in technological design," says Melanie Mitchell, a computer scientist at Portland State University in Oregon. But she's not quite ready to jump on the slime mold express. "This paper uses only one relatively simple example," she cautions. "It's not obvious that similar experiments would work as well for matching other transport networks."


Types of slime mold

Most slime mold are smaller than a few centimeters, but the very largest reach areas of up to thirty square meters, making them the largest undivided cells known. Many have bright colors such as yellow, brown, and white.

A common slime mold which forms tiny brown tufts on rotting logs is Stemonitis. Another form which lives in rotting logs and is often used in research is Physarum polycephalum. In logs it has the appearance of a slimy webwork of yellow threads, up to a few feet in size. Fuligo forms yellow crusts in mulch.

The Protostelids life cycle is very similar to the above descriptions, but these are much smaller, the fruiting bodies only forming one to a few spores.

The Dictyosteliida, cellular slime molds, are distantly related to the plasmodial slime molds and have a very different life style. Their amoeba do not form huge coenocytes and remain individual. They live in similar habitats and also feed on microorganisms. When food runs out and they are ready to form sporangia, they do something radically different. They release signal molecules into their environment, by which they find each other and create swarms. These amoeba then join up into a tiny multicellular slug like coordinated creature which crawls to an open lit place and grows into a fruiting body. Some of the amoebae become spores to begin the next generation, but some of the amoebae sacrifice themselves to become a dead stalk, lifting the spores up into the air.

The Acrasidae, have a similar life style to Dictyostelids, but their amoebae behave differently and are of uncertain taxonomic position.

The Plasmodiophorids also form coenocytes but are internal parasites of plants (e.g., club root disease of cabbages).

Finally the Labyrinthulomycetes are marine and form labyrynthine networks of tubes in which amoebae without pseudopods can travel.


E pluribus plasmodium

“The slime mold is really fascinating because it is simultaneously one and many,” says Megan Dobro, a Hampshire biology professor who participated in the symposium. “There are all these individuals who are committed to acting towards the best interest of the community.”

A plasmodium of Physarum nuclei moves by oscillating its cytoplasm back and forth every 50 seconds, shifting its center of gravity to move at a little more than a third of an inch an hour. When food is nearby, the plasmodium forms a network of slender tubules that branch out in search of it, eventually finding the optimal path.

Mathematicians and computer scientists have taken interest in slime molds’ ability to solve optimization problems in geometry and information delivery. In a 2010 study, Japanese scientists arranged oat flakes in the pattern of cities near Tokyo around a Physarum plasmodium, and within 26 hours the slime mold had formed a network that was strikingly similar to the Tokyo rail system. Similar experiments have been conducted with the motorways of Britain, Canada, Spain, and ancient Rome.

Scientists have shown that slime molds also exhibit rudimentary learning behavior. Shock a plasmodium at regular intervals, and it will alter its behavior in anticipation of the next one. Expose a plasmodium to a repellent but harmless stimuli, and it will eventually ignore it. Some scientists suggest that slime mold behavior may even shed light on the origins of intelligence.

Under some conditions, a plasmodium will produce a stalk that disperses cells to become new slime molds, but to do so, some individual cells within the stalk must sacrifice themselves for the good of future generations.

“This is behavior that humans don't seem to do,” Professor Dobro says. “And I mean not just the self-sacrifice for future generations, but even the thinking about future generations.”


Seeing the Beautiful Intelligence of Microbes

The slime mold Physarum polycephalum forms a network of cytoplasmic veins as it spreads across a surface.

All images by Scott Chimileski and Roberto Kolter (except where indicated)

Authors
John Rennie
Lucy Reading-Ikkanda

Intelligence is not a quality to attribute lightly to microbes. There is no reason to think that bacteria, slime molds and similar single-cell forms of life have awareness, understanding or other capacities implicit in real intellect. But particularly when these cells commune in great numbers, their startling collective talents for solving problems and controlling their environment emerge. Those behaviors may be genetically encoded into these cells by billions of years of evolution, but in that sense the cells are not so different from robots programmed to respond in sophisticated ways to their environment. If we can speak of artificial intelligence for the latter, perhaps it’s not too outrageous to refer to the underappreciated cellular intelligence of the former.

Under the microscope, the incredible exercise of the cells’ collective intelligence reveals itself with spectacular beauty. Since 1983, Roberto Kolter, a professor of microbiology and immunobiology at Harvard Medical School and co-director of the Microbial Sciences Initiative, has led a laboratory that has studied these phenomena. In more recent years, it has also developed techniques for visualizing them. In the photographic essay book Life at the Edge of Sight: A Photographic Exploration of the Microbial World (Harvard University Press), released in September, Kolter and his co-author, Scott Chimileski, a research fellow and imaging specialist in his lab, offer an appreciation of microorganisms that is both scientific and artistic, and that gives a glimpse of the cellular wonders that are literally underfoot. Imagery from the lab is also on display in the exhibition World in a Drop at the Harvard Museum of Natural History. That display will close in early January but will be followed by a broader exhibition, Microbial Life, scheduled to open in February.

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High magnification of the slime mold Physarum polycephalum shows the cytoplasm pumping furiously through its huge single cell. This cytoplasmic streaming allows the slime mold to push forward toward nutrients and potentially carpet a surface.

The slime mold Physarum polycephalum sometimes barely qualifies as a microorganism at all: When it oozes across the leaf litter of a forest floor during the active, amoeboid stage of its life cycle, it can look like a puddle of yellowish goo between an inch and a meter across. Yet despite its size, Physarum is a huge single cell, with tens of thousands of nuclei floating in an uninterrupted mass of cytoplasm. In this form, Physarum is a superbly efficient hunter. When sensors on its cell membrane detect good sources of nutrients, contractile networks of proteins (closely related to the ones found in human muscle) start pumping streams of cytoplasm in that direction, advancing the slime mold toward what it needs.

But Physarum is not just reflexively surging toward food. As it moves in one direction, signals transmitted throughout the cell discourage it from pushing counterproductively along less promising routes. Moreover, slime molds have evolved a system for essentially mapping their terrain and memorizing where not to go: As they move, they leave a translucent chemical trail behind that tells them which areas are not worth revisiting.

After Physarum explores an area and finds it lacking in nutrients, it leaves behind a chemical trail as a kind of externalized memory that tells the slime mold not to go back there.

When bacteria were first observed through a microscope, suspended in liquid on slides, in their simplicity they seemed like the archetypes of primitive, solitary cells. The truth, however, is that in the wild, most bacteria are highly gregarious. Some bacteria do swim through their environment as lonely individuals but most bacterial cells — and most species of bacteria — prefer to live in compact societies called biofilms anchored to surfaces. (The individual swimmers often represent offshoots of biofilms, seeking to colonize new locations.)

In a high-magnification scanning electron micrograph of a Pseudomonas aeruginosa biofilm, the individual rod-shaped bacteria are interlinked by hairlike structures called pili. Bacillus bacteria secrete an extracellular matrix that encases the cells and helps them form a more structured community.

Roberto Kolter and Steve Minsky (Bacillus)

Moreover, biofilms are not just dense accumulations of bacterial cells. They have elaborate functional structures, inside and out, that serve the cells’ collective destiny, as can be seen in the images below of Pseudomonas aeruginosa. The biofilm is stained with Congo red dye, which bonds to the extracellular matrix proteins that the bacteria secrete as a scaffolding for their community. The deeply wrinkled surface of the biofilm maximizes the area through which the bacteria can absorb oxygen it also probably helps them collect nutrients and release waste products efficiently.

As this Pseudomonas biofilm expands, it develops a more complex internal structure. Bacteria in different parts of its mass may also develop more specialized functions.

Within the biofilm, the bacteria divide the labor of maintaining the colony and differentiate into forms specialized for their function. In this biofilm of the common soil bacterium Bacillus subtilis, for example, some cells secrete extracellular matrix and anchor in place, while some stay motile cells at the edges of the biofilm may divide for growth, while others in the middle release spores for surviving tough conditions and colonizing new locations.

The wrinkled structure of this Bacillus subtilis biofilm helps to ensure that all the bacteria in it have access to oxygen (left). A digital scanned model of the biofilm helps illustrate how the bacterial community can vary its structure in three dimensions (right).

One might wonder why natural selection would have favored this collective behavior instead of more rampant individualism among the cells. Part of the answer might be what evolutionary theorists call inclusive fitness: In so far as the bacteria within a biofilm are related, individual sacrifices are offset by the increases in fitness to each cell’s millions of cousins. But it may also be that every role within the biofilm has its advantages: Cells at the edge are most exposed to dangers and must reproduce furiously to expand the biofilm, but they also have access to the most nutrients and oxygen. Cells on the inside depend on others for their vital rations but they may survive longer.

The surfaces that biofilms grow across are not always solid. These B. subtilis are forming a pellicle — a kind of floating biofilm at the interface between water and air. The genetic pathways involved in forming a pellicle are essentially the same as those used in growing across stones, though they may respond to the changes in their habitat by altering the precise mix of proteins in the extracellular matrix as needed.

Bacteria can grow across nonsolid surfaces, too, as this B. subtilis culture shows by forming a pellicle, or floating biofilm, across the air-liquid interface in a beaker.

Expansive growth is not the only way in which microbial communities can move. Below, B. subtilis is engaging in a behavior called dendritic swarming, in which cells rapidly push outward in branching columns that can efficiently pave a surface. Biofilms swarm when they detect that they are in environments rich in nutrients: Swarming helps a biofilm exploit this valuable territory before any competing communities can.

At least two important changes in the differentiation of the cells in a biofilm take place to enable swarming. First, motile cells on the periphery of the film develop extra flagellae, which enables them to swim more energetically. Second, some edge cells also begin to secrete surfactant, a slippery material that helps the motile cells slide more rapidly over the surface.

When biofilms grow in flat laboratory dishes, the dendritic columns of swarming biofilms remain neatly distinct: They extend and coil in and around one another but they do not cross. That seems to be in part because the surfactant piles up around the biofilm branches as a barrier. Similarly, some bacteria can swarm in more terraced structures under laboratory conditions. What the implications of that option are for bacteria in nature is still a mystery.

These bacteria are engaging in the behavior called dendritic swarming, which allows a microbial community to expand rapidly into desirable, resource-rich environments.

Another type of behavior demonstrated by biofilms growing under laboratory conditions is spiral migration, demonstrated in the time-lapse video below of Bacillus mycoides. These bacterial cells grow in long chains or filaments that curl either clockwise or counterclockwise. The specific advantages of this spiraling movement are still under investigation, according to Chimileski, but they must be considerable because B. mycoides excels at taking over available environments. “Bacillus mycoides is one of the easiest bacterial species to cultivate from the soil,” he explained. When scientists isolate microbes from soil and grow them on agar dishes, particularly at room temperature, “the mycoides will often spread across the entire plate and overtake all of the other organisms. For this reason, it is considered if anything a kind of ‘nuisance species’ for many microbiologists.”

These geometric and presumably functional patterns that biofilms produce in culture are intriguingly beautiful. Yet Chimileski notes that there is much left to discover when it comes to translating behaviors seen in the lab to natural microbial communities.

Chimileski points out that “most natural biofilms are multi-species ecosystems and cells inside natural biofilms usually grow more slowly.” He continued, “I like to think of the way we grow bacteria in a petri plate, where a single species is by itself and has everything it needs to grow at optimal temperatures, as ‘turning up the volume’ on the biology of the organism.” Under laboratory conditions, researchers can study which genes are involved in complex multicellular behaviors and they can measure the benefits to the fitness of the bacterial species. But in natural environments, biofilms don’t usually get to form exactly the same patterns as in the lab because of limited nutrients or competition with other species. “So the same biology might be occurring on a particle of soil in your backyard at smaller size scales and over longer time periods,” he said, even if it is less easy to visualize.

Spiral migration is a behavior favored by the highly successful soil bacterium Bacillus mycoides. Communities of these cells expand by forming long filaments of cells that coil either clockwise or counterclockwise — an orientation that is strain-specific and genetically determined.

Biofilm behaviors testify to the capacity and openness of bacterial to form collectives — but that openness has limits, as shown in this culture with several cohabiting biofilms. Here, adjacent biofilms that consist of the same bacteria or closely related strains comfortably merge. But the adjacent biofilms made up of more divergent bacteria keep themselves distinct and may even try to eliminate or control each other.

Biofilms are so intolerant of other strains and species because they invest considerably in the production of surfactant, extracellular matrix and other molecules that bacteriologists classify as public goods — ones that the bacteria secrete for other members of their community. The bacteria guard these jealously because unrelated freeloading cells could benefit strongly by using them first.

Biofilms rebuff such freeloaders in different ways. For example, the B. subtilis colonies in this image adopt a strategy of “kin discrimination,” in which they secrete antibiotic compounds that are toxic to other species but not to their own. Proteus mirabilis bacteria defend their interests in a different way based on “self-recognition”: The P. mirabilis biofilms examine encroaching cells, stab any from a different species with a spearlike structure and inject them with poisons that will kill almost all but closely related species.

Several different strains of B. subtilis grow side by side in this dish. Because the biofilms discriminate against dissimilar strains of bacteria, they may merge compatibly with close relatives but form boundaries against others.

The colors appearing in the biofilm culture of Streptomyces coelicolor in the video below reflect natural pigments that the bacteria produce. The value of the pigments for the biofilms is not entirely clear, but it is probably not tied to their color. Rather, these pigment molecules are often bioactive in various ways. “The blue pigment seen in this video is actinorhodin, which is technically an antibiotic,” Chimileski said, but added that the term is misleading in this context. “Killing or growth inhibition usually occurs only at very high concentrations relative to what is out in nature.” For that reason, he said, there is “an emerging view that killing is probably not the ecological function of many or most antibiotics. Rather, these bioactive molecules act as signals or developmental cues” to other cells.

That view is echoed in a note from Gleb Pishchany, another research fellow in Kolter’s laboratory who studies how diverse types of bacteria cohabit. “An intriguing possibility is that in natural ecosystems, Streptomyces use pigments and other bioactive molecules” at “lower concentrations as signals that are exchanged among multispecies microbial communities,” he wrote. The pigments may help cohabiting assortments of bacteria rein in one another’s less neighborly instincts, and thereby maintain a more cooperative and fruitful communal existence.

In this powdery colony of Streptomyces coelicolor, the pigmentation comes from actinorhodin, a molecule with antibacterial effects. Biofilms may use bioactive pigments as signals for controlling the behaviors of other microorganisms in their shared environment.

These striking photographs of microbe communities were captured by DSLR cameras. Chimileski collects his still images with macro lenses while working at the bench, while the videos are made in an incubator dedicated to time-lapse microscopy. He sets the camera to snap a picture every 10 minutes, although he increases the frequency to every minute or two for behaviors happening more quickly, such as the movements of slime molds. As a result, the movements of the microbes in these videos are typically accelerated between 5,000 and 50,000 times their actual speeds. Chimileski does not use false color to beautify the images: Aside from using dyes to stain the extracellular matrix in some cultures, he shows the natural coloration of the microorganisms.

Chimileski typically grows bacterial colonies at 30°C, a temperature at which he can collect images of slower growing species for several weeks. Although the heat and humidity suited to biofilm growth are less than ideal for cameras, he said the equipment is rated for more extreme conditions. The few cameras that have malfunctioned did so for a mechanical reason: The number of shots that he needs to document microbial behaviors is so large that the shutters on the cameras eventually break down after hundreds of thousands of clicks.


Complex structures' organization studied in slime mold

Researchers in Japan think they have found an answer to the fundamental biological question of how individual cells know which way to position themselves within a complex, multicellular body. Depending on a cell's purpose in the larger structure, contact or diffuse chemical signals direct it to its final destination.

The journey from egg and sperm to a fully grown body requires more than just multiplication. Plants, animals, and people are all made of trillions of cells, carefully organized into larger structures like tissues and organs. Somehow, each cell knows where it belongs -- the left side of the heart, the inner lining of the colon, and so on -- and generally stays put.

"It's close to impossible to dissect what's happening while cells position themselves in multicellular organisms because there are so many players: different cell types, different molecules inside cells, different chemical signals outside the cells, cell growth, programmed cell death," said Professor Satoshi Sawai from the University of Tokyo, an expert in biological physics, a field that uses the principles of physics to understand living systems.

The slime mold system

Slime molds provide a simpler system to understand cell positioning. Slime molds are amoebas, but are similar in size and shape to human white blood cells and share the fundamental aspects of cell dynamics, such as migration and engulfment of disease-causing pathogens.

Individual cells of the slime mold Dictyostelium discoideum can exist independently, living freely in the soil and eating bacteria and fungi. When food is scarce, independent slime mold cells clump together and function as a multicellular organism.

When slime mold cells clump together, sometimes 100 cells, other times 10,000 cells, they differentiate into two distinct types.

The first type, pre-stalk cells, eventually forms a column that supports a sphere composed of the second type, pre-spore cells. Researchers call this two-part structure a fruiting body. The pre-stalk cells will die as the pre-spore cells eventually float off in the wind to a better environment where they can grow and divide again as independent amoebas.

Inside the clump, before the fruiting body takes shape, cells attach to form long trains and swirl around, immersed in a chemical signal that they secrete. First identified in the 1970s, this diffusive chemical, called cAMP, attracts cells.

Traditionally, the degree of attraction to cAMP signals was thought to separate the cells into pre-stalk and pre-spore cells. More recent genetic experiments revealed, however, that molecules related to adhesion, or cell-to-cell touch, may also be important.

"What's great about slime mold is that you can take individual cells out of the larger structure and they still do their thing by behaving naturally in a relatively simple setup that mimics the multicellular environment," said Sawai.

Two types of signals

In their new experiments, the researchers took cells out of a multicellular clump and tracked how the individual cells migrate in response to artificial touch and cAMP signals.

When cell trains formed, the leader cell moved in the direction of cAMP. The follower cells were not pulled along, but rather actively pushed leader cells forward.

"Cell-cell contact activates processes for cell movement. The follower cells are the engine and the leader cells are the steering wheel, always pointing in the direction of the chemical signal," said Sawai.

Researchers also placed individual pre-stalk or pre-spore cells with beads coated with an adhesion molecule that appears to function in the tail end of cells. All cells attached to follow the bead as in a cell train. Researchers then added cAMP to the experiment. Pre-stalk cells released the bead and moved towards the cAMP source. Pre-spore cells, however, ignored cAMP and held fast to the bead.

Sawai's research team demonstrated that head-to-tail touch between cells directs their migration, but cAMP somehow overrides this contact only in pre-stalk cells.

"Many people think you have to go to Mars to look for the fundamental rules of what makes life. But we can look at all the still-unexplored branches of the tree of life here on Earth. Slime mold gives us hints at what to look for to understand the mechanistic logics underlying more complex species," said Sawai.

This discovery of the importance of cell-cell contact to activate cell movement and organization will open new possibilities to study cell-pattern formation in events such as embryo development or spread of breast cancer.

The results are published in the Proceedings of the National Academy of Sciences of the United States of America.


Abstract

When aggregating amoebas of the cellular slime mold Dictyostelium discoideum are disaggregated and morphogenesis is reinitiated, the amoebas will reaggregate in less than 1 10 th the original time. When aggregating amoebas are disaggregated and resuspended either in full nutrient medium or in buffered salts solution containing dextrose, they retain this developmentally acquired capacity to rapidly reaggregate for approximately 1 hr and then lose it completely in a synchronous and discrete step which we have referred to as the “erasure event.” In this report, it is demonstrated that micromolar concentrations of cAMP completely block this transition from the developmental to vegetative state, and that other cyclic nucleotides also inhibit it, but they do so at 20-fold higher concentrations. Neither the hydrolysis products of cAMP nor the vegetative chemoattractant folic acid inhibit dedifferentiation at concentrations as high as 10 −3 M, demonstrating a specificity for cyclic nucleotides and cAMP in particular. The addition of cAMP at any time during the lag period preceding the erasure event inhibits it and addition immediately after the erasure event reverses it. Since cAMP may inhibit the transition from the developmental to vegetative state intracellularly or extracellularly, we have also examined the intracellular concentration of cAMP and the levels of cAMP binding sites on the cell surface during the erasure process. Evidence is presented that the majority of cAMP binding sites on the cell surface are not necessary for the inhibition of erasure by cAMP. The results of these latter studies are discussed in terms of alternative models for the involvement of cAMP in the transition from the developing to vegetative state.

This investigation was supported by Grant PCM 78-15763 from the National Science Foundation and Grant GM 25832 from the National Institutes of Health.

Robert Finney and Bernice Slutsky were trainees under Cellular and Molecular Training Grant GM 07228.


Reproduction in Protists

As far as protists reproduction process is concerned, some of these organisms are known to resort to asexual reproduction, while others resort to sexual reproduction. Discussed below are the details about how protists reproduce asexually and sexually. This will help you get a better understanding of the overall process of reproduction in members of Kingdom Protista.

Asexual Reproduction

Like other single-celled organisms, such as archaea and bacteria, asexual reproduction is the primary method of reproduction for protists. These organisms generally opt for binary fission method of asexual reproduction wherein a single parent produces an off-spring without having to depend on fertilization by another organism. In other words, the parent organism literally divides itself into two cells. As only one parent is involved, the newly formed organism inherits the genes of its parent and not from the two organisms as is the case in organisms which resort to sexual reproduction.

While binary fission is predominant method of asexual reproduction in single-celled protists, there do exist some protists which resort to yet another method of asexual reproduction, i.e. reproducing by producing spores. Fungus-like slime molds and water molds are apt examples of protists which resort to spore form state to make the reproduction possible. These protists produce spores and release them in the air. These spores, in favorable conditions, eventually hatch into amoeba-like cells, which grow by feeding on bacteria, and mate when they encounter the correct mating type to form zygotes.

Sexual Reproduction

It was initially believed that protists only reproduce asexually, however recent studies have revealed that they also resort to sexual reproduction, especially under stressful conditions. When there is extreme environmental stress, which makes survival difficult, some protist organisms exchange genetic material with the intention of producing an off-spring which will have the ability to survive in the prevailing extreme environment. In case of sexual reproduction, the two organisms involved go through the process of meiosis wherein a reproductive cell with half the set of DNA belonging parent, known as gamete, is created. Further when the gametes of two parent organisms come in contact, a zygote is formed. This zygote has a full set of DNA, courtesy half a set of DNA from either parent.

While that was all about the reproduction process, there also exist several other facets of their lives that we are not aware of. It may come as a surprise for many, but we do come in direct contact with various protists which act as pathogens in our body. One of the best example of the same is Plasmodium falciparum, the protozoan parasite which causes malaria. Taking that into consideration, study of protists and the role they play in the environment becomes all the more interesting – as well as important.