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Social Amoebas or Cellular Slime Molds: Fascinating Organisms

Linda Crampton is a writer and experienced science teacher with an honors degree in biology. She enjoys writing about science and nature.

Dictyostelium discoideum in a laboratory

Dictyostelium discoideum in a laboratory

Interesting and Unusual Creatures

Social amoebas or cellular slime molds are fascinating organisms that spend part of their lives as one-celled creatures and the rest joined together to form a superorganism. The single-celled form has some features in common with the amoeba that lives in pond water. The multicellular superorganism crawls to a new area and then produces fruiting bodies for reproduction. The crawling structure is called a grex or a slug.

The organisms are sometimes known as dictyostelids. Researchers are discovering that the separate and the joined stages in the life of a dictystelid have some intriguing features. They are of great interest to biologists who study cell communication and molecular biology. The study of dictyostelid behaviour may reveal some facts related to our own biology.

Life cycle of a social amoeba or cellular slime mold

Life cycle of a social amoeba or cellular slime mold

Potentially Confusing Names and Features

Social amoebas are not the same as the unicellular creatures known as amoebas, though as mentioned above they do share some features with these organisms. In addition, when social amoebas (or cellular slime molds) join to form a slug, they don't have the same features as slime molds. The latter organisms are sometimes referred to as plasmodial slime molds to avoid confusion with the cellular type. "Slug" is another confusing name because cellular slime molds are not closely related to the familiar slugs that we see in gardens. Their combined form may take the shape of a slug, however.

The multicellular slug of a cellular slime mold is often visible to the naked eye but is tiny. Like the slug, the plasmodium of a plasmodial slime mold is made by the union of cells. Unlike the slug of a cellular slime mold, it isn't multicellular because the membranes of the cells that made it break down. The plasmodium is essentially a mass of cytoplasm containing multiple nuclei, though it does have an outer covering and is sometimes coloured. It's often clearly visible to the unaided eye and frequently has a different shape from the slug of a cellular slime mold.

Fuligo septica is a plasmodial slime mold.

Fuligo septica is a plasmodial slime mold.

The Amoeboid Stage of a Dictyostelid

Like the amoebas studied in schools, social amoebas are one-celled creatures that move by extending projections called pseudopods, into which their cytoplasm flows. They are predators that surround and trap their prey with the pseudopods. The prey enters a food vacuole, which digests the captured organism.

Social amoebas can be found around the world. The individual organisms live in the upper layer of the soil, on leaf detritus, and on animal dung. They feed on bacteria. They reproduce by binary fission, or the process of splitting in half. Social amoebas seem to spend most of their life as separate organisms. If they run out of food, however, a dramatic change takes place. Tens of thousands of the organisms stream towards a common point, forming a growing mound. The mound eventually tips over to form a slug-like structure, or a grex.

John Tyler Bonner was an American biologist who died in 2019. He was a pioneer in the study of social amoebas and has made many films of their activity. Some of his clips of the different stages in the life cycle are shown in the video below.

The Slug or Grex Stage of a Dictyostelid

The dictyostelid slug is attracted to heat, light, and humidity. It moves to the surface of the soil and then travels to a new area that may have a better source of bacteria for food. When it finds a suitable spot, it pushes its front tip into the substrate, forming a stalk, and lifts the rest of its body into the air. The structure is now called a fruiting body instead of a grex or slug.

The cells in the sorus (the expanded section at the top of the fruiting body) change into spores and are released into the environment. Spores have a protective wall and are more resistant to environmental stresses than the amoebas are. A spore releases an amoeboid cell after it lands on a suitable substrate. The stalk of the fruiting body dies. In essence, the amoeboid cells that formed the stalk give up their lives to elevate and save the other cells in the fruiting body.

Dictyostelium discoideum is a social amoeba and a model organism. Many of the discoveries about social amoebas have been made in this species. It was discovered on decaying leaves in 1933 in North Carolina. The species is sometimes referred to as Dicty.

Founder Cells and Slug Production

Many questions surround the life cycle of Dictyostelium discoideum and other social amoebas. A lot of them concern the slug, which is an unusual structure. One question of interest is the cause of the amoeba movement towards a common point during the formation of a slug. Researchers have discovered that at least part of the answer is the production of a chemical called cyclic AMP, or cyclic adenosine monophosphate.

The first cells that release the chemical are called the founder cells. When another cell detects the chemical, it moves towards a founder cell and in turn releases cyclic AMP itself. As a result, other cells are attracted by the chemical and move towards it. As the process is repeated, a train of cells following a founder cell forms. These cells eventually join to form a slug.

Although the alternate name for social amoebas is cellular slime molds, unlike true molds the organisms are not fungi. Their classification is controversial, but they are often considered to be protists.

The Function of Sentinel Cells

As a slug migrates, it may encounter dangerous bacteria and toxins. Fortunately, the slug contains sentinel cells. These absorb both bacteria and toxins and are eventually sloughed off the multicellular structure as it moves. Other cells then take over the role of being a sentinel. Sentinel cells have been likened to the cells of the immune system in our body, which work to protect us from infection.

Farmer Slugs

Bacteria in Farmer Slugs

In many slugs formed in the wild, the fruiting body that forms is more or less free of bacteria due to the action of the sentinel cells. About a third of the slugs that have been examined not only retain a significant number of bacteria but actually seem to encourage their presence, however.

The slugs in the smaller group gather bacteria, transport them without harming them, and harvest (eat) them only at the appropriate time. Some of the bacteria enter the spores in the sorus, providing food for the amoeboid cells that develop from the spores. The process has been likened to a primitive form of agriculture, and the slugs are known as farmers.

Competition Between Slugs

Researchers have made an interesting discovery about dictyostelid slugs consisting of clones (genetically-identical organisms). The slugs are farmers. They contain bacteria that produce a toxin that inhibits the growth of non-farmer slugs. In this case, cooperation happens within the slug and competition happens between different slugs. The features of farmers seem to be complex. To some extent, they also seem to vary according to the circumstances. More research is required to understand their behaviour.

Our results suggest that successful farming is a complex evolutionary adaptation because it requires additional strategies, such as recruiting third parties, to effectively defend and privatize the crops.

— David Queller and Joan Strassmann, via the Washington University in St. Louis and phys.org

Symbiotic Bacteria and Toxin Resistance

A research team from the Washington University in St. Louis has found that farmer slugs have fewer sentinel cells than non-farmer slugs, which might be considered a disadvantage. The researchers found a symbiotic and helpful bacterium named Burkholderia in farmer slugs, however. Symbiotic organisms live together. In this case, the bacterium protected the farmers from toxins.

The researchers discovered that when farmer slugs with Burkholderia were challenged with a toxin, they produced the same number of viable spores as when they weren't exposed to the toxin. On the other hand, the non-farmers produced fewer viable spores when challenged with a toxin. When the Burkholderia bacteria in the farmers were killed by an antibiotic, the farmers behaved just like the non-farmers with respect to their response to toxin exposure.

Fruiting bodies of Dictyostelium discoideum growing on black agar

Fruiting bodies of Dictyostelium discoideum growing on black agar

The Role of Lectins in Bacteria Protection

Bacteria and other microbes live in our intestine. They form a community known as the intestinal microbiome. The microbes in the community are known to have important benefits for us and may influence our lives in additional ways which have not yet been discovered. Some social amoebas appear to have the equivalent of a microbiome. There are some puzzling aspects of this microbiome.

One unanswered question is how a slug knows that some bacteria that enter it should be destroyed and others should be kept alive. How does a farmer slug "know" which bacteria to kill and which to keep?

Recent research at the Baylor College of Medicine suggests that chemicals called lectins may play a role in the protection process. They found that two proteins belonging to a class of lectin molecules called discoidins were a hundred times more concentrated in farmers than in non-farmers. Discoidins bind to sugars, including the ones found on the surface of bacteria. They coat desirable bacteria in the slug, protecting them from destruction.

Lectin-coated bacteria inside the amoeba remain alive longer than bacteria without coating, and can transfer genetic material to the amoeba.

— Dr. Adam Kuspa, Baylor College of Medicine

Production of DNA Nets

The Baylor College researchers have made another interesting discovery. They've found that social amoebas—or at least the ones in their study—can create nets of DNA (deoxyribonucleic acid) containing antimicrobial granules. The nets trap and destroy bacteria. Both of the Baylor College discoveries are quite recent. More research is definitely needed, but the initial discoveries are intriguing.

Potential Benefits of Studying Social Amoebas

Many unanswered questions about the biology of social amoebas exist, and many discoveries need to be clarified. Though researchers are making progress in identifying and understanding the activities in the organisms and their slugs, their knowledge is incomplete. It's interesting to discover that such small and apparently simple organisms as social amoebas are not so simple after all.

The organisms have eukaryotic cells (ones containing membrane-bound organelles), as we do. In addition, we make many of the same chemicals that they produce. Communication via chemicals is important in the human body, as it is between social amoebas. Discoveries in the organisms may therefore be helpful to biologists studying human cells, molecules, and genes.

There are currently believed to be two groups of cellular slime molds: the dictyostelids discussed in this article and the lesser known and apparently smaller group known as acrasids. There is still a lot to be discovered about living things. Learning more about both groups of social amoebas would almost certainly be very interesting. It would be wonderful if the studies also helped us increase our knowledge about nature and human biology.

References

  • Introduction to slime molds from the University of California Museum of Paleontology
  • Changing from an amoeba to a grex from Indiana Public Media
  • Sentinel cells, symbiotic bacteria, and toxin resistance from PubMed, National Institutes of Health (Abstract)
  • Amoebas farm bacteria and carry guards to protect crops from the phys.org news service
  • Lectins help social amoebas establish their own microbiome from Bayer College of Medicine

© 2018 Linda Crampton

Comments

Linda Crampton (author) from British Columbia, Canada on August 22, 2018:

Thank you, Dianna. I always appreciate your visits.

Dianna Mendez on August 22, 2018:

Your bring interesting thoughts to us through your writing. Could not have imagined amoebas being social!

Linda Crampton (author) from British Columbia, Canada on August 01, 2018:

Hi, Manatita. The actions and potential actions of gut bacteria are very interesting to explore and consider. The chemicals that they make and their possible influence on us are especially interesting. I haven't read David Grundy's book, though I've heard that he makes some controversial claims. I'll look out for it now that you've mentioned it.

manatita44 from london on August 01, 2018:

The American David Grundy has written a book called the Plant Paradox. He speaks of lectins in a similar way. He also talks of both bad and good bacteria communicating via neuro- transmitters to the brain.

I believe they use prebiotics to strengthen the probiotics and make them more dominant. It seems to be the dorminant bacteria that gets more messages to the brain, telling it what to do.

Eat more or less junk, for example. Educational article on slugs or amoebas.

Linda Crampton (author) from British Columbia, Canada on July 31, 2018:

Thank you so much, Peggy. I appreciate your kindness and your comment a great deal. I agree with you—life forms are very interesting!

Peggy Woods from Houston, Texas on July 31, 2018:

You always put so much work into your articles and make them understandable even to people who may not have studied biology. Those videos were fascinating to watch. Life forms are so interesting! Now I know information about social amoebas thanks to you.

Linda Crampton (author) from British Columbia, Canada on July 31, 2018:

Thanks for the comment, Larry.

Larry Rankin from Oklahoma on July 31, 2018:

Always educational.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

Hi, Flourish. I agree. There is so much to learn about human biology and the biology of other organisms. I suspect that we'll find many more similarities between living things.

FlourishAnyway from USA on July 30, 2018:

I was very interested in the comparison to the intestinal microbiome. There is so much we don’t know about the human body and nature. It’s not surprising that there are parallels between them.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

Thanks for the funny comment, Heidi! I hope you have a great week, too.

Heidi Thorne from Chicago Area on July 30, 2018:

Well, that really is a social network! Wonder if they have an amoeba version of Facebook? :)

Fascinating, as always. And as we continue the search for extraterrestrial life, knowing how these organisms behave should certainly provide insight as we discover new life forms. (I think we will.)

Have a great week!

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

Thank you, Pamela. I'm hoping that researchers discover more about social amoebas soon.

Pamela Oglesby from Sunny Florida on July 30, 2018:

I don't ever remember reading about social amoebas, and the new studies are really interesting. Thanks for another interesting article.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

I hope the discoveries are useful too, Mary. They are certainly interesting!

Mary Norton from Ontario, Canada on July 30, 2018:

These recent findings about social amoebas are really interesting and useful to studies on human cells and hopefully, this can advance our knowledge of the human body.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

Hi, Bill. Thank you for the amusing and kind comment.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

I appreciate your visit and comment, Alan.

Bill Holland from Olympia, WA on July 30, 2018:

Social Amoebas...I thought you were talking about me. lol Jana is correct, your articles are always a pleasure to read. Thank you!

jonnycomelately on July 30, 2018:

Linda, great study and valuable opening of doors so we can explore some more. Thank you.

Linda Crampton (author) from British Columbia, Canada on July 30, 2018:

Thank you very much, Jana.

Jana Louise Smit from South Africa on July 30, 2018:

Your articles are always a pleasure to read. Thank you for this one. :)