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Bacteria with bodies – multicellular prokaryotes

The views expressed are those of the author and are not necessarily those of Scientific American.

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Bacterial cells are fundamentally different to the cells of multicellular animals such as humans. They are far smaller, with less internal organisation and no nucleus (they have DNA but it is not packaged safely within a membrane). Because of this bacteria are almost exclusively single-celled organisms, with their own autonomy and often mobility.

Of course many bacteria form large interlinked structures such as biofilms and colonies. These show impressive cellular organisation, but they cannot really be considered one single multicellular organism. In order to be considered a multicellular creature, and organism must fulfil certain criteria:

  • Cells must stick together! This sounds fairly obvious but it does involve mechanisms for cellular adhesion
  • Cells must be able to communicate. In an multicellular body the cells must remain in communication, and change in response to conditions that affect the whole body
  • Dependency. Cells must be dependent on the surrounding cells for survival, otherwise the body is just a large colony.
  • Differentiation. The cells of the body specialise at different tasks. In most cases this is terminal differentiation – i.e once the cell has specialised it cannot return to it’s unspecialise state.

Are there some bacteria that can do all that? Not very many of them can, true, or there would be large multicellular bacterial ‘animals’ roaming the plains. But there are a number of photosynthetic bacteria are able to form truly multicellular structures, albeit rather small ones.

A multicellular filamentous cyanobacteria. Image from the reference below.

Those long chains are technically all one organism, a photosynthesising cyanobacteria. The outer cell wall surrounds the whole organism in one continual envelope, and fulfills the first requirement for multicellularity, keeping the cells together. The arrows point towards larger cells which fulfill the both the third and the fourth. These larger cells are very different from the ones surrounding them; they have differentiated to form specialised cells whose only job is to take up inorganic nitrogen from the surroundings and ‘fix’ it into a usable organic form.

This is a very important development, as the enzyme required to fix nitrogen does not work in the presence of oxygen, which is vital for respiration. That’s why most animals and plants can’t fix nitrogen and instead rely on food sources, or surrounding soil bacteria for the organic form. Bacteria have different ways to respond to this problem. Some rely on outside food sources, others become totally anaerobic (not using any oxygen at all) and some, like the cyanobacteria, have differentiated to form special nitrogen-fixing cells.

(There is a third strategy, which is to become a nitrogen fixing bacteria by night, and an aerobically respiring bacteria by day, but this requires huge amounts of energy as it means that the cell has to do a complete enzyme turnover every twelve hours)

The differentiated cell is called a heterocyst. It has a thicker cell wall to stop oxygen diffusing into the cell, and all cellular processes that might produce oxygen have been removed. Once the cell has turned into a heterocyst it cannot change back again, and is completely dependant on the cells surrounding it for the products of respiration, which it cannot carry out by itself as the process requires oxygen. Likewise, the surrounding cells are dependant on the heterocyst for the provision of nitrogen.

A representation of the metabolic tasks being shared between the heterocysts and surrounding cells. Sugars go from surrounding cells to the heterocyst, and nitrogen goes from heterocyst to surrounding cells. Figure (c) me.

The cells also communicate between themselves, using a feedback system of chemical messages to determine which of them will differentiate into a nitrogen cell and which ones will stay as normal respiring cells. They can also choose to differentiate into hormogonia, which are little lines of very tiny cells that act as invasive reproducing particles. Hormogonia have some pretty awesome properties, they can glide through slime, scuttle around with pili and even float on water due to internal gas vesicles. Unlike the nitrogen-producing cells though, hormogonia are not terminally differentiated, and can turn back into normal cells once they’ve reached a good destination to reproduce in.

Can this be considered ‘true’ multicellular behaviour? There are arguments either way, but as far as I’m concerned this is a multicellular bacteria. It’s certainly the closest a bacteria can get to loosing it’s single-celled autonomy and existing within a larger organism.

This post is based on an older post from my previous blog, Life of a Lab Rat.

Flores, E., & Herrero, A. (2009). Compartmentalized function through cell differentiation in filamentous cyanobacteria Nature Reviews Microbiology, 8 (1), 39-50 DOI: 10.1038/nrmicro2242

S.E. Gould About the Author: A biochemist with a love of microbiology, the Lab Rat enjoys exploring, reading about and writing about bacteria. Having finally managed to tear herself away from university, she now works for a small company in Cambridge where she turns data into manageable words and awesome graphs. Follow on Twitter @labratting.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. jack.123 9:43 pm 11/17/2011

    Have there been any studies of virises grouping in a same way?

    Link to this
  2. 2. drjonz 8:51 am 11/18/2011

    Hey Labrat,

    Have you ever thought about bacteria thinking? How does quorum sensing trigger just so many of the colony to begin committing suicide and building biofilm, or increasing their rate of mutation to cope with a threat? For that matter, how do they KNOW there’s a threat?

    We think of bacteria as totally OTHER, so we can focus on killing them. Nathan Sharon wrote about using sugars to talk to them. I think it’s a better approach and write about it in THE BOIDS AND THE BEES if your interested. I think xylitol does a pretty good job there.

    Link to this
  3. 3. S.E. Gould in reply to S.E. Gould 11:48 am 11/18/2011

    @Jack.123: virus’s are not cells, just small bits of DNA wrapped up in a protein. They also have no autonomy of their own once they are out of a host-cell. Any aggregations they form are more likely to be large crystalline mostly ‘dead’ structures rather than the moving living colonies/bodies of bacteria.

    @drjonz: Thinking is a whole new area, one of philosophy and Consciousness and one that science is completely unable to answer! Quorum sensing is a wonderful and elegantly complex process where small chemical signals can build up into a sophisticated instructions. Quite what they all mean, and what signals trigger their deployment is still being worked out.

    I’ve never tried talking to my bacteria. Swearing at them, yes :p In all seriousness any time you introduce something into a bacterial medium could be considered ‘communicating’ – adding a signal to turn on a gene, or adding a chemical to stimulate protein synthesis. Even adding high quantities of antibiotic is communicating “please die”. They don’t always answer how you want!

    Link to this

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