This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
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.
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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.
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.
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.
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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