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.
Primary source: Nature Reviews Microbiology