Bacterial multicellularity

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We generally think of Bacteria (and Archaea and protists, for that matter) as being unicellular - every bug for itself. But this is not a very sophisticated viewpoint. It's true that Bacteria don't develop into large interdependent clusters that walk around talking to one another on cell phones, but multicellularity is a spectrum, not just an "is" or "is not". Most Bacteria communicate by secreting small compounds called phermones; cell-to-cell communication is a kind of multicellularity, in the sense that the cells act and react as a population rather than individually. Many Bacteria form aggregates of various kinds during their life cycle in which cells differentiate. In some cases, there is no doubt that the bacterium is multicellular, for example the filamentous cyanobacteria in which some cell undergo terminal differentiation in nitrogen-fixing heterocysts, or the colonies of Streptomyces that undergo complex morphological development and programmed cell death. Cells in a group specializing into different forms, and especially if that differentiation is terminal (those cells will not contribute to future generations) is the hallmark of multicellularity.

Anabaena
This filament of Anabaena is a clear example of bacterial multicellularity - the small light-colored cells are heterocycsts, which are terminally-differentiated cells specialized for nitrogen fixation.
Jason Oyadomari, http://www.keweenawalgae.mtu.edu/index.htm

One distinction between multicellular behavior in Bacteria (and Archaea and unicellular eukaryotes) in contrast to plants/animals/fungi is that the groups of cells are often composed of more than one species; for example, the photosynthetic mats. Here we have complex layers of different kinds of cells, working together to make a living; a tissue, of a sort, made up of cells of various kinds specialized for different functions. Not much different than the tissue of a plant of animal, except that the cells that make it up are of different species.