Read more: 鈥�Not so simple: Bugs that break all the rules鈥�
OK, so bacteria can be more socially complex than we thought. At least we can count on them being only single-celled. Or can we? When food is scarce, slime bacteria come together to form fruiting bodies that produce spores. Their height gives the spores a better chance of finding fresh pastures to exploit than if individual bacteria turned directly into spores.
Fruiting bodies can be simple mounds of cells, all of which then turn into spores. Others, like that of Stigmatella aurantiaca, are more complex branching structures in which some cells form the support structure and only some get to form spores. This kind of division of labour is one of the hallmarks of multicellularity.
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Some biologists refer to this as multicellular behaviour rather than true multicellularity, because normal growth occurs when the cells are separate. However, cyanobacteria in the genera Anabaena (pictured right) and Nostoc tick all the boxes. When nitrogen is low, about 1 in 10 of these bacteria, which grow in long filaments of cells, specialise in fixing nitrogen. Once the cells have changed from a normal cell to a nitrogen-fixing one there is no going back and they become totally dependent on the other cells for energy. In turn, these cells depend on them for nitrogen.
Such examples were thought to be exceptional, but multicellular traits are turning out to be widespread. The biofilms formed by many bacteria, for instance, are not mere piles of cells but organised structures. There is even some within them.
Perhaps the biggest surprise is the discovery that . In plants and animals, programmed cell death is vital 鈥� it is what separates our fetal fingers as we grow, for instance, and cells infected by viruses try to kill themselves stop the virus replicating. Programmed cell death appears to play similar roles in bacteria, too. It helps form the structure of some fruiting bodies and biofilms, and is also used to stop viruses spreading. In other words, many bacteria sacrifice themselves for the greater good.
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