Field of Science

Social Evolution in Bacteria - SGM series

This is the fourth post in my latest SGM series.
The social behaviour of bacteria is something that I get very excited about. From the wolf-pack hunting strategies of Myxococcus xanthus to the terminal differentiation of cyanobacteria, it's something that I never get tired of writing about. As well as providing interesting quirks of bacterial behaviour, living within a colony also gives new scope for exploring the evolution of bacteria; not just as single entities but as a fully functioning social group.

One of the differences of living within a social colony as opposed to alone means that altruistic-type behaviour has to be adopted. Bacteria living within a biofilm need to excrete the sticky goo that holds the biofilm together, which is problematic because synthesising and secreting goo takes up a lot of energy. So within this colony, there will be 'cheaters' - those bacteria that live in the surrounding goo produced by others, while making none themselves.

A bacterial biofilm, showing individual bacteria in green. Image taken from the FEI website, shown there courtesy of Paul Gunning, Smith & Nephew

As with all colonies, cheating might benefit the individual but has no benefit for the colony as a whole. Too many cheaters and there won't be any biofilm. And recently an even more subtle form of cheating has been shown within the biofilms of the bacteria Pseudomonas aeruginosa, with bacteria that don't just refuse to make vital sticky chemicals, but also abstain from the entire process of forming a biofilm.

Bacteria use a complex communication system called quorum sensing in order to determine how many other bacteria they are surrounded by. Once enough bacteria are present, all signalling their existence, the biofilm will start to form. However some bacteria isolated from the biofilm were shown not to be taking part in any quorum sensing at all. Quorum sensing appears to be quite a burden for a growing cell - cells with the quorum sensing genes knocked out tend to grow a lot faster that the socially conscious cells that allow biofilms to form.

The paper that goes through this (reference one) highlights it as a form of social cheating, with bacteria avoiding quorum sensing to benefit themselves while mooching off the quorum sensing behaviour of others. I'm not entirely certain that this is the case though. It may just be an good example of job allocation within the bacterial society. Clearly not all bacteria are required to be continually quorum sensing, so why should they all have to? Would it not be more sensible to have some exempt from that task, so that they can concentrate on growing, dividing, and spreading the colony? This may be more a case of tax-breaks than of benefit-cheats.

Social evolution doesn't just take place within species, but also between them, and like every other organism bacteria are in a constant state of coevolution with both their 'prey' and their predators. Most predator-prey interactions take long periods of time to study, but the beauty of bacteria is that you can go through several generations in the course of one week's growth. Studies of the bacteria Pseudomonas fluorescens and its bacteriophage parasite showed that both the bacteria and the bacteriophage evolved far quicker when interacting together than they did when competing against a non-changing opponent.

Bacteriophage surrounding a bacteria. Image from wikimedia commons

'Evolve' here means that the bacteria and the bacteriophage showed a greater change in their genetic makeup, and a greater genetic divergence from bacteria not pitted against the phages. Unsurprisingly, the genes that changed the most were those involved in host-phage interaction. This study (reference 2) is also a great example of the usefulness of whole genome sequencing. Whole populations of bacteria and phage were allowed to evolve both together and separately and then just sent away for sequencing with the results analysed at the end.

You really can't be an anti-evolutionist while studying bacteria. They just do it so damn quickly and often you can see it happening.

Sandoz, K., Mitzimberg, S., & Schuster, M. (2007). From the Cover: Social cheating in Pseudomonas aeruginosa quorum sensing Proceedings of the National Academy of Sciences, 104 (40), 15876-15881 DOI: 10.1073/pnas.0705653104

Paterson S, Vogwill T, Buckling A, Benmayor R, Spiers AJ, Thomson NR, Quail M, Smith F, Walker D, Libberton B, Fenton A, Hall N, & Brockhurst MA (2010). Antagonistic coevolution accelerates molecular evolution. Nature, 464 (7286), 275-8 PMID: 20182425

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