Clumping is good; controlled clumping is better.
August 26, 2011 § 1 Comment
When I pouted last week about the fact that other writers had beaten me to the punch in discussing an interesting recent paper on the fitness benefits of clumping in yeast, I had somehow failed to notice that another, similarly fascinating, paper on a related topic had just come out from the Bassler lab (Nadell and Bassler 2011. A fitness trade-off between local competition and dispersal in Vibrio cholerae biofilms. PNAS doi:10.1073/pnas.1111147108). This paper is looking at the formation of biofilms in the bacterium Vibrio cholerae, a nasty little bug that has been a major evolutionary force in the development of modern sewage systems. One of the factors that makes V. cholerae hard to get rid of is the fact that it can, when it chooses, grow in biofilms; it can produce a structural matrix called extracellular polysaccharide (EPS) in which the bacterial cells are embedded. EPS production has a number of benefits, including offering bacteria from many species the opportunity to collaborate and behave as a community. The puzzling thing, though, is that these community benefits are available to everyone, not just the bacteria who do the work of producing the EPS. This is a classic set-up for “cheating”; in theory, if some bacteria can gain the benefits of EPS production without paying the price for it, then those “cheating” bacteria would be expected to grow faster than the poor exploited EPS producers. At some point, the EPS producers (still struggling to build community, no doubt) would die out, and the whole system would collapse. The theoretical arguments seem very persuasive, but actually EPS-producing bacteria show no signs of going away. So clearly we need a new theory.
Kevin Foster and colleagues (including Carey Nadell, the first author of this paper) have been working for some time now on the possibility that EPS production, in addition to its benefits for the community, offers direct benefits to the cells that produce it. If you simulate the growth of EPS-producing microbes in three dimensions, including the way that nutrients and oxygen diffuse and are consumed, you can see that producing EPS can help a lineage of cells to push itself above the masses and get access to better conditions, incidentally suffocating non-EPS producing cells. This line of argument suggests that, far from being a happy “all for one, one for all” type commune, microbial biofilms are a balancing act between cooperation and competition — much like some other societies you might be aware of. It also suggests that, though there are some conditions in which “cheaters” (non-EPS producing cells, though now they look lazy and stupid rather than cunning) can win, especially when a group of cells is colonizing a new area, if a biofilm persists for a long time the EPS-producing cells have a strong advantage. And a particularly clear prediction from the 3D modeling is that, in a mixture of EPS-producers and non-producers, the EPS-producing lines should end up in skyscraper-like towers (reaching towards better oxygen conditions), suffocating the cheaters.
Nadell and Bassler set out to examine these predictions in V. cholerae, which you might call a facultative EPS producer: it uses quorum sensing mechanisms to turn on EPS production at low cell density, and repress production at high cell density. First, they measured the cost of EPS production in V. cholerae by measuring the growth of otherwise matched EPS-producing and non-producing strains in liquid culture. EPS-producing strains grew a whopping 25% slower than the non-producers; indeed, EPS production costs a lot, and you can imagine why cheating might look attractive. In biofilms, though, the EPS-producing cells do better: they grow more successfully than non-producers as a monoculture, and non-producers do even less well in mixed cultures of producers and non-producers. By labeling the strains with fluorescent proteins, the authors were able to track the composition of the biofilm over time in 3D; it’s like watching a Le Corbusier-designed urban landscape go mad. And indeed, the EPS-producing cells are the ones building the towers, while the non-producers suffocate at ground level. Suddenly cheating doesn’t look like such a great strategy.
But, the authors point out, growing in one location is not the only task that a community of microorganisms must succeed at in evolution. Cells that are permanently stuck in one place will do poorly, however good they are at exploiting the resources they have. Nadell and Bassler wondered whether EPS production might be a disadvantage for dispersal. And it is. The authors established mixed biofilms of EPS producers and non-producers in a microfluidic chamber, with medium flowing through the chamber to provide nutrients, and connected the effluent flow from this chamber to a second, empty chamber. Even though the EPS-producing cells do better than non-producers in the biofilm, most of the cells that escape to the new chamber and start to colonize it are non-producers. This could be the explanation for why V. cholerae has evolved its strategy of switching on EPS production at low density (just after colonizing a new area) and switching it off at high density (when the resources in an established colony are beginning to run thin). The delay between becoming low-density and switching on EPS production could be an important parameter in this system — when you’re colonizing a completely new area, it might well be advantageous to grow as fast as possible, so switching on EPS production too soon could be a disadvantage. Nadell and Bassler point out that the long-term dynamics of competition between producers and non-producers are likely to depend on how often the cells need to migrate to new areas; and though the ability to stick together is good, the ability to switch clumping on and off may be even better.
Nadell CD, & Bassler BL (2011). A fitness trade-off between local competition and dispersal in Vibrio cholerae biofilms. Proceedings of the National Academy of Sciences of the United States of America PMID: 21825170