Applying the handicap principle to biofilms: condition-dependent signalling in Bacillus subtilis microbial communities

Bacteria in nature often reside in differentiated communities termed biofilms. These communities, which are composed of a number of functionally-distinct cell types, are an interesting example of division of labour in microbes, and as such have been used as a system to study the evolution of cooperation. The structured population of the biofilm also plays a critical role in the persistence of infections, and biofouling of medical and industrial devices. Biofilm formation involves several stages of differentiation, which are mediated by extracellular factors secreted by cells composing the biofilm. The developmental model of biofilm formation describes this process mechanistically: specific subpopulations of cells synthesize signals within the biofilm, and promote the differentiation of other subpopulations. The handicap principle suggests that signals function because they provide reliable information regarding the state of the signaller; here, we apply the handicap principle to signalling among cells composing Bacillus subtilis biofilms, emphasizing the perspective of secreted factors as sources of information rather than solely as mediators of development. Such information could facilitate competition among phenotypically-similar cells composing the biofilm, affecting local organizational patterns within defined subpopulations.