Cell-cell interactions are common to all living systems. Bacteria are no exception, and numerous mechanisms that use secreted products as signaling molecules are known. Among these, the so-called “quorum sensing” systems are perhaps the best studied. In quorum sensing, all bacterial cells within a population produce secreted molecules. Only when population densities are high is there a response to these compounds, thus allowing the bacteria to coordinate their behavior. However, it is clear that there is much more to bacterial cell–cell interactions than simply counting numbers and coordinating behavior. Secreted molecules also play key roles in microbial development so that different cell fates can arise and coexist within a single-species population. In addition, in settings where multiple species coexist, their interactions often are mediated through extracellular compounds. Development in one microbe can be influenced by small molecules secreted by other species.
Almost all quorum sensing studies have been performed under laboratory conditions – far removed from how bacteria actually live. What happens in the “wild” when rival bacterial gangs contest the same territory?
Interspecies interactions that result in Bacillus subtilis forming biofilms are mediated mainly by members of its own genus. PNAS USA November 10 2011. doi: 10.1073/pnas.11036301
Many different systems of bacterial interactions have been described. However, relatively few studies have explored how interactions between different microorganisms might influence bacterial development. To explore such interspecies interactions, we focused on Bacillus subtilis, which characteristically develops into matrix-producing cannibals before entering sporulation. We investigated whether organisms from the natural environment of B. subtilis – the soil – were able to alter the development of B. subtilis. To test this possibility, we developed a coculture microcolony screen in which we used fluorescent reporters to identify soil bacteria able to induce matrix production in B. subtilis. Most of the bacteria that influence matrix production in B. subtilis are members of the genus Bacillus, suggesting that such interactions may be predominantly with close relatives. The interactions we observed were mediated via two different mechanisms. One resulted in increased expression of matrix genes via the activation of a sensor histidine kinase, KinD. The second was kinase independent and conceivably functions by altering the relative subpopulations of B. subtilis cell types by preferentially killing noncannibals. These two mechanisms were grouped according to the inducing strain’s relatedness to B. subtilis. Our results suggest that bacteria preferentially alter their development in response to secreted molecules from closely related bacteria and do so using mechanisms that depend on the phylogenetic relatedness of the interacting bacteria.