| Microbes have evolved many strategies to adapt to changes in environmental conditions and population structures, including cooperation and competition. Identified in Escherichia coli, Contact Dependent Growth Inhibition (CDI) systems are wide-spread, polymorphic interbacterial competition systems. The current model for CDI states that CDI+ bacteria inhibit the growth (or kill) CDI- bacteria upon cell-cell contact by delivering the variable toxic C-terminus of a large exoprotein into the cytoplasm of target bacteria, where it functions to degrade DNA or tRNA molecules. Immunity proteins, CdiIs, protect bacteria against CDI-mediated competition in an allele-specific manner. The biological relevance of this phenomenon, i.e., how CDI systems contribute to the survival of bacteria in their natural habitat, however, is unknown. Here we characterize the predicted CDI systems of Burkholderia species, encoded by genes bcpAIOB . By contrast to what has been observed in E. coli, the B. thailandensis bcpAIOB genes only mediate interbacterial competition on a solid surface, which occurs in a defined spatiotemporal manner. We further establish that, in addition to mediating interbacterial competition, the B. thailandensis CDI system exoprotein BcpA, along with eDNA, is required for biofilm development. We show using a bcpA-gfp fusion that within the biofilm, only a subset of cells express the genes strongly at any given time. Constitutive production of the system's immunity protein in the entire bacterial population did not alter biofilm formation, indicating a CDI-independent role for BcpA in this process. We also identify a role for the polymorphic nature of CDI systems during biofilm formation: the variability of toxin domains (BcpA-CTs) functions to mediate kind discrimination and the allele-specificity of immunity proteins (BcpIs) functions as a mechanism for self-recognition. Furthermore, we show that CDI systems mediate competitive exclusion by not allowing non-self bacteria to enter a pre-established biofilm. Collectively, our data suggest that CDI systems perform a dual function in the development of sociomicrobiological communities: they mediate cooperation by building microbial biofilms that are composed of only self bacteria and they mediate competition by inhibiting the incorporation of non-self bacteria into the community. |
