The Role of Outer-Membrane Receptor Binding in Contact-Dependent Growth Inhibition Systems

The contact-dependent growth inhibition (CDI) system is a bacterial mechanism which mediates competition and cell-cell adhesion. CDI was originally described in an isolate of Escherichia coli wherein a small, three-gene locus was sufficient to elicit growth inhibition of neighboring Escherichia coli K-12 populations upon cell-cell contact. Subsequently, CDI systems were identified in a number of alpha-, beta-, and gamma-proteobacteria. In this study, I compare and contrast bacterial CDI systems with other bacterial toxin delivery systems. I seek to characterize the effective species range of the Escherichia coli CDI system in hopes of understanding how CDI systems are disseminated and how they influence microbial growth and competition in complex environs. I go on to demonstrate that the effective species ranges of CDI systems are determined by two factors: variability in outer-membrane receptor proteins and the utilization of specific receptor-binding sequences within CdiA proteins. Furthermore, I demonstrate that the CDI system of Escherichia coli isolate EC93 mediates biofilm formation via a previously undescribed cell-cell adhesion mechanism. Finally, I speculate on the role of bacterial CDI systems in bacterial competition and group behaviors and offer a model of CDI propagation and dissemination.