Contact-dependent growth inhibition (CDI) is a phenomenon by which bacterial cell growth is regulated by direct cell-to-cell contact via the CdiA/CdiB two-partner secretion system. Characterization of mutants resistant to CDI allowed us to identify BamA (YaeT) as the outer membrane receptor for CDI and AcrB as a potential downstream target. Notably, both BamA and AcrB are part of distinct multi-component machines. The Bam machine assembles outer membrane beta-barrel proteins into the outer membrane and the Acr machine exports small molecules into the extracellular milieu. We discovered that a mutation that reduces expression of BamA decreased binding of CDI+ inhibitor cells, measured by flow cytometry with fluorescently labelled bacteria. In addition, alpha-BamA antibodies, which recognized extracellular epitopes of BamA based on immunofluorescence, specifically blocked inhibitor-target cells binding and CDI. A second class of CDI-resistant mutants identified carried null mutations in the acrB gene. AcrB is an inner membrane component of a multidrug efflux pump that normally forms a cell envelope-spanning complex with the membrane fusion protein AcrA and the outer membrane protein TolC. Strikingly, the requirement for the BamA and AcrB proteins in CDI is independent of their multi-component machines, and thus their role in the CDI pathway may reflect novel, import-related functions.
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