β-helical protein PerA orchestrates antibiotic uptake across theCaulobacterouter membrane via converging stress signaling systems

While large membrane-impermeable antibiotics cannot traverse a lipid barrier, spacious importers such as TonB-dependent receptors (TBDRs) can mistakenly ferry antibiotics across the bacterial outer membrane (OM). We discovered that loss of PerA, an enigmatic β-helix protein in the OM of the oligotrophic α-proteobacteriumCaulobacter crescentus, reprograms the OM TBDR proteome from ChvT that imports the glycopeptide antibiotic vancomycin to an uncharacterized TBDR (BugA) that confers sensitivity to the polypeptide antibiotic bacitracin. Both antibiotics are large zinc-binding molecules that target the peptidoglycan, echoing the zinc stress response induces destabilization of PerA. Inactivation of PerA launches two conserved and interwoven envelope stress programs that remodel the OM with TBDRs, a tripartite multidrug efflux pump and periplasmic proteases. Thus, unanticipated entry routes for antibiotics emerge in stressed diderm bacteria that may be treatable with membrane impermeable antimicrobials owing to an underlying transcriptional stress response pathways coordinated by a novel type of OM regulator.