Role of the cell envelope stress regulators BaeR and CpxR in control of RND-type multidrug efflux pumps and transcriptional cross talk with exopolysaccharide synthesis in Erwinia amylovora.

The purpose of this study was to identify the role of the cell envelope stress-sensing systems BaeSR and CpxARP in regulation of multidrug efflux and exopolysaccharide synthesis in Erwinia amylovora. We have previously reported that BaeR activates transcription of the RND-type efflux pumps AcrD and MdtABC. In this study, we found that a cpxR-deficient mutant was highly susceptible to β-lactams, aminoglycosides and lincomycin, whereas a baeR mutant showed no change in antimicrobial sensitivity. However, overexpression of BaeR in a mutant lacking the major RND pump AcrB increased resistance of E. amylovora to several compounds that are not substrates of AcrD or MdtABC. Furthermore, we observed that overexpression of BaeR significantly increased amylovoran production. Moreover, the expression of RND-type efflux pumps was changed in regulatory mutants of exopolysaccharide production. Our data suggest that BaeSR and CpxARP regulate additional mechanisms, beside efflux, which are responsible for antimicrobial resistance of E. amylovora.