Different effects of transcriptional regulators MarA, SoxS and Rob on susceptibility of Escherichia coli to cationic antimicrobial peptides (CAMPs): Rob-dependent CAMP induction of the marRAB operon

Abstract

Cationic antimicrobial peptides (CAMPs), a component of the mammalian immune system, protect the host from bacterial infections. The roles of the Escherichia coli transcriptional regulators MarA, SoxS and Rob in susceptibility to these peptides were examined. Overexpression of marA, either in an antibiotic-resistant marR mutant or from a plasmid, decreased bacterial susceptibility to CAMPs. Overexpression of the soxS gene from a plasmid, which decreased susceptibility to antibiotics, unexpectedly caused no decrease in CAMP susceptibility; instead it produced increased susceptibility to different CAMPs. Deletion or overexpression of rob had little effect on CAMP susceptibility. The marRAB operon was upregulated when E. coli was incubated in sublethal amounts of CAMPs polymyxin B, LL-37 or human b-defensin-1; however, this upregulation required Rob. Deletion of acrAB increased bacterial susceptibility to polymyxin B, LL-37 and human b-defensin-1 peptides. Deletion of tolC yielded an even greater increase in susceptibility to these peptides and also led to increased susceptibility to human a-defensin-2. Inhibition of cellular proton-motive force increased peptide susceptibility for wild-type and acrAB deletion strains; however, it decreased susceptibility of tolC mutants. These findings demonstrate that CAMPs are both inducers of marA-mediated drug resistance through interaction with Rob and also substrates for efflux in E. coli. The three related transcriptional regulators show different effects on bacterial cell susceptibility to CAMPs.

Low pH Enhances the Action of Maximin H5 against Staphylococcus aureus and Helps Mediate Lysylated Phosphatidylglycerol-Induced Resistance

“…Acid conditions have also been shown to promote resistance to AMPs in soil bacteria 92 and to inhibit the antibacterial activity of endogenous human peptides, such as defensins and LL-37, to organisms found in lungs 110,111 . On the other hand, PMF dependent efflux pumps also appear able to import porcine, bovine and other AMPs, thereby promoting their activity [112][113][114] . The promotion of antimicrobial action by low pH has been reported for a number of AMPs, including human hepcidins 115 , gads 116 clavanins and styelins 117 from marine organisms, and a range of synthetic peptides [118][119][120][121] , along with the anticancer activity of a variety of AMPs 16 .…”

The CpxR/CpxA Two-component Regulatory System Up-regulates the Multidrug Resistance Cascade to Facilitate Escherichia coli Resistance to a Model Antimicrobial Peptide

“…3) was not present in its promoter region. It is known that the marA gene encodes MarA, a regulator that activates transcription of both tolC as well as the pump component genes, including acrAB (19). Therefore, we investigated whether the CpxR/CpxA system could activate transcription of the mar-RAB operon (marked as mar, hereinafter).…”

“…However, mutation of acrAB or tolC eliminates this resistance in this constitutive marA strain (19). In addition, deletion of tolC in a marA constitutive expression strain causes a higher susceptibility to CAMPs than deletion of acrAB (19), suggesting that an additional TolC-dependent tripartite efflux system(s) other than AcrAB/TolC should contribute to this resistance. AcrAB and other resistance-nodulation-cell division superfamily efflux pumps probably use these export systems to mediate efflux of CAMPs because a loss of proton motive force in the presence of an uncoupler, i.e.…”

“…In contrast, transcriptional repressor MarR, which binds at the operator region (marO), is only released once a compound such as salicylate binds, allowing for transcription (8,9,18). Interestingly, the marRAB operon can also be activated by sublethal concentrations of CAMPs via Rob (which is required for polymyxin B-induced up-regulation of micF) and facilitate resistance at least in part by overexpressing the AcrAB/TolC efflux pump (19,20).…”

“…Although deletion of marA had no apparent phenotypic effect regarding susceptibility to CAMPs, constitutive expression of marA by either a point mutation in marR or a plasmid harboring a wild-type copy of marA raised bacterial resistance to multiple CAMPs, which represent various classes including ␣-helical and ␤-sheet CAMPs (such as LL-37 and HBD-1, respectively) as well as cyclic lipopeptide polymyxin B (19). However, mutation of acrAB or tolC eliminates this resistance in this constitutive marA strain (19).…”