Bile Salts Modulate Expression of the CmeABC Multidrug Efflux Pump in Campylobacter jejuni

Abstract

<p>CmeABC, a multidrug efflux pump, is involved in the resistance of <em>Campylobacter jejuni</em> to a broad spectrum of antimicrobial agents and is essential for <em>Campylobacter</em> colonization in animal intestine by mediating bile resistance. Previously, we have shown that expression of this efflux pump is under the control of a transcriptional repressor named CmeR. Inactivation of CmeR or mutation in the <em>cmeABC</em> promoter (P_{<em>cmeABC</em>}) region derepresses <em>cmeABC</em>, leading to overexpression of this efflux pump. However, it is unknown if the expression of <em>cmeABC</em> can be conditionally induced by the substrates it extrudes. In this study, we examined the expression of <em>cmeABC</em> in the presence of various antimicrobial compounds. Although the majority of the antimicrobials tested did not affect the expression of <em>cmeABC</em>, bile salts drastically elevated the expression of this efflux operon. The induction was observed with both conjugated and unconjugated bile salts and was in a dose- and time-dependent manner. Experiments using surface plasmon resonance demonstrated that bile salts inhibited the binding of CmeR to P_{<em>cmeABC</em>}, suggesting that bile compounds are inducing ligands of CmeR. The interaction between bile salts and CmeR likely triggers conformational changes in CmeR, resulting in reduced binding affinity of CmeR to P_{<em>cmeABC</em>}. Bile did not affect the transcription of <em>cmeR</em>, indicating that altered expression of <em>cmeR</em> is not a factor in bile-induced overexpression of <em>cmeABC</em>. In addition to the CmeR-dependent induction, some bile salts (e.g., taurocholate) also activated the expression of <em>cmeABC</em> by a CmeR-independent pathway. Consistent with the elevated production of CmeABC, the presence of bile salts in culture media resulted in increased resistance of <em>Campylobacter</em> to multiple antimicrobials. These findings reveal a new mechanism that modulates the expression of <em>cmeABC</em> and further support the notion that bile resistance is a natural function of CmeABC.</p>