Multidrug efflux systems belonging to the resistance-nodulation-cell division (RND) family are ubiquitous in Gram-negative bacteria and are critical for antimicrobial resistance. This realization has led to efforts to develop efflux pump inhibitors (EPI) for use as adjuvants for antibiotic treatment of resistant organisms. However, the functions of RND transporters extend beyond antimicrobial resistance to include physiological functions that are critical for pathogenesis, suggesting that EPIs could also be used as antivirulence therapeutics. This was documented in the enteric pathogen Vibrio cholerae, in which EPIs were shown to attenuate the production of the critical virulence factors cholera toxin (CT) and the toxin-coregulated pilus (TCP). In this study, we investigated the antivirulence mechanism of action of the EPI phenyl-arginine-β-naphthylamide (PAβN) on V. cholerae. Using bioassays, we documented that PAβN inhibited virulence factor production in three epidemic V. cholerae isolates. Transcriptional reporter studies and mutant analysis indicated that PAβN initiated a ToxR-dependent regulatory circuit to activate leuO expression and that LeuO repressed the expression of the critical virulence activator aphA to attenuate CT and TCP production. The antivirulence activity of PAβN was found to be dependent on the ToxR periplasmic sensing domain (PPD), suggesting that a feedback mechanism was involved in its activity. Collectively, the data indicated that PAβN inhibited V. cholerae virulence factor production by activating a ToxR-dependent metabolic feedback mechanism to repress the expression of the ToxR virulence regulon. This suggests that efflux pump inhibitors could be used as antivirulence therapeutics for the treatment of cholera and perhaps that of other Gram-negative pathogens.
Keywords: RND; cholera; efflux; efflux pump inhibitors; pathogenesis; virulence; virulence inhibition.