Elevated concentrations of polymyxin B elicit a biofilm-specific resistance mechanism in Vibrio cholerae

Abstract Vibrio cholerae can form biofilms both in the aquatic environment and in the human intestine, facilitating the release of hyper-infectious aggregates. Due to the increasing antibiotic resistance that impedes treatment of infections, alternatives need to be found. One of these alternatives is antimicrobial peptides, including polymyxin B (PmB), which is already used to treat infections caused by antibiotic-resistant bacteria. In this study, we first investigated the resistance of V. cholerae O1 El Tor strain A1552 to various antimicrobials under aerobic and anaerobic conditions. An increased resistance to PmB is observed in anaerobiosis, with a 3-fold increase in the dose required for 50% growth inhibition. We then studied the impact of the PmB on the formation and the degradation of V. cholerae biofilms to PmB. Our results show that PmB affects more efficiently biofilm formation under anaerobic conditions. On the other hand, preformed biofilms are susceptible to degradation by PmB at concentrations close to the minimum inhibitory concentration (MIC), resulting in approximately 50% reduction of the biomass. At higher concentrations, we observed less degradation and an opacification of the biofilm structures within 20 minutes post-treatment, suggesting a densification of the structure. This densification does not seem to result from the overexpression of matrix genes but rather from the release of DNA through cellular lysis, forming a protective shield that limits the penetration of the PmB into the biofilm. Importance Vibrio cholerae is an intestinal pathogen capable of forming biofilms and resisting antimicrobials both in the aquatic environment and during infection. Understanding and determining the resistance of V. cholerae to antimicrobials during the infection is crucial to improve patient care. During the infection and in the aquatic environment, V. cholerae form biofilms, structures that are known for their significance in antimicrobial resistance. In this study, we investigated the antimicrobial resistance of V. cholerae in both aerobic and anaerobic conditions, in their planktonic and biofilm forms. The major finding of this study is the identification of a resistance mechanism specific to elevated concentrations of polymyxin B, a last-resort antimicrobial used in the treatment of infections caused by multidrug-resistant Gram-negative bacteria. This resistance mechanism likely involves the lysis of bacterial cells on the surface of the biofilm, resulting in the release of DNA that provides a protective shield against PmB for bacteria within the biofilm matrix.