Characterization of Campylobacter jejuni response to the mutant prevention concentration hypothesis
de Souza Amado, Joaquim Eustaquio
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In this age of antimicrobial resistance concerns, a new hypothesis has been proposed that could serve to increase the lifespan of our antimicrobial stock. It is termed the mutant prevention concentration (MPC) hypothesis, and it states that in every interaction between microbe and antibiotic there is a concentration that will prevent first step spontaneous mutants from appearing after approximately 10^10 CFUs of bacteria are exposed to the drug of choice. Additional research is needed before this hypothesis could be useful in practical determinations to help establish day-to-day treatment of bacterial infections both in humans and animals. There are many factors, beside the classical and known antimicrobial targets, that are involved in antimicrobial resistance acquisition and facilitation of such events. Approximately 10^10 CFU of Campylobacter jejuni 81116 and three laboratory derived mutants carrying insertional deletions respectively at cmeB (efflux pump), cmeR (regulator of the efflux pump) and one presenting a Thr-86-Ile mutation in the gyrA gene, were exposed to 2-fold increasing concentrations of nalidixic acid, norfloxacin, enrofloxacin and ciprofloxacin. Mutation frequencies were recorded at each concentration along with the mutant prevention concentration (MPC), and minimum inhibitory concentrations (MICs) for isolated colonies selected over several trials. Susceptibility results (MICs) were also recorded for each isolate before exposure to each antimicrobial. Susceptibility measurements before exposure of the parent and its transformants to each fluoroquinolone showed a 2-fold decrease for the cmeB DNA sequence deleted transformant. Additionally, the absence of CmeB protein decreased the MPC consistently by 2-fold for every fluoroquinolone tested. This protein absence also translated into statistically significant mutation frequency decrease. When CmeB protein was over-expressed in the cmeR sequence insertionally deleted transformant, there were no apparent impacts on MPC or on mutation frequencies. The data from these studies reveal how MPC could be used as a tool to predict the usefulness of an antimicrobial coupled with the MIC methodology. It also shows that MPC could be another tool for deciding which dose of an antimicrobial to use and which antimicrobial is the one of choice.