Rifampicin− Resistant Salmonella and Shiga–toxin−Producing Escherichia coli Exhibit Comparable Lactic Acid and Chlorine Stress Phenotypes to Wild-Type Strains

Using bacteria with rifampicin resistance can be useful when working with food matrices that have complex microbiota, though there is potential for repeated exposure to rifampicin during the selection process to introduce undesired mutations that lead to different phenotypic responses compared to the wild-type. In this study, we evaluated phenotypic responses of wild-type and rifampicin-resistant mutants for five strains each of Salmonella and Shiga-toxin producing Escherichia coli (STEC) that have been commonly used in low-moisture food challenge studies. No significant differences (P > 0.05) in growth rate and maximum density were observed between wild-type and rifampicin-resistant mutants for all isolates evaluated. Other than STEC O26:H11, no significant (P > 0.05) differences were observed between the wild-types and rifampicin-resistant mutants exposed to either 0.5% lactic acid or 45 ppm free chlorine solutions, and the magnitude of differences was within 0.5 log CFU/mL. Examining the single-nucleotide polymorphisms (SNPs) between whole-genome sequences revealed a limited number of variants with predicted moderate- or high-impact on genes. A consistent mutation occurred in rpoB, which is the primary target of rifampicin, while other SNPs appeared sporadically, often in hypothetical genes. These findings suggest that using these rifampicin-resistant strains produces comparable results to their wild-types and are suitable for low-moisture food challenge studies.