Indexed on: 23 Apr '16Published on: 23 Apr '16Published in: Foodborne pathogens and disease
The aim of this study was to investigate the mechanism of rifampicin resistance in Listeria monocytogenes strains isolated from different types of food and the impact of specific mutations in the rpoB gene on susceptibility to different antimicrobial agents and on fitness cost. Fifteen spontaneous rifampicin-resistant strains were selected. The DNA regions corresponding to clusters I-II, III, and N-terminal end of the rpoB gene of Escherichia coli were amplified and sequenced, leading to the identification of 10 different substitutions, nine of which (Ser466Pro, Gln470Lys Asp473Asn, Gly479Asp, His483Tyr/Arg/Asp, Arg486His, and Leu490Pro) were located in cluster I and one (Pro521Leu) in cluster II. From among these mutations, substitutions at positions 466, 470, 486, 490, and 521 have not been described for L. monocytogenes. Only substitutions at positions 470, 479, 483, and 486 lead to resistance to very high concentrations of rifampicin (minimum inhibitory concentration [MIC] ≥256 μg/mL) and rifabutin (MIC 128 μg/mL). Furthermore, mutations at positions 473, 490, and 521 had different effects on susceptibility to rifampicin compared to other bacterial species. A correlation between rifampicin resistance and susceptibility to a wide range of antimicrobials was determined. Substitutions in RpoB did not change the susceptibility of the mutants to different antimicrobials. The fitness of the mutants was assessed by paired competition experiments. Mutations at positions 470 and 479 were not associated with a reduction in fitness level. There was no correlation between the MIC of rifampicin and fitness cost. The risk of transmission of resistant strains through the food chain highlights the need for monitoring resistance, identifying mutant organisms, their genotypes, and their altered phenotypes to understand their dissemination.