Susceptibility of Listeria monocytogenes strains isolated from dairy products and frozen vegetables to antibiotics inhibiting murein synthesis and to disinfectants

The susceptibility of 96 strains of Listeria monocytogenes isolated from food to antibiotics and disinfectants currently used in human therapy, veterinary, medicine and food industry was determined by a standard operating procedure - broth dilution method. Antimicrobial agents included the beta-lactams ampicillin and penicillin, the lantibiotic nisin, and the disinfectants benzalkonium chloride and chlorhexidine gluconate. Among the studied strains we found 13 strains with 8-fold, 7 strains with 16-fold and 2 strains with 32-fold decreased susceptibility to ampicillin, as determined by MIC, compared to wild type reference strain. Interestingly, the mentioned strains were isolated from frozen vegetables and soups, none of the isolates from dairy products showed any elevated resistance to the studied antimicrobial agents. The occurrence in food products of strains with increased resistance to ampicillin is disquieting, especially since beta-lactams are the most frequent antibiotic of choice in the therapy of infections caused by the pathogen.