Effect of sub-lethal concentrations of biocides on the susceptibility to antibiotics of multi-drug resistant Salmonella enterica strains

Ten multi-drug resistant Salmonella enterica isolates belonging to ten different serotypes were exposed to increasing sub-inhibitory concentrations of three biocides widely used in food industry facilities (trisodium phosphate, sodium nitrite, or sodium hypochlorite). Cultures were tested, before and after exposure to biocides, against 31 antibiotics of clinical significance by means of a standard disk-diffusion technique (CLSI). Exposed cultures displayed reduced susceptibility to a range of antibiotics, as compared with not exposed cultures. The impact of biocide exposure on reduced susceptibility to antibiotics was dependent on the Salmonella strain and the antibiotic family tested, susceptibility to aminoglycosides and cephalosporins being the most strongly affected. Results in the present study suggest that extensive use of biocides at sub-lethal concentrations could contribute to the emergence of antibiotic resistance in multi-drug resistant Salmonella enterica strains and therefore represent a public health risk. The intra-specific differences observed in antibiotic susceptibility underline the need to screen a wide range of strains.