Abstract
The human food-borne pathogen Listeria monocytogenes is capable of persisting in food processing plants despite cleaning and sanitation and is likely exposed to sublethal biocide concentrations. This could potentially affect susceptibility of the bacterium to biocides and other antimicrobial agents. The purpose of the present study was to determine if sublethal biocide concentrations affected antibiotic susceptibility in L. monocytogenes. Exposure of L. monocytogenes strains EGD and N53-1 to sublethal concentrations of Incimaxx DES (containing peroxy acids and hydrogen peroxide) and Triquart Super (containing quaternary ammonium compound) in four consecutive cultures did not alter the frequency of antibiotic-tolerant isolates, as determined by plating on 2× the MIC for a range of antibiotics. Exposure of eight strains of L. monocytogenes to 1 and 4 μg/ml triclosan did not alter triclosan sensitivity. However, all eight strains became resistant to gentamicin (up to 16-fold increase in MIC) after exposure to sublethal triclosan concentrations. Gentamicin-resistant isolates of strains N53-1 and 4446 were also resistant to other aminoglycosides, such as kanamycin, streptomycin, and tobramycin. Gentamicin resistance remained at a high level also after five subcultures without triclosan or gentamicin. Aminoglycoside resistance can be caused by mutations in the target site, the 16S rRNA gene. However, such mutations were not detected in the N53-1-resistant isolates. A combination of gentamicin and ampicillin is commonly used in listeriosis treatment. The triclosan-induced resistance is, hence, of great concern. Further investigations are needed to determine the molecular mechanisms underlying the effect of triclosan.