Effect of low doses of biocides on the susceptibility of Listeria monocytogenes and Salmonella enterica to various antibiotics of clinical importance

Abstract

The use of subinhibitory concentrations of biocides in food processing environments requires special attention because it is related to potential increases in antibiotic resistance. In this study, we determined the effect of exposure to low doses of four biocides (sodium hypochlorite, SHY; benzalkonium chloride, BZK; peracetic acid, PAA; trisodium phosphate, TSP) on the resistance to 10 antibiotics and on the hydrophobicity of the cellular surface of a strain of Listeria monocytogenes serotype 1/2 b (LM) and a strain of Salmonella enterica serotype Agona (SA), both of meat origin. The cultures were exposed at 37 °C in tryptone soya broth to increasing concentrations of the biocides, starting with half the minimum inhibitory concentration (MIC/2) and incrementing by 1.5 times the concentration until growth was no longer observed, calculating the MIC of the antibiotics before (control cultures) and after exposure. After exposure to TSP, LM was able to grow in the presence of a concentration of the biocide 2.53 times higher than the MIC of unexposed cultures. No adaptation was observed for SHY, BZK or PAA. SE demonstrated adaptation to BZK (it tolerated a concentration 1.13 times higher than the MIC for the unadapted strain) and PAA (2.53 times). LM cultures presented increased resistance (from susceptibility to reduced susceptibility, from susceptibility to resistance, or from reduced susceptibility to resistance) to erythromycin (strains exposed to BZK, PAA and TSP), chloramphenicol (all compounds), tetracycline (BZK) and fosfomycin (all compounds). Regarding SA, after exposure its resistance to gentamicin (all compounds) and enrofloxacin (BZK, PAA and TSP) increased. The cell surface hydrophobicity (determined through the microbial adhesion to solvents -MATS- test) increased (LM exposed to PAA and TSP; SA exposed to BZK) or decreased (SA exposed to PAA) after contact with the biocides. These findings suggest that the use of biocides at subinhibitory concentrations can contribute to the increase in bacterial resistance to antibiotics, in addition to modifying the hydrophobicity of the cellular surface, which is related to the capacity of bacteria to form biofilm.