Effect of exposure to sub-inhibitory concentrations of biocides on the susceptibility to antibiotics of sessile cells of Listeria monocytogenes and Salmonella enterica

Abstract

The effect of sub-inhibitory concentrations of several disinfectants on the antibiotic resistance of sessile cells of Listeria monocytogenes and Salmonella enterica was investigated. Biofilms were formed in the presence of biocides at MIC/2. Five compounds were used: peracetic acid (PAA), benzalkonium chloride (BZK), sodium hypochlorite (SHY), polyvinyl pyrrolidone iodine (PVI) and bis(3-aminopropyl)amine (BAPA). Two strains of L. monocytogenes (LM30 and LM28) and S. enterica (S. Typhimurium -ST- and S. Enteritidis -SE-) were used. Mono-species biofilms (MSB) and mixed biofilms (MB, LM30 + ST and LM28 + SE) were tested. LM30 went from susceptible to resistant to tetracycline and erythromycin after exposure to BAPA in MB. LM28 went from susceptible to resistant to tetracycline in the presence of PVI in MSB, from susceptible to intermediate to chloramphenicol after exposure to PAA in both MSB and MB, and from susceptible to resistant to ampicillin after exposure to SHY and BAPA in MB. ST went from intermediate to resistant to gentamicin after exposure to SHY, PVI and BAPA (MSB), or to PVI and BAPA (MB). This strain went from susceptible to intermediate to cefoxitin after exposure to BZK and PVI (MSB) or to BZK, PVI and BAPA (MB). For fosfomycin, ST went from susceptible to intermediate after exposure to PAA, PVI and PAPA (MSB) or to SHY, BZK, PVI and BAPA (MB). In the case of cephalothin, ST moved from intermediate to resistant category in MSB exposed to BAPA. SE moved from susceptible to intermediate to fosfomycin after contact with PAA in both MSB and MB. By contrast, in several occasions, strains increased their susceptibility to antibiotics after exposure to biocides. In some additional cases, contact with disinfectants caused significative modifications in the MIC of various antibiotics without the strains changing from one category to another. These findings show that contact with low concentrations of biocides can modify the antibiotic resistance of sessile cells of L. monocytogenes and S. enterica. The need to use adequate doses of disinfectants in food facilities in order to avoid changes in bacterial resistance is highlighted.