This study was intended to investigate the effect of contact with concentrations close to the minimum inhibitory concentration (MIC) (0.5, 1, and 1.5 MIC; MIC of planktonic cells was determined using a microdilution broth method) of sodium hypochlorite (SHY) or benzalkonium chloride (BZK) during the process of formation of biofilm (24 h), upon the architecture and viability of the biofilms formed by four L. monocytogenes isolates of molecular serotype 1/2a: S2-1 (BZK-susceptible strain; MICBZK = 3.0 ppm), S2-2 (BZK-resistant strain qacH positive; MICBZK = 13 ppm), CDL 69 (BZK-resistant strain bcrABC positive; MICBZK = 10 ppm), and S2BAC (BZK-resistant laboratory mutant of S2-1, with multidrug resistance phenotype; MICBZK = 9 ppm). Images were examined through confocal laser scanning microscopy after staining with SYTO 9 and Propidium Iodide. Biovolume values in the observation field (14,161 μm2) in the absence of biocides ranged from 103,928.3 ± 6,730.2 μm3 (S2BAC) to 276,030.9 ± 42,291.9 μm3 (S2-1). Exposure to SHY at 0.5 MIC reduced (p < 0.05) the biovolume of biofilms formed by S2-1 and S2BAC and did not modify (p > 0.05) the biovolume of biofilms by S2-2 and CDL 69. Exposure to sub-MICs of BZK decreased (p < 0.05; S2-1) or enhanced (p < 0.05; S2-2, CDL 69 and S2BAC) biofilm development. Exposure to biocides at 1 or 1.5 MIC inhibited biofilm formation. This study provides clear evidence that BZK at sub-MICs can enhance the biofilm-forming ability of BZK-resistant L. monocytogenes strains. Because biofilms contribute to the persistence of bacteria throughout the food chain and represent a major source of food contamination, our findings suggest the importance of avoiding sub-MICs of disinfectants in food-handling environments.
Keywords: Listeria monocytogenes; benzalkonium chloride; biofilms; sodium hypochlorite; sub-MICs.