This research project aimed to investigate if foodborne pathogens, e.g., Listeria monocytogenes, develop tolerance to biocides and whether the rotation of different biocides can help prevent the development of biocide tolerance.
Results showed that L. monocytogenes could increase its tolerance to bezalkonium chloride (BC), a quaternary ammonium compound, but not to sodium hypochlorite (SH) or peracetic acid (PAA). However, BC minimum inhibitory concentrations (MIC) increased only by a few mg/mL, having no impact on the efficacy of recommended industrial concentrations (hundreds to thousands mg/ml). Also, survival of L. monocytogenes isolates with BC tolerance genes (qacH, bcrABC, emrC and emrE) was not improved when exposed to industrial concentrations of BC. Overall, L. monocytogenes isolates (n=240) showed limited variation in their sensitivity to biocides. Rather, biocide efficacy was more affected by residual organic matter and/or bio-film, demonstrating the need for proper cleaning prior to biocide application.
To investigate the effect of biocides on industrially relevant bacteria, floor drain samples from the food industry were used to develop a representative (31 species, 24 genera) biofilm model. Biocide treatments on this model showed that rotation between two or three biocides at industrial concentrations is not necessary to prevent increased biocide tolerance compared to single use of the same biocide. Individual biocides affect, which bacteria dominate in the regrowing biofilm. While rotation schemes based on PAA or SH with BC used once a week were among the best-performing biocide schemes, schemes using one biocide (SH or PAA) had similar performance, indicating rotation is not necessary.