The potential for biocide tolerance in Escherichia coli and its impact on the response to food processing stresses

Biocides are used at all stages of the farm-to-fork continuum to reduce or eliminate both pathogens and spoilage microorganisms. Currently there is limited understanding of the mechanisms which contribute to biocide tolerance. Also, the impact of this phenotype may affect other risk reduction measures applied across the food chain. Tolerance to one or more biocides may contribute to an increase in the persistence of foodborne pathogens and other bacteria in the food chain. A panel of verocytotoxigenic and non toxigenic Escherichia coli strains were screened for their tolerance to eight commercial biocides and three biocidal active compounds (triclosan, benzalkonium chloride and chlorhexidine). Minimum inhibitory concentrations (MIC) for the commercial biocides were lower than the working concentration recommended by the manufacturers, while the MICs for the biocidal active components ranged from 0.78 to 12.5 μg/ml. As a means of exploring the associated phenotypes, mutant strains were selected which had an increased tolerance to biocides. A stepwise broth method was used to isolate biocide tolerant mutants. No stable mutants could be selected when commercial biocide preparations were used. In contrast, three stable mutants were isolated displaying an increased tolerance to the biocidal active component triclosan with growth observed at >8 mg/ml in comparison to the MIC for the wildtype strain (6.25 μg/ml). One isolate showed an increased tolerance to benzalkonium chloride. The responses of both isogenic wildtype and mutant isolates were compared to stresses often encountered across the food production chain. The strains were exposed to acid at pH 2 and 4, and to temperatures of 55 and 62 °C. No significant differences were observed in the survival of the wildtype and mutant strains under these environmental conditions, except in the case of E. coli O103 (T11 2EF1) at 55 °C (P < 0.001) and E. coli O157 (T3 5H5) at 62 °C (P < 0.05).