Inhibition of Listeria monocytogenes from Ready-to-Eat Foods and Food Processing Environments in British Columbia by Quaternary Ammonium Based Biocides and Triclosan

Introduction: Listeria monocytogenes is a foodborne pathogen, causing severe disease and mortality (20-40%) in susceptible populations. L. monocytogenes is a significant problem to the ready-to-eat (RTE) food industry due to its ability to persist in food processing environments (FPEs). Frequent use of sanitizers in FPEs is concerning, as it may select for resistant L. monocytogenes.  Biocide resistance may facilitate persistence among some strains of L. monocytogenes and increase the risk of product contamination.

Purpose: To assess the resistance of L. monocytogenes from RTE foods and FPEs to common biocides.

Methods: The minimum inhibitory concentration (MIC) of benzalkonium chloride (BAC), E-san (ES), and triclosan (TRI) was determined for L. monocytogenes (n = 42) isolates from RTE foods or RTE FPEs. Isolates were grown overnight on blood agar plates at 37°C and one colony was suspended in Mueller-Hinton broth. Aliquots were spotted onto Mueller-Hinton agar with 2% sheep blood containing appropriate concentrations of biocide. Growth was examined following incubation at 37°C for 24-48 hours. An established resistance breakpoint (10µg/ml) was used for all biocides. BAC and ES resistant isolates were PCR screened for bcrABC.

Results: Resistance to BAC between Lineage I and Lineage II isolates (4.3% and 51.5%, respectively) did differ significantly (P = 0.00014). Differences (P < 0.001) between lineages existed for TRI, with 95.7% of Lineage I isolates and 39.3% of Lineage II isolates being resistant. Lineage I and Lineage II isolates differed (P = 0.00014) in resistance to ES; specifically, 4.3% and 51.5% of Lineage I and Lineage II isolates, respectively, displayed resistance. Eighteen isolates had high MICs for BAC and ES (20-25µg/ml); fifteen of these were positive for bcrABC, all of which were Lineage II.        

Significance: While MICs remain below recommended concentrations for these biocides, isolates with increased resistance to sanitizers are a cause for concern as it may contribute to persistence in the FPE.