Assessing manufacturers' recommended concentrations of commercial sanitizers on inactivation of Listeria monocytogenes

Listeria monocytogenes is the causative agent of listeriosis in humans. Its wide distribution in the environment is one of the main reasons it is a source of food poisoning. L. monocytogenes also forms biofilms, making cleaning and sanitation difficult. Microtiter plate assays have previously been used to assess biofilm formation of L. monocytogenes and to evaluate the antimicrobial activity of disinfectants in suspension. In this study, we have used a microplate assay to evaluate the minimum sanitizer concentrations required for a 5 log10 CFU/ml decrease of L. monocytogenes cells in suspension and in biofilm. Twenty-one sanitizers were tested against 20 strains of L. monocytogenes. The results showed that all tested sanitizers achieved a 5-log10 reduction of viable cells in suspension at concentrations below the manufacturers' recommended concentrations, of a biguanide-based product. When tested against L. monocytogenes in biofilm, only the peroxyacetic acid, chlorine dioxide and acidified sodium chorite-based products gave a 5-log10 decrease, within or close to the manufacturers' recommended concentrations. No relationship was observed between susceptibility to chemical sanitizers and other characteristics of the isolates: pulsotypes, sources, biofilm forming ability or persistence. This work suggests that caution must be taken in selecting an appropriate sanitizer for use in food processing plants and an effective cleansing programme is required to ensure thorough inactivation and removal of L. monocytogenes in biofilms.

► The microplate assay method is useful to compare bactericidal effects of sanitizers. ► Sanitizers had similar performance on the inactivation of Listeria in suspension. ► Three classes of sanitizers achieved a 5-log reduction against Listeria biofilm. ► Persistent Listeria strains did not show resistance to any sanitizer tested.