A Comparative Study of Enumeration Methods for Listeria monocytogenes on Naturally Contaminated Ready-to-Eat Foods

Introduction: Listeria monocytogenes is a foodborne pathogen that is the causative agent in the human disease listeriosis. In foods and in food processing environments, L. monocytogenes is exposed to various stressors that possibly decrease its viability in terms of culture recovery with standard laboratory enrichment methods. Highly selective culture media used to facilitate growth of L. monocytogenes over competitive background flora may impact recovery of sub-lethally injured cells.

Purpose: Since it is expected that healthy individuals ingest small numbers of this bacterium without suffering any apparent ill effects, some countries, including Canada, have established quantitative guidelines for L. monocytogenes in ready-to-eat (RTE) food products. This study compared two enumerative methods for L. monocytogenes; the Health Canada method MLFP-74, which is based on direct plating without enrichment recovery, and the Most Probable Number (MPN) method, which utilizes enrichment steps, following the Health Canada method MFHPB-30.

Methods: Forty-five RTE meat and nine RTE sprout samples obtained from regulatory surveillance programs that tested positive for L. monocytogenes with MFHPB-30 were enumerated using both methods.

Results: The MLFP-74 method, with a detection limit of 5 CFU/g, resulted in all samples tested having less than the minimum detection limit (<MDL) of L. monocytogenes. The MPN method, with a minimum detection limit set to <0.03 mpn/g found that eight of the 45 RTE meat samples had countable levels of L. monocytogenes, which ranged from 0.03 mpn/g to 2.9 mpn/g. All other meat sample results were <MDL. Five of the nine sprout samples had countable L. monocytogenes levels which ranged from 0.04 mpn/g to 0.43 mpn/g. All other results for sprouts were <MDL.

Significance: These naturally-contaminated samples contained low levels of L. monocytogenes. There was no notable difference in results obtained by the two enumerative methods. A comparison involving more highly contaminated products would offer a greater opportunity to assess the potential for recovery issues.