Evaluation of selective enrichment media for accurate most probable number enumeration of Listeria monocytogenes in ready-to-eat seafood products.

Abstract

Listeria monocytogenes is a foodborne bacterium that can cause listeriosis. Ready-to-eat (RTE) seafood products are among the food categories associated with listeriosis. Because contamination levels of L. monocytogenes in RTE seafood products are often below 10 CFU/g, accurate enumeration of low bacterial concentrations is essential for reliable risk assessment of these products. A most probable number (MPN) method is widely used for enumerating low bacterial concentrations in food samples; however, the choice of selective enrichment medium strongly affects recovery, especially in the presence of competing microflora and injured cells. In this study, five types of RTE seafood products were inoculated with low concentrations of L. monocytogenes, and four selective enrichment media including half Fraser broth, Listeria enrichment broth (UVM and FDA formulation), and buffered Listeria enrichment broth (BLEB) were compared. When samples were incubated in BLEB for 48 h, the MPN values agreed with the inoculation levels in 93-100% across strains and food matrices. Under these conditions, MPN estimates for heat-injured inocula were accurate in 93% of cases. Among the media evaluated, incubation in BLEB for 48 h yielded the most accurate and consistent MPN estimates across strains and food matrices, supporting its suitability for quantitative analysis of low levels of L. monocytogenes.