Enterobacteriaceae complicate the recovery of Listeria monocytogenes from food product enrichments using buffered Listeria enrichment broth

Buffered Listeria enrichment broth (BLEB) is the preferred enrichment formulation of the U.S. Food and Drug Administration (FDA) for the recovery of Listeria monocytogenes. BLEB, which relies on sodium nalidixate, cycloheximide and acriflavine-HCl for selectivity, permits the growth of all foodborne species of Listeria and several non-Listeria species including some members of the family Enterobacteriaceae. Five species of Enterobacteriaceae and four strains of non-pathogenic Listeria species were evaluated to determine which ones most suppressed to L. monocytogenes 48 hour populations in BLEB; these were subsequently used in spiked-food enrichment competition studies. L. monocytogenes recovery was complicated by the simultaneous presence of both Listeria innocua and Enterobacteriaceae competitors. The overall mean L. monocytogenes enrichment populations (n=15 per matrix) were 4.2 ± 0.6, 3.1 ± 0.4, and 2.4 ± 0.7 log CFU/mL for guacamole, asadero cheese, and refrigerated crabmeat, respectively. The overall mean L. innocua enrichment populations (n=15 per matrix) were 8.0 ± 0.7, 6.4 ± 0.7, and 6.5 ± 0.3 log CFU/mL for the same three matrices, respectively. L. monocytogenes was not recovered from any spiked-food enrichment when using either Oxford or chromogenic agars. L. innocua was recovered from all enrichments. Matrix-free competition assays using non-pathogenic Listeria species and Citrobacter braakii, simultaneously, indicate the potential for Enterobacteriaceae competitors to contribute considerably to both L. monocytogenes population suppression and to large Listeria inter-species population differentials. In some instances, Enterobacteriaceae competitors suppressed L. monocytogenes populations more than the non-pathogenic Listeria species. The ability to recover L. monocytogenes from food matrices with complex microflora, including multiple species of Listeria, is an important issue for regulatory agencies since detection without recovery is considered a false positive test result.https://doi.org/10.21423/jrs-v04n01p010 (DOI assigned 7/23/2019)