Purpose: This study establishes the final population densities of select strains of L. monocytogenes and L. innocua following selective enrichment of spiked mung bean sprouts. Additionally, this study estimates the inter-species population differential (Δlog CFU/ml) that results when two Listeria species are present together during the enrichment.
Methods: Portions of mung bean sprouts were spiked (1 to 5 CFU/g) with a L. monocytogenes strain or a L. innocua strain or both together. Ten strains of each species were studied. Selective enrichment was performed using the FDA-BAM method. PALCAM agar was used to estimate the Listeria populations of single-species spiked sprout enrichments. qPCR was used to enumerate L. monocytogenes in the double-spiked enrichments.
Results: Substantial L. monocytogenes population suppression (≈ 3 Δlog CFU/mL) and large Listeria inter-species population differentials (≈ 3 Δlog CFU/ml) were observed. The mean post-enrichment L. monocytogenes population of singly-spiked sprouts was 6.1±1.2 log CFU/ml. In matrix-free enrichments, the mean population was 9.3±0.2 log CFU/ml. The mean post-enrichment populations of doubly-spiked sprouts were 4.7±1.1 and 7.6±0.2 log CFU/ml, for L. monocytogenes and L. innocua, respectively.
Significance: Detection of L. monocytogenes and recovery of the organism from high microbial load food products may be hindered as a result of microbial competition. Continued research toward the improvement of L. monocytogenes enrichment methods is needed to enhance the capabilities of regulatory agencies to detect and recover L. monocytogenes amidst a complex microflora.