P2-94 Listeria Inter-species Competition during the Selective Enrichment of Spiked Mung Bean Sprouts

Tuesday, August 2, 2016
America's Center - St. Louis
Kaitlin Cauchon, U.S. Food and Drug Administration-ORA, Jefferson, AR
Ronald Smiley, U.S. Food and Drug Administration-ORA, Jefferson, AR
Anthony Hitchins, U.S. Food and Drug Administration (retired), Rockville, MD
Introduction: Listeria monocytogenes can be difficult to recover following selective enrichment when non-pathogenic Listeria species are concomitantly present in the test sample. Naturally high microbial levels, in foods such as mung bean sprouts, can further complicate recovery because of variation in the growth rate under selective conditions.

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.