Purpose: This study evaluates the level of inter-species competition between L. monocytogenes and Listeria innocua during selective enrichment of spiked mung bean sprouts, using three regulatory test methods.
Methods: Portions of mung bean sprouts were spiked (one to five CFU/g) with L. monocytogenes (n=10 strains total) and L. innocua (n=1 strain). Selective enrichment was performed using the FDA BAM, the USDA-FSIS, or the EN ISO 11290.1 method. Quantitave PCR (qPCR) was used to enumerate L. monocytogenes in the double-species spiked sprout enrichments. Listeria innocua was enumerated using PALCAM agar.
Results: Following the selective enrichment of spiked mung bean sprouts, the L. innocua/L. monocytogenes population differentials were 2.8±1.1, 3.3±1.3, and 3.6±1.4 Δ log CFU/ml for the FDA BAM, USDA-FSIS, and EN ISO 11290-1 methods, respectively. In matrix-free enrichments, the population differentials were 1.7±0.9, 3.4±1.2, 2.1±1.2 Δ log CFU/ml for the same three methods, respectively. Considerable strain dependent variation was observed for L. monocytogenes with all three enrichment methods. The populations of L. innocua was always greater than L. monocytogenes.
Significance: The resident microorganisms on the sprouts contributed to the final population differentials observed between L. innocua and L. monocytogenes in both the FDA BAM and EN ISO 11290-1 methods. Resident microorganisms may negatively impact recovery of L. monocytogenes from high microbial load foods, following selective enrichment when multiple Listeria species are present. Continued improvement of selective enrichment for L. monocytogenes is needed to enhance the capabilities of regulatory laboratories to recover this organism amidst a complex microflora.