Delivery of Selective Agents via Time-Delayed Release Tablets Improves Recovery of Listeria monocytogenes Injured by Acid and Nitrite

Listeria selective enrichment media are designed to enhance the isolation of the organism and increase the chances of detection. Drawbacks include the requirements for prolonged sample incubation (48 to 72 h) and manual addition of selective agents, which may be a source of contamination. Modified Listeria recovery broth (mLRB) is a proprietary enrichment medium formulated to facilitate the recovery of injured cells; its selective agents are incorporated into a format that allows delayed release until 6 h of incubation. We evaluated the change in cell populations over time for acid- and nitrite-injured Listeria monocytogenes in mLRB with the selective agents added manually at 0 h (mLRBS0) and 6 h (mLRBS6). Recovery of injured cells in mLRB plus time-delayed tablets (mLRBTD) was also compared with that in enrichment media recommended by the U.S. Department of Agriculture (University of Vermont broth), the U.S. Food and Drug Administration (buffered Listeria enrichment broth), and the International Organization for Standardization (demi-Fraser broth). Nitrite- or acid-injured Listeria at approximately 10 CFU/ml were inoculated into each broth medium, and Listeria populations were enumerated at various times from 12 to 48 h of incubation at 37°C. Analysis of variance revealed that acid-injured Listeria populations in mLRBS6 at 24 h were significantly higher (P < 0.05) than those in mLRBS0; however, the differences in populations on these two media were not significant for nitrite-injured cells. Cell populations of four strains of Listeria inoculated into mLRBTD were significantly higher at 24 h than when those strains were enriched in buffered Listeria enrichment broth, demi-Fraser broth, and University of Vermont broth. Comparison between artificially contaminated milk and meat samples with a four-strain cocktail of Listeria resulted in cell populations that were significantly higher (P < 0.05) at 24 h on mLRBTD for contaminated meat than on mLRB for contaminated milk. Delivery of selective agents via time-delayed release tablets into mLRB maximizes recovery of acid- and nitrite-injured Listeria and saves analyst time during food sample analysis.