Initial Cell Concentration and Selective Media Effects on the Isolation of Listeria monocytogenes from Enrichment Cultures of Inoculated Foods

Abstract

To facilitate rapid quantitation of Listeria monocytogenes in artificially inoculated dairy products, seafoods, and other foods, the recently revised U.S. Food and Drug Administration methodology for Listeria in foods was further abbreviated. The ability to isolate L. monocytogenes was measured in terms of the number of inoculated colony forming units (cfu)/g of food. Isolation values for the foods tested, using modified McBride's agar (MMA), were <1, 1–10, and >11 inoculated cfu/g in 30, 44, and 26% of test samples, respectively. The corresponding percentages with lithium chloride-phenylethanol-moxalactam (LPM) agar were 41, 51, and 8%, respectively. In eight (21%) instances, isolation values on LPM agar were significantly superior to those on MMA. The superiority of LPM was most dramatic when MMA isolation values were >10 initial cfu/g. However, LPM was not an infallible remedy for MMA deficiencies even at >10 cfu/g. Ease of isolation was apparently not related to food type. For 10 different test portions of Brie cheese, isolation values ranged from 0.3 to > 10,000 of inoculated cfu/g with MMA and 0.3 to 2500 of inoculated cfu/g with LPM agar.