Isolation of Listeria species from precooked chilled foods

Cook-chill food prepared commercially was introduced to serve our patients and staff in 1989. The presence of Listeria monocytogenes in samples taken before regeneration was examined over two periods. For period A (April-November 1989), c. 0.2 g of food was examined; 2.7% of 992 meat samples and 0.6% of 1084 samples of other types of food were found to contain the organism. As a result, cold sliced meat was withdrawn from the patients' menu. For period B (February-July 1990), following the introduction of new guidelines, 25 g of food was examined after enrichment. Of the 854 meat samples examined 9% were positive for L. monocytogenes, and of the 1465 samples of other types of food examined 2.29% were positive. The significant increase between the two time periods occurred in the number of samples containing less than 10 colony forming units (cfu) g-1 of L. monocytogenes. The proportion of positive samples that contained 10-10(3) cfu g-1 remained the same for both periods (0.7%). Isolation of the organism was significantly associated with samples which had total viable counts of greater than 10(4) cfu g-1. There was no specific pattern of distribution of different serotypes in our isolates of L. monocytogenes. In the absence of evidence that small inocula of less than 10 cfu g-1 in cook-chill food before regeneration could be harmful to patients, we question whether the extra cost involved in following the Department of Health guidelines and examining 25 g of food for the presence of L. monocytogenes is justified.