Abstract

A study was carried out to test the effect of natural food samples on the isolation of L.monocytogenes from microbiological reference materials. Each of the 30 participating laboratories were given a free choice of enrichment method(s) for the detection of L.monocytogenes, the type(s) of food samples and the number of samples tested. To test the effect of inoculum size reference materials with two levels of contamination (ca 5 and ca 100 cfu/capsule) were provided. Listeria monocytogenes was isolated from 87.8 and 97.8% of the capsules containing 5 and 100 cfu respectively when examined without the addition of food. In the presence of foods the proportions of samples yielding the organism were 63.4% and 72.5% respectively. There was no relation between the isolation methods used or the type of food tested. The results, however, were laboratory dependent. This confirms the need for the performance of individual laboratories to be tested with reference materials both with and without the addition of food samples.