The influence of cheese matrix, microbial flora and enrichment broths on the detection of Listeria monocytogenes by PCR and the Vidas assay (R)

The detection of Listeria monocytogenes using either IDF-standard 143:1995 or ISO standard 11290-1. 1996 is time-consuming and thus inappropriate for online-control of milk and milk products in L. monocytogenes surveillance p rograms. Therefore, the reliability of a simple polymerase chain reaction a ssay and the Vitek Immune Diagnostic assay (Vidas assay(R)) were evaluated as rapid test procedures useful for detection of L, monocytogenes from enri chment brothes and factors which might impair detection efficiency were des cribed. Six Listeria enrichment broths were artificially spiked either with (i) a L . monocytogenes strain, (ii) a L. monocytogenes strain and competing flora and, in a third step, (iii) L. monocytogenes, competing flora and sterile c heese matrix. Listeriae were grown and samples were regularly drawn after 1 2, 24 and 48 h of incubation and subjected to polymerase chain reaction (PC R) and the Vidas assay(R). Also, viable cell counts of Listeria were reveal ed by plating samples at similar time points to selective media. Results de monstrated a negligible inhibitory effect of competing flora or selective b roth components on PCR performance. Infrequent detection rates were observe d when cheese matrix was added. False negative detection results were appar ently dependent on brand of cheese used thus indicating an increased inhibi tory effect of high fat content-containing brands of cheeses. The effect wa s strongest in blue veined cheese. The Vidas assay(R) was found appropriate for screening L. monocytogenes as positive results were observed in 72,5 a nd 97,5% of all experiments after 24 and 48 h of enrichment, respectively. The detection limit was calculated as 5.8 to 6.3 log(10) cfu/ml.