Comparison of methods for optimum detection of stressed and low levels of Listeria monocytogenes

Twelve laboratories across Canada participated in this third part of a comparative study of modified versions of the ‘FDA’ and ‘USDA’ methods for the detection of Listeria monocytogenes in food and environmental samples. Both methods were modified by the inclusion of additional plating media, as well as by the use of modified Fraser broth in the modified ‘FDA’ method, and additional 24 h incubation of the enrichment broth in the modified ‘USDA’ method. Approximately 88–94% of the positive samples were detected after 24 h of enrichment. Incubation of the ‘USDA’ enrichment broth for 48 h significantly ( P ) improved recovery of L. monocytogenes . The modified ‘FDA’ and ‘USDA’ methods were comparable (i.e. not significantly different, P>0·05 ) in their ability to isolate stressed and low levels of L. monocytogenes . The pH of the enrichment broths varied greatly after 24 and 48 h incubation. However, the ‘USDA’ enrichment broth maintained its pH closer to neutrality which may aid in the isolation of this micro-organism, especially injured cells, from some foods. Modified Fraser broth (MFB) proved to be useful as a secondary enrichment broth, as shown by the significantly greater number ( P ) of isolates being recovered from this broth, even though it is not very selective with a 74% false-positive rate. This improved recovery may result from the combination of the use of MFB with the selective plating media. During qualitative testing, Oxford agar (OXA) proved to be the significantly better medium, with lithium chloridephenylethanol-moxalactam medium (LPM), modified Oxford agar (MOX) and Palcam agar (PAL) being comparable in isolating stressed and low levels of this micro- organism from a variety of samples. Quantitative counts of stressed and non-stressed cultures showed that LPM, OXA and PAL were comparable (not significantly different, P>0·05 ) in their ability to recover stressed cells of this micro-organism.