A Canadian Comparative Study of Modified Versions of the "FDA" and "USDA" Methods for the Detection of Listeria...

Eleven laboratories across Canada took part in a comparative study of modified versions of the "FDA" and "USDA" methods for the detection of Listeria monocytogenes in foods and environmental samples. Both were modified by the inclusion of additional plating media and the use of modified Fraser broth in the modified "FDA" method. Approximately 92% of the positive samples were detected after 24 h of enrichment. Testing of routine samples by the participating laboratories showed no significant difference (p<0.05) in ability to isolate L. monocytogenes by either the modified "FDA" or "USDA" methods. However, the modified "FDA" method isolated significantly more positives (16.8%) from the spiked foods/controls than the modified "USDA" method (p<0.05). For all samples tested by both methods in the same laboratory, again the modified "FDA" method significantly out performed the "USDA" version by approximately 6% (p<0.05). However, the spiked foods/controls tested by both methods in the same laboratory showed no difference (p<0.05) in their ability to isolate L. monocytogenes . Overall, the modified "FDA" and "USDA" methods were comparable (within 1.0%) in their ability to isolate this microorganism. The "USDA" preenrichment broth maintained its initial pHbetter. Modified Fraser broth, in principle, proved to be useful as a screening tool but is not very selective. Oxford agar proved to be marginally better than lithium chloride-phenylethanol-moxalactam medium and significantly (p<0.05) better than modified Oxford agar in isolating L. monocytogenes .