Rapid determination of Listeria monocytogenes by automated enzyme-linked immunoassay and nonradioactive DNA probe.

A rapid and reliable analytical method was developed to detect and confirm the presence of Listeria monocytogenes in raw and partially processed foods. Forty-nine food samples (25 mixed cut vegetable salad, 12 smoked salmon, and 12 sterile smoked salmon) were individually inoculated with high levels [10-100 colony forming units (cfu)/25 g sample] and low levels (1-10 cfu/25 g sample) of L. monocytogenes, and were screened using the Vitek Immuno Diagnostic Assay (VIDAS) Listeria monocytogenes (VIDAS LMO)]. Positive test results were confirmed as L. monocytogenes by nonradioactive DNA probe. All samples inoculated with high levels of L. monocytogenes were detected by VIDAS and 96% were confirmed as L. monocytogenes by DNA probe. VIDAS LMO detected 89% of samples inoculated with low levels of L. monocytogenes, and 87% of these were confirmed as positive by DNA probe. In addition, 12 other samples (4 from each of mixed cut vegetable salad, smoked salmon, and sterile smoked salmon) were inoculated with high levels of L. ivanovii, L. seeligeri, L. welshimeri, L. innocua, L. grayi, and L. murrayi. Samples were assayed by the same protocol and all gave negative results. Compared with the cultural method, the VIDAS LMO nonradioactive DNA probe combination is highly specific, discriminates between L. monocytogenes and all other Listeria species, and reduces analytical time.