Polymerase chain reaction detection of Listeria monocytogenes on frankfurters using oligonucleotide primers targeting the genes encoding internalin AB.

A polymerase chain reaction (PCR) assay targeting the genes encoding internalin AB (inlAB) was developed for detecting Listeria monocytogenes in pure cell cultures and on artificially contaminated frankfurters. Four sets of oligonucleotide primers were evaluated. The set targeting a 902-bp region of the inlAB gene was the most specific. This PCR product was detected in 51 L. monocytogenes strains belonging to four different serogroups (1/2a, 1/2b, 1/2c, and 4b). In contrast, the PCR product was not detected in other Listeria spp. (Listeria innocua, Listeria ivanovii, Listeria seeligeri, Listeria welshimeri, or Listeria grayi) or in gram-positive, non-Listeria bacteria, indicating that the primer set was highly specific for L monocytogenes. The detection limit of the PCR assay was 10(5) CFU per ml of pure cell culture. However, the assay could detect as few as 10(1) CFU of L. monocytogenes in 25 g of frankfurter with 16 h of enrichment in modified Listeria enrichment broth at 30 degrees C. The total assay time including enrichment was approximately 24 h. These results suggest that the PCR assay can be used to rapidly detect L. monocytogenes on frankfurters and possibly other types of ready-to-eat meat products.