Quantification of Listeria monocytogenes in ready to eat vegetables using MPN method associated to quantitative Real-Time PCR

The rapid detection of human pathogens in fresh or minimally processed vegetables is a challenge for food security. In this work, a total of 22 samples of ready to-eat salads (RTE), containing different varieties of salads, of different brands were purchased from several supermarkets located in Puglia, Italy, during period 2010–2011. Samples were analysed to determine whether microbial contamination, and in particular pathogenic bacteria, such as Listeria monocytogenes, were present in these food products. The occurence of Listeria spp. was tested by the immunoassay Listeria Rapid Test (Oxoid). L. monocytogenes positive samples and artificially contaminated samples of minimally processed leafy vegetables were evaluated by the Most Probable Number (MPN) combined with quantitative real-time PCR (qRTPCR). Positive MPN enrichment tubes were analyzed by RT-PCR with primers specific for L. monocytogenes using the commercial preparation TaqMan® Listeria monocytogenes Detection Kit. Real-time PCR assay was also performed on six non-Listeria monocitogenes species. The obtained results showed a good exclusivity and inclusivity. Moreover, qRT-PCR was faster to perform with MPN results obtained in about 48 h for the quantitative qRTPCR analysis developed.