IDENTIFICATION OF NON-LISTERIA SPP. FROM SELECTIVE AGAR MEDIA USED FOR THE DETECTION OF LISTERIA MONOCYTOGENES IN READY-TO-EAT VEGETABLES

Listeria monocytogenes species, frequently isolated from soil, vegetation and water, is consistently associated with human illness. With growth temperature from 0°C to 45°C, it is a key foodborne pathogen in chilled, refrigerated and ready-to-eat foods. Food safety criteria applicable to ready-to-eat foods able to support the growth of L. monocytogenes are established in Regulation (EC) No. 2073/2005, amended in 2007. There are current ISO horizontal methods for the detection and enumeration of L. monocytogenes in food. ISO 11290-1:1996 is a method for the detection, while ISO 11290-2:1998 is a method for enumeration. Both methods were amended in 2004 to include modified media. However, conventional methods of detection for Listeria spp. can take up to five days to obtain a result, not fully compatible with the very short shelf-life of ready to eat vegetables. Aim of the study was the detection of L. monocytogenes in ready-to-eat sliced artichokes by conventional plating method and the identification of a representative number of presumptive pathogen isolates by genus- and species-specific PCR reactions and by 16S rRNA gene sequencing. Listeria spp. detection was performed, in parallel, on DNA extracted directly from artichoke samples by real-time PCR with the mericon Listeria spp Kit (QIAGEN S.r.l., Italy), which is commercialized for the specific and sensitive detection of several Listeria species in enrichment cultures of food samples. The results evidenced that none of the presumptive Listeria isolates on selective media belonged to Listeria spp. and, according to sequencing results, the isolates represented 4 different bacterial species belonging to Cellulosimicrobium, Microbacterium and Curtobacterium genera.