Detection and enumeration of viable Listeria monocytogenes cells from ready-to-eat and processed vegetable foods by culture and DVC-FISH

Listeria monocytogenes is an important agent of foodborne disease, showing low prevalence but high mortality. There is evidence that vegetables are important vehicles of transmission, especially those minimally processed (fresh, fresh-cut vegetables under modified atmosphere packaging (MAP) or frozen) and directly exposed to consumers. The aim of this work was to detect and enumerate viable L. monocytogenes cells by culture and molecular methods from vegetable foods.A total of 191 vegetable samples (fresh, frozen and fresh-cut under modified atmosphere packaging) were studied. L. monocytogenes was detected and identified by selective plating, PCR and DVC-FISH.An isolation rate of 4.19% was obtained by culture, with a higher incidence in frozen vegetables. Six isolates belonged to serotype 1/2a and 2 to 4b. Counts were below <100 cfu/g for the eight positive samples, according to the food safety criteria established for the RTE by Commission Regulation EC No 1441/2007.Multiplex PCR method yielded a greater number of positive samples (10.47%). DVC-FISH technique determined that viable cells of L. monocytogenes were present in 32.98% of the samples, containing up to 4.97 log10 viable cells/g.

► A high percentage of RTE and processed vegetable is positive for L. monocytogenes.
► DVC-FISH is a useful tool for rapid detection of viable L. monocytogenes.
► All the L. monocytogenes isolates showed the presence of the lmo virulence gene.