Palcam agar base

Three strains of Listeria monocytogenes, kindly provided by the laboratory of Microbiology and Biotechnology of Foods of Agricultural University of Athens (Food Microbiology Culture Collection-FMCC), were used in the present study. The strains, namely FMCC-B-129, FMCC-B-131 and FMCC-B-133, were originated from Greek industries. More specifically, the three strains were isolated from RTE frozen minced meat meal, conveyor belt of RTE frozen food and from soft cheese, respectively. The pure cultures were stored in −80 °C in brain heart infusion broth (BHI, LabM, Lancashire, UK) supplemented with 20% (v/v) glycerol. The strains were subcultured twice in BHI broth for 24 h and 18 h at 37 °C, before use. Bacterial cells were harvested separately by centrifugation at 10,000 g for 10 min, washed in ¼ strength Ringer’s solution (this step was performed twice), and finally resuspended in 10 mL of the aforementioned solution. The three-strain cocktail was prepared by mixing the three strains in equal volumes. The final mixture was used to inoculate ham slices at an approximate level of 4 log CFU/g. Inoculum size was confirmed by serial dilutions and plating on Palcam Agar base (LabM, Lancashire UK), for Listeria spp., incubated at 30 °C for 48 h.

Samples (10 g) of ham slices were weighed aseptically, added to sterile quarter strength Ringer’s solution (LabM, Lancashire, UK) (90 mL), and homogenized in a stomacher (Stomacher 400, Circulator, Seward) for 60 s at room temperature. The resulting suspensions were serially diluted in the same diluent and 1 or 0.1 mL samples of the appropriate dilutions were poured or spread, respectively, on the following agar media: de Man–Rogosa–Sharp Agar (MRS, Oxoid, Hampshire, UK) for LAB, incubated at 30 °C for 72 h; Plate Count Agar (LabM, Lancashire, UK) for TVC, incubated at 30 °C for 48 h; STAA Agar Base (Oxoid, Hampshire, UK) for Brochothrix thermosphacta, incubated at 25 °C for 48 h; Rose Bengal Chloramphenicol Agar (LabM, Lancashire, UK) for yeasts/molds incubated at 25 °C for 5 days; Violet Red Bile Glucose Agar (Oxoid, Hampshire, UK) for Enterobacteriaceae, incubated at 37 °C for 24 h, Pseudomonas Agar Base (LabM, Lancashire, UK), for Pseudomonas spp. incubated at 25 °C for 48 h, as well as Palcam Agar Base (LabM, Lancashire, UK), for Listeria spp. incubated at 30 °C for 48 h.