Listeria selective enrichment supplement

For the selective enrichment of Listeria spp. 1 ml of cell suspension was added to 9 ml Buffered-Listeria-Enrichment-Broth (BLEB) (Merck KGaA, Darmstadt, Germany). After 2 h of preincubation at 30°C Listeria-Selective-Enrichment supplement (Merck KGaA, Darmstadt, Germany) was added and the culture was further incubated at 30°C. After 24 h, 48 h, and 7 days 0.1 ml of the enrichment culture were transferred to 10 ml Half-Fraser-Bouillon (Merck KGaA, Darmstadt, Germany) and incubated aerobically at 37°C. Enrichment culture dependent detection of Listeria spp. was conducted after 24 and 48 h using Oxford-Agarplates (Merck KGaA, Darmstadt, Germany) and Palcam-Agarplates (Merck KGaA, Darmstadt, Germany) inoculated with the second enrichment culture. In case of a detection of Listeria spp. Rapid‘L.mono medium (Bio-Rad Laboratories GmbH, Munich, Germany) plates were used to identify L. monocytogenes. After inoculation, the identification plates were incubated at 37°C for 48 h.
In order to enrich S. enterica cells, 1 ml of cell suspension was added to 9 ml of BPW and incubated aerobically 37°C for 24 h. Afterwards cultures were streaked out on Xylose-Lysine-Deoxycholat (XLD) agar plates (Fluka, Buchs, Switzerland) and incubated at 37°C for 24 h in order to identify Salmonella spp.

Isolation and identification of L. monocytogenes were carried out using the method of Food and Drug Administration (FDA) [83 ]. Briefly, 25 g of each sample was mixed with 225 ml of Listeria enrichment broth (Merck, Darmstadt, Germany). The cultures were incubated at 30 °C for 4 h for the enrichment. Then, Listeria-Selective Enrichment Supplement (Merck, Darmstadt, Germany) was added to the broth and incubated for 44 h. A loopful from the enrichment broth was streaked onto Palcam Listeria Selective agar (Merck, Darmstadt, Germany) and incubated for 48 h at 35 °C. The grey-green colonies with a black center and black halo were subjected to the confirmatory tests such as Gram staining, motility in SIM Medium and biochemical test (catalase, oxidase, hemolysis on blood agar, urea, nitrate reduction, MR-VP, CAMP test, esculin hydrolysis, and fermentation of glucose, mannitol, maltose, xylose, and rhamnose) (Table S1) [83 ].