Investigation of the presence of Listeria monocytogenes in white cheese samples presented for consumption in Istanbul

Gram positive, facultative anaerobic, non-spore-forming, rod-shaped Listeria can be found in a variety of environments such as soil, silage, water, sewage and foods, because of they are tolerant to low pH, low temperature and high salt concentration. This genus consists seventeen different types: Listeria monocytogenes, Listeria seeligeri, Listeria welshimeri, Listeria ivanovii, Listeria innocua, Listeria grayi, Listeria rocourtiae, Listeria marthii, Listeria weihenstephanensis, Listeria fleischmannii, Listeria floridensis, Listeria cornellensis, Listeria aquatica, Listeria grandensis, Listeria riparia, Listeria newyorkensis, Listeria booriae. Among these bacteria, only L. monocytogenes and L. ivanovii showed pathogenic features. L. monocytogenes is an important food-borne pathogen and causes an infection named listeriosis in humans. This infection can be caused by consuming cheese, milk, unwashed raw vegetables and fruits, cooked and ready-to-eat foods contaminated with L. monocytogenes. Even though L. monocytogenes infection is encountered less frequently than other foodborne diseases, it is the second highest mortality among all foodborne pathogens. In this study, 119 white cheese samples purchased from markets and bazaars in Istanbul were investigate for the presence of L. monocytogenes. In first place, Listeria spp. isolated from white cheese samples according to standard recommended by FDA-BAM. Subsequent biochemical tests were performed for identification: Gram staining, oxidase, catalase and beta hemolysis reactions, nitrate reduction, motility test on SIM medium, acid production from rhamnose, mannitol, ribose, xylose and α-methyl-D mannoside, methyl red, voges-proskauer and CAMP tests. Strains identified at the genus level were confirmed by PCR method using primers belonging to the iap and hlyA gene regions, and the susceptibility of the identified strains against antibiotics was performed by Kirby-Bauer disc diffusion method. Listeria spp. was isolated from 3 (% 2.52) of 119 cheese samples analyzed. The 7 Listeria spp. obtained from these 3 samples were found to contain the iap gene region but not the hlyA gene region. Sequence analysis using the 16S rRNA gene region showed that these isolates were L. seeligeri. As a result of antibiotic susceptibility tests, 6 (% 85.71) of 7 L. seeligeri isolates were resistant to ciprofloxacin and 3 (% 42.85) were resistant to penicillin. All strains were susceptible to amikacin, chloramphenicol, rifampin, gentamicin, ampicillin, cefaclor, amoxicillin/clavulanic acid, trimethoprim-sulfamethoxazole, clarithromycin, vancomycin and tetracycline antibiotics. According to the findings of this study, it was determined that white cheese samples produced with pasteurized milk were contaminated with L. seeligeri. Listeria contamination in cheese made with pasteurized milk showed that production, packaging, distribution and storage practices following the pasteurization process should be followed more strictly. It is recommended to apply controls at each stage to prevent contamination.