Microbiological Quality and Presence of Foodborne Pathogens in Full-Fat Cow’s Yellow Cheese in Bulgaria

Cheese plays a significant role in human nutrition, being one of the most popularready-to-eat foods produced worldwide. Cheese is rich in microbiota and more than 400 species of LAB, Gram (+) and Gram (-) bacteria, yeasts and molds have been identified in it so far. The rich nutritional content of cheese also favors the development of foodborne pathogenic bacteria. The aim of our study was to investigate the microbiota in commercially branded full-cream yellow cheese with a high market share as well as to search for pathogenic bacteria in it. An amount of different bacterial groups in 1 g of cheese was assessed using SPS agar (Clostridium), MCA agar (Staphylococcus), KEAA (fecal enterococci), YGS (yeasts), Candida agar (Candida spp.) and Nutrient broth (total number). Randomly picked isolates were subcultured on Columbia agar with 5% sheep blood; McConkey agar (BD, USA); Sabouraud and BBL CHROMagar Candida (BD, USA). Identification of isolates was carried out using Crystal (BD, USA) system; VITEK 2, bioMerieux, France, API 20 C AUX Candida (bioMerieux, France). Our results revealed that the total number of bacteria in the investigated sample was 6.9x105. Cheese was negative for clostridia and coliforms. It was positive for yeasts but within the standard limit – 2x102 CFU/g, including Candida spp. - 2x102 CFU/g. Alarmingly, a high amount of fecal enterococci - 1.5x105 CFU/g and staphylococci – 2x104 CFU/g was detected. Bacteria isolated from the cheese sample were further identified as Staphylococcus simulans, Enterococcus avium, Candida krusei and Cryptococcus neoformans, and Staphylococcus simulans, associated with endocarditis of chickens in humans. A putative reason for the staphylococcal contamination of cheese could be mastitis in the cows whose milk was processed. Another isolate, Enterococcus avium, was reported to be responsible for bacteremia and brain abscesses in humans and is known to be resistant to antibiotics. Yeasts, including Candida spp., complied with the standard but even at a small amount they could be harmful to immunocompromised consumers, children or pregnant women. Pathogens as Candida krusei and Cryptococcus neoformans were detected in the investigated sample.