Survival of Escherichia coli O157:H7 in Galotyri cheese stored at 4 and 12 °C

Abstract

Post-process contamination of fresh acid-curd cheeses with Escherichia coli O157:H7 may pose a risk considering the low infectious dose and the ability of the pathogen to survive in acidic foods. To evaluate its survival in Galotyri, a traditional Greek acid-curd cheese, portions (0.5 kg) of two commercial fresh products, one artisan (pH 3.9±0.1) and the other industrial (pH 3.7±0.1), were inoculated with approximately 3.0 or 6.5 log cfu g −1 of a five-strain cocktail of E. coli O157:H7, including rifampicin-resistant derivatives of the strains ATCC 43895 and ATCC 51657, and stored aerobically at 4 and 12 °C. Survival was monitored for 28 days by plating cheese samples on tryptic soy agar with 100 mg l −1 rifampicin (TSA+Rif), SMAC and Fluorocult E. coli O157:H7 agar media. The pathogen declined much faster ( P < 0.05 ) in the industrial as compared to the artisan cheeses at both temperatures. Thus, while E. coli O157:H7 became undetectable by culture enrichment after 14 days at 4 °C in industrial samples, irrespective of the inoculation level, populations of 1.4–1.9 and 4.2–5.1 log cfu g −1 survived after 28 days in the corresponding artisan cheeses with the low and high inocula, respectively. Survival was longer and greater ( P < 0.05 ) on TSA+Rif than on SMAC and Fluorocult, indicating the presence of acid-injured cells. Interestingly, survival of E. coli O157:H7 after 14–28 days in cheeses was better at 12 °C than at 4 °C, probably due to yeasts which grew on the surface of temperature-abused cheeses. The large difference in the pathogen's inactivation between the industrial and artisan cheeses at 4 °C could not be associated with major differences in pH or type/concentration of organic acids, suggesting another anti- E. coli O157:H7 activity by the industrial starter. The high survival of the pathogen in artisan Galotyri under conditions simulating commercial storage should be of concern.