Decline in Mycobacterium bovis and Brucella abortus populations during the maturation of experimentally contaminated parmesan-type cheese

Abstract

Brazilian legislation allows the manufacture of raw milk cheese with a maturation exceeding 60 days at room temperature above 5°C, but there is a lack of solid scientific evidence on the efficacy of this maturation process in inactivating important pathogens that may be present in milk, such as Mycobacterium bovis and Brucella abortus. Thus, the objectives of this study were to produce parmesan-type cheese experimentally contaminated with M. bovis and B. abortus and evaluate the survival of these pathogens along 2-month maturation. Parmesan-type cheese was manufactured in the laboratory using whole pasteurized milk with or without inoculation with M. bovis (SB1033) or B. abortus (1119-3) and matured at 18°C for up to 63 days. M. bovis was inoculated in Stonebrink-Leslie medium supplemented with antibiotics and incubated at 37°C for 45 days, and B. abortus was incubated in Farrel medium at 36°C for 3 days. The average D18°C value, weighted by variance, was 37.5 ± 5.3 days for M. bovis and 5.9 ± 0.7 days for B. abortus. The average physicochemical parameters in the cheese at the end of the study were as follows: pH = 4.89, water activity = 0.976, and moisture percentage = 43.1%. The pH might have contributed to the reduction in the population of B. abortus but seems not to have influenced the population of M. bovis. We conclude that the duration of the maturation process influences the size of the surviving populations of M. bovis and B. abortus, and that the shortening of the maturation duration might not ensure a decline in pathogen levels to safe levels. Thus, complementary studies considering the effect of several other technological aspects on the survival of these pathogens are required, including the effect of the lactic acid bacterial population, salt content, and temperature of maturation.