Persistence ofMycobacterium paratuberculosisduring Manufacture and Ripening of Cheddar Cheese

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Abstract
Model Cheddar cheeses were prepared from pasteurized milk artificially contaminated with high 104to 105CFU/ml) and low (101to 102CFU/ml) inocula of three differentMycobacterium paratuberculosisstrains. A reference strain, NCTC 8578, and two strains (806PSS and 796PSS) previously isolated from pasteurized milk for retail sale were investigated in this study. The manufactured Cheddar cheeses were similar in pH, salt, moisture, and fat composition to commercial Cheddar. The survival ofM. paratuberculosiscells was monitored over a 27-week ripening period by plating homogenized cheese samples onto HEYM agar medium supplemented with the antibiotics vancomycin, amphotericin B, and nalidixic acid without a decontamination step. A concentration effect was observed inM. paratuberculosisnumbers between the inoculated milk and the 1-day old cheeses for each strain. For all manufactured cheeses, a slow gradual decrease inM. paratuberculosisCFU in cheese was observed over the ripening period. In all cases where high levels (>3.6 log10) ofM. paratuberculosiswere present in 1-day cheeses, the organism was culturable after the 27-week ripening period. TheDvalues calculated for strains 806PSS, 796PSS, and NCTC 8578 were 107, 96, and 90 days, respectively. At low levels of contamination,M. paratuberculosiswas only culturable from 27-week-old cheese spiked with strain 806PSS.M. paratuberculosiswas recovered from the whey fraction in 10 of the 12 manufactured cheeses. Up to 4% of the initialM. paratuberculosisload was recovered in the culture-positive whey fractions at either the high or low initial inoculum.