Thermal Tolerance ofMycobacterium paratuberculosis

Abstract
Dvalues (decimal reduction time; the time required to kill 1 log concentration of bacteria) were determined for both human and bovine strains (Dominic, Ben, BO45, and ATCC 19698) ofMycobacterium paratuberculosisin 50 mM lactate solution (pH 6.8) and in milk at four temperatures (62, 65, 68, and 71°C). ViableM. paratuberculosisorganisms were quantified by a radiometric culture method (BACTEC). Thermal death curves for theM. paratuberculosisstrains tested were generally linear, withR2of ≥0.90, but a few curves (R2, 0.80 to 0.90) were better described by a quadratic equation. The human strains (Dominic and Ben) had similarDvalues in milk and in lactate solution. However,Dvalues for the bovine strains (BO45 and ATCC 19698) were significantly different depending on the menstruum.Dvalues for low-passage clinical strains (Dominic, Ben, and BO45) were lower than those of the high-passage laboratory strain (ATCC 19698). TheDvalue based on pooled data for clinical strains ofM. paratuberculosisin milk at 71°C (D71°C) was 11.67 s. PooledD62°C,D65°C, andD68°Cof clinicalM. paratuberculosisstrains in milk were 228.8, 47.8, and 21.8 s, respectively. TheZvalue (the temperature required for the decimal reduction time to traverse 1 log cycle) of clinical strains in milk was 7.11°C. TheDvalues of clumped and singleM. paratuberculosiscells were not significantly different. TheDvalues of allM. paratuberculosisstrains tested were considerably higher than those published forListeria,Salmonella, andCoxiellaspp. and estimated forMycobacterium bovis, indicating thatM. paratuberculosisis more thermally tolerant. This study supports the premise thatM. paratuberculosismay survive high-temperature, short-time pasteurization when the initial organism concentration is greater than 101cells/ml.