THERMAL INACTIVATION OF MYCOBACTERIUM-PARATUBERCULOSIS IN MILK

Thermal inactivation of Mycobacterium paratuberculosis, a suspected hu man pathogen, was determined in ultrahigh-temperature whole milk. Thre e strains of M. paratuberculosis were examined for survival at tempera tures from 55 to 75 degrees C using a submerged glass capillary tube m ethod, Clumped and declumped suspensions of the cultures were used to determine the rate of heat inactivation and survival at pasteurization temperatures. Methods for declumping M. paratuberculosis included the use of glass beads, vortexing and passing the cells through a 26-gaug e needle. The latter procedure was found to be superior over other met hods and did not affect the viability of cells. Capillary tubes filled with milk containing 4 x 10(6) to 3 x 10(7) CFU/ml were heated at tem peratures ranging from 55 to 75 degrees C. At 55 degrees C, minimal th ermal inactivation was observed for clumped and declumped cells. At 58 degrees C, thermal inactivation ranging from 0.3 to 0.7 log reduction was observed for both clumped and declumped suspensions. D values at 60 degrees C ranged from 8.6 to 11 min and 8.2 to 14.1 min for clumped and declumped cells, respectively. At 63 degrees C, the D values rang ed from 2.7 to 2.4 and 1.6 to 2.5 min for clumped and declumped cells, respectively. Survival of M. paratuberculosis at initial levels rangi ng from 44 to 10(5) CFU/ml at pasteurization treatment (63 degrees C f or 30 min and 72 degrees C for 15 s) was also determined. No survivors were observed after incubating plates for up to 4 months on Middlebro ok 7H11 agar and up to 2 months on Herrold's egg yolk medium. The sens itivity of the plating method was 1 CFU/250 mu l. These results demons trate that low levels of M. paratuberculosis, as might be found in raw milk, will not survive pasteurization treatments.