Letters in Applied Microbiology 1998, 26, 166–170 Effect of high-temperature, short-time (HTST) pasteurization on milk containing low numbers of Mycobacterium paratuberculosis I.R. Grant 1 , H.J. Ball 2 and M.T. Rowe 1 1 Department of Food Science (Food Microbiology), The Queen’s University of Belfast, and 2 Veterinary Sciences Division, Department of Agriculture for Northern Ireland, Belfast, Northern Ireland, UK 1647/97: received 22 September 1997 and accepted 30 September 1997 I.R. GRANT, H.J. BALL AND M.T. ROWE. 1998. The efficacy of high-temperature, short-time (HTST) pasteurization (72 °C/15 s) when low numbers (¾ 10 3 cfu ml -1 ) of Mycobacterium paratuberculosis are present in milk was investigated. Raw cows’ milk spiked with Myco. paratuberculosis (10 3 cfu ml -1 , 10 2 cfu ml -1 , 10 cfu ml -1 , and 10 cfu 50 ml -1 ) was subjected to HTST pasteurization using laboratory pasteurizing units. Ten bovine strains of Myco. paratuberculosis were tested in triplicate. Culture in BACTEC Middlebrook 12B radiometric medium detected acid-fast survivors in 14·8% and 10% of HTST-pasteurized milk samples at the 10 3 and 10 2 cfu ml -1 inoculum levels, respectively, whereas conventional culture on Herrold’s egg yolk medium containing mycobactin J detected acid-fast survivors in only 3·7% and 6·7% of the same milk samples. IS900-based PCR confirmed that these acid-fast survivors were Myco. paratuberculosis. No viable Myco. paratuberculosis were isolated from HTST- pasteurized milk initially containing either 10 cfu ml -1 or 10 cfu 50 ml -1 . recently, in the examination of milk from asymptomatic cows, INTRODUCTION Sweeney et al. (1992) cultured the organism from nine of 77 Mycobacterium paratuberculosis causes paratuberculosis, com- (11·6%) milk samples and reported a titre of 2–8 cfu 50 ml -1 monly known as Johne’s disease (JD), in cattle, sheep, goats of milk, and Streeter et al. (1995) cultured the organism from and other ruminants (Cocito et al. 1994). Although not cur- three of 126 (2·3%) milk samples. The study by Sweeney rently classified as a zoonotic agent, Myco. paratuberculosis has et al. (1992) suggests a low incidence of Myco. paratuberculosis been identified in intestinal biopsy tissue from a proportion of in the milk of naturally infected cattle, at least in asymptom- patients with Crohn’s disease (CD) (Chiodini 1989). Whether atic animals. this indicates a causative role for Myco. paratuberculosis in There has been increased interest in the efficacy of current CD, or simply a complicating infection, is still the subject of milk pasteurization conditions as a result of the above find- much debate. However, if Myco. paratuberculosis is implicated ings, and the possible association of Myco. paratuberculosis in CD, then milk has been suggested as a possible vehicle of with Crohn’s disease. Two recent studies (Chiodini and Her- transmission of the organism from cattle to humans (Hermon- mon-Taylor 1993; Grant et al. 1996), which investigated the Taylor 1993; Thompson 1994). Detectable quantities of efficacy of holder (63·5 °C/30 min) and high-temperature, Myco. paratuberculosis have been reported in the milk of both short-time (HTST, 71·7 °C/15 s) pasteurization when high clinically affected (Taylor et al. 1981) and asymptomatic cattle numbers (10 4 and 10 7 cfu ml -1 ) of Myco. paratuberculosis (Sweeney et al. 1992; Streeter et al. 1995) with JD. Taylor were present in milk, showed that neither holder nor HTST et al. (1981) cultured Myco. paratuberculosis from milk of nine pasteurization completely inactivated Myco. paratuberculosis of 26 (35%) cows confirmed to have advanced JD. More when the organism was present at levels - 10 4 cfu ml -1 . The numbers of Myco. paratuberculosis added to the milk in these Correspondence to: Dr Irene Grant, Department of Food Science (Food pasteurization studies were probably unrealistically high and Microbiology), The Queen’s University of Belfast, Newforge Lane, Belfast BT9 5PX, Northern Ireland, UK (e-mail: I.Grant@qub.ac.uk). unlikely to be encountered in naturally infected milk which © 1998 The Society for Applied Microbiology