Mycobacterium paratuberculosis heat resistance

This letter is submitted in response to the paper of Grant et al. (1999). A number of scientists have recently published papers demonstrating that Mycobacterium avium subsp. paratuberculosis (MP), the agent of Johne's disease of cattle, is resistant to heat (Chiodini and Hermon‐Taylor 1993; Hope et al. 1996; Meylan et al. 1996; Sung and Collins 1998). Grant et al. (1999) also suggested that the traditional high temperature short time pasteurization of milk (HTST, 72 °C for 15 s) could be insufficient to protect public health. Furthermore, their data imply that there would be a greater kill of MP if the heating time was increased by 5–10 s at 72 °C as opposed to increasing the temperature by 13 °C. The suggestion that MP might be involved in Crohn's disease has not yet been demonstrated. Of course, if a link between MP and Crohn's disease was established and if pasteurization was chosen as a sanitary measure for risk management, knowledge of both the concentration of viable MP cells in raw milk and their thermal resistance would have to be determined in order to achieve an acceptable level of public health protection. Yet the zoonotic character of MP is not established, information on its concentration in raw milk is scarce, and other scientists ( Stabel et al. 1997; Keswani and Frank 1998) have published heat resistance results that contradict those of Grant et al. The two groups argue on the way heat resistance measurements should be performed. Heat resistance assessment is a laboratory activity that almost stopped in research laboratories 20 years ago. This letter is therefore intended to re‐open the debate and to review evidence that originally suggested that such studies were often futile.