Heat resistance of Mycobacterium avium subsp. paratuberculosis in milk

Mycobacterium avium subsp. paratuberculosis (MAP) is the known cause of Johne's disease in cattle and has been implicated as a possible cause of Crohn's disease in humans. When present in milk in high numbers the organism has been shown to survive laboratory pasteurisation treatments. This apparent heat resistance was investigated by: (1) using a combined acid fast/viability stain to visualise viable MAP in milk at different stages during pasteurisation; (2) comparing the thermal resistance of clumped and declumped MAP cells; and (3) assessing the lethality of a range of time/temperature heat treatments. The results provide circumstantial evidence that the tendency for the organism to clump contributes to its apparent heat resistance. Processing strategies to reduce the possibility of its survival in commercially pasteurised milk are suggested.