A bulletin for the Australian Food Industry    February 1999

Contents: Preventing meat-borne contamination | Safe food supply for Australia | Tainting of foods | Fresh Cuts industry guidelines | Labelling genetically modified food | Mycobacterium paratuberculosis in milk

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Mycobacterium paratuberculosis in milk

Considerable attention is being devoted to studies on the heat resistance of M. paratuberculosis following unconfirmed evidence that the organism can survive in commercially pasteurised milk. M. paratuberculosis is the causative organism of Johne's disease, a chronic enteritis of cattle and other animals, and has been linked to Crohn's disease in humans. It was claimed as long ago as 1913 that 'the histological characteristics of the two diseases are so similar as to justify a proposition that they may be the same' (IFST 1998). A definitive cause of Crohn's disease has still to be established and there is evidence both for and against the involvement of M. paratuberculosis.

Milk pasteurisation is arguably the most important single food related public health initiative of the twentieth century. Pasteurisation processes were originally designed to inactivate Coxiella burnetti and M. tuberculosis, the two most heat resistant vegetative pathogens likely to be present in raw milk.

The concept that the number of any given microorganism which will survive a given heat process is proportional to the number of that organism heated, is well established. Commercial heating processes in the food industry are designed to inactivate the level of target organisms likely to be encountered in foods with a generous safety margin which will vary with the circumstances.

It is difficult in the laboratory to duplicate exactly the heat treatments given to foods commercially. This is particularly the case where short hold times are involved as in HTST - high temperature short time pasteurisation (72° C for 15 seconds) or where the commercial process involves rapid flow rates and agitation. There is thus conflicting data emerging from laboratory studies on the heat resistance of M. paratuberculosis. However it does seem on the basis of a number of studies that M. paratuberculosis is more heat resistant than either C. burnetti or M. tuberculosis.

In a recent study (Letters in Applied Microbiology 26 1998 166-177), Grant and co-workers used initial populations of 10³ cfu/ml, 10² cfu/ml, 10 cfu/ml or 10 cfu/50 ml. The lowest of these inoculum levels was chosen on the basis of population levels recorded in the milk of asymptomatic cattle in the U.K.

No viable M. paratuberculosis were isolated from HTST pasteurised milk initially containing either 10 cfu/ml or 10 cfu/50 ml. A negative result was recorded when no viable organisms could be detected in 50 ml of the pasteurised milk. This tends to support other studies where unrealistically high initial populations of the organism survived standard pasteurisation. In these studies initial populations of 10⁴ cfu/ml or greater of M. paratuberculosis were used. There is no evidence that M. paratuberculosis poses a problem in Australia. However the outcome of the survey in the UK by the Ministry of Agriculture, Fisheries and Food will be studied with interest by the dairy industry around the world.