Methods for detection and enumeration of viable Mycobacteriuum paratuberculosis from milk and milk products

A review of the relevant scientific literature and information on methodology for milk volunteered by various research groups around the world was carried out. This revealed that, firstly, there is a paucity of published information on the subject of methods for the detection and enumeration of this organism from milk, and, secondly, there is no consensus on what the optimum method for the recovery of M. paratuberculosis from milk is. Isolation of this organism from milk is generally achieved by chemical decontamination prior to culture. The use of chemical decontamination lias implications in relation to percentage recovery of M. paratuberculosis cells present in a milk or cheese sample and, consequently, accurate enumeration of the organism, and also the successful recovery of sub-lethally injured cells which may occur in pasteurized milk. Novel methods of detection, which circumvent the need for chemical decontamination, such as immunomagnetic separation, use of ChemScan and solid phase cytometry are currently being investigated and some success has been reported to date. However, the problem with most novel approaches is a lack of specificity and there is scope for refinement of some of these methods and development of further methods. PCR methods for M. paratuberculosis in milk are currently being developed and evaluated in a number of laboratories world-wide, including quantitative PCR using Taqman technology. The major drawback of many of these PCR methods is that although they can detect the presence of M. paratuberculosis in a milk or cheese sample they cannot confirm cell viability. RT-PCR and NASBA methods for M. paratuberculosis, which have the potential to distinguish viable and dead cells, are in the early stages of development at laboratories in Ireland and France. The dairy industry must recognize that method development and evaluation for M. paratuberculosis takes a long time (18-24 months) and a 'gold standard' test for the detection of viable M. paratuberculosis in milk or milk products is unlikely to be identified in the near future.