Survival of Mycobacterium avium subspecies paratuberculosis in retail pasteurised milk

Area of Science:

• Food safety and microbiology
• Veterinary public health
• Bacteriology

Background:

• Mycobacterium avium subspecies paratuberculosis (MAP) is a pathogen of concern in milk.
• Pasteurisation is a key process for eliminating milkborne pathogens.
• Previous surveys have investigated MAP in milk with varying detection methods.

Purpose of the Study:

• To survey retail pasteurised milk for viable MAP using the Phage-PCR assay.
• To compare the efficacy of Phage-PCR with other detection methods for MAP.
• To investigate the source and distribution of MAP in milk.

Main Methods:

• Survey of 368 retail semi-skimmed pasteurised milk samples in England (May 2014 - June 2015).
• Utilisation of the Phage-PCR assay for viable MAP detection.
• Comparison of Phage-PCR results with culture and direct PCR methods.

Main Results:

• 10.3% of surveyed milk samples contained viable MAP cells.
• Phage-PCR detected lower cell numbers and increased MAP-positive samples compared to other methods.
• MAP appears to be shed directly into milk within the udder, not solely from faecal contamination.
• MAP cells were found to be clustered and potentially located within somatic cells prior to lysis.

Conclusions:

• Pasteurisation does not fully eliminate viable MAP in milk.
• The Phage-PCR assay offers enhanced sensitivity for MAP detection in milk.
• MAP's presence within somatic cells may confer protection against heat inactivation during pasteurisation.
• Intracellular MAP within somatic cells is a potential explanation for viable MAP in retail milk.