B12002: Review of raw milk analysis methods and assessment of their effectiveness as pathogen markers and indicators of farm hygiene

Study Duration: July 2002 to June 2004

Contractor: Direct Laboratories

This projects seeks to review theoretically and appraise practically microbiological, biochemical and other methods used to enumerate levels of E. coli, coliforms, faecal enterococci and other appropriate bacterial groups in order to assess their fitness as indicators of raw milk quality. Enumeration of total bacterial levels and somatic cell count are currently used in the dairy industry to assess raw milk quality because there is a perception that total bacterial levels are an indicator of faecal contamination. Faecal contamination is the most likely route of pathogen introduction into raw milk. However, these methods may not be appropriate as pathogen markers because bacteria, and in particular coliforms, colonise raw milk by a number of routes. Common vectors are coliform mastitis, incorrectly cleaned milking equipment, contaminated water supplies, dirty bulk tanks and transport containers, as well as through faecal contamination of cows' udders and teats. Although low somatic cell counts can sometimes be used to eliminate coliform mastitis as a reason for high bacterial numbers, total bacterial counts do not generally correlate with the presence or absence of pathogens or provide a good indicator of farm hygiene.

A list of practical, inexpensive, appropriate microbiological testing methods for enumerating coliforms, E. coli and faecal enterococci in milk will be generated by reviewing technical peer-reviewed scientific literature, informal discussions with experts from the dairy industry, and critical evaluation of non peer-reviewed resources. An initial theoretical evaluation of compiled methods by expert panel will be undertaken, and those protocols considered most likely to be appropriate for assessing the hygienic quality of raw milk selected for further evaluation. A specific emphasis of the theoretical evaluation will also be to identify any method that has been used previously as an indicator of pathogen presence. The most promising methods will be appraised by laboratory studies to ensure that each method is robust, reproducible and produces reliable results. A secondary evaluation of the best methods will then be undertaken. A representative selection of raw milk samples will be tested over 1 year to ensure account is taken of seasonal variation in bacterial levels. Duplicate aliquots of samples will also be tested for the presence of known milk-borne pathogens (e.g. campylobacter, VTEC, salmonella and Mycobacterium paratuberculosis).

In addition, to identify any correlation between pathogen presence and groups of indicator organisms, the relationship between coliform levels, E. coli counts and numbers of other specific types of bacteria (e.g. faecal enterococci) in raw milk and the hygiene standard at production facilities will be studied. Hygiene levels on farms will be determined using a standardised operating method and proforma and assessment by experienced dairy hygiene consultants.

The project was commissioned to determine if there were any microbiological indicators capable of providing a specific correlation to the audit score recorded during the Dairy Hygiene Inspectorate�s visual assessment of on farm dairy hygiene standards.

The bacterial indicators that were identified as potentially useful by this study were total bacterial counts, psychrotrophs, coliforms and pseudomonas. Only coliforms, which were related to milking plant cleanliness, had significant relationships which were preserved across both the summer and winter seasons. One of the difficulties encountered during this study was the high degree of variation in bacterial levels in milk, which meant the relationships identified are not suitable for 'snapshot' determinations of farm standards based on a single analyses. Total bacterial counts are currently used by dairy companies as farm hygiene and milk quality indicators. This study was unable to identify any alternative bacterial groups which provided a better measure of on-farm hygiene practices and the likelihood of pathogens being present in raw milk. The current approach to dairy farm inspection remains a valid method to deliver effective dairy farm hygiene enforcement.

The final report is available from the Agency's Information Centre.
To obtain a copy, please contact the Enquiry Desk, Information Services, Food Standards Agency (tel: 020 7276 8181/8182 or email: infocentre@foodstandards.gsi.gov.uk)

Contact: For any enquiries concerning this research project, please contact the relevant Programme contact or email: science@foodstandards.gsi.gov.uk