Microbiology of raw milk in New Zealand

The results of this study demonstrate the occurrence of the non-spore-forming pathogens, Staphylococcus aureus, Escherichia coli (total count and O157:H7), Listeria, Campylobacter and Salmonella, in New Zealand's raw milk supply. Samples of raw milk were collected monthly within five major dairying regions over one year. Each month, samples from five randomly selected farm vats in each region were collected for analysis (297 samples in total). Methods based on plate count techniques were used to enumerate S. aureus and E. coli. Enrichment methods in combination with a modified most probable number detection method were used to monitor samples for the presence of E. coli O157:H7, Listeria, Campylobacter and Salmonella. Salmonella was not detected in this study, and Campylobacter was isolated once (0.34%). E. coli was present at < 100 cfu/ml in 99% of samples and exceeded 103 cfu/ml in 0.7% of samples. E. coli O157:H7 was not detected whereas non-pathogenic E. coli O157 strains (i.e. lacking genes for stx1, stx2, eae and Hly A) were detected in 1% of samples. S. aureus was not detected (< 1 cfu/ml) in 21% of samples; levels were > 1 but < 100 cfu/ml in 60% of samples and on one occasion (0.34%) S. aureus exceeded 104 cfu/ml. L. monocytogenes was isolated from 0.68% of samples and L. innocua was present in 4% of samples. The results demonstrate that raw milk sampled from farm vats in New Zealand, as in other countries, inevitably contains recognised pathogens and, hence, control by pasteurisation or an equivalent treatment of raw milk remains paramount. Even so, the prevalence of most of these pathogens was lower than those reported in many of the studies performed in other countries.

► Raw milk samples from vats on individual farms in New Zealand contained a variety of pathogens.
► Neither Salmonella nor E. coli O157:H7 were isolated.
► Campylobacter and Listeria monocytogenes were detected at low levels.
► S. aureus and E. coli levels were usually <100 cfu/ml although higher counts did occur.