Raw milk collected from dairy farms has to travel to processing facilities before it is transformed into the delicious dairy products we consume. Because the milk from many dairy cows is collected together, any contamination issue from a single animal can lead to spoilage or contamination. How related are the microbiota of raw milk and the cows from which they come? A new study in Applied and Environmental Microbiology addresses this question using high- throughput DNA sequencing technologies to compare the milk production environment and the raw milk microbiota.
Determining raw milk microbiota can help prevent spoilage and foodborne illness Source
Many factors, including animal caretaker hygiene and microbiome, herd health, and herd environment, can influence the composition of milk microbiota. Previous studies using microbial culturing techniques had shown that milk contamination was strongly influenced by herd environment – particularly the barns used to house the herd. Culture techniques may miss some microbial members, though, and so first author Conor Doyle, working with senior scientist Paul Cotter, investigated the microbiome using 16S rRNA gene sequencing.
The authors used the fact that the Irish dairy farm uses pastures to graze animals for most of the year, but keeps herds indoors during the winter, to test their hypothesis that these housing differences may influence the microbes the dairy cows are exposed to, and in turn, their milk. They took environmental samples from feces, animal bedding, grass, and surface soil, as well as teat swab samples, which were collected after teat preparation. DNA was prepared from these samples, and compared to samples prepared from milk collected from cows kept indoors or outdoors.
As expected, the biggest difference in raw milk microbiota composition was observed between indoor- and outdoor-kept cows. The differences in temperature, diet, and proximity to neighboring animals may all play a role in the observed communities: milk from the indoor cows was more likely to have higher proportions of gut-associated bacteria, while milk from outdoor cows was more likely to have higher proportions of environmental bacteria. This confirmed that the largest influence in raw milk microbiota composition was the cow housing method.
What about non-environmental influences? Cows undergo teat prep before milk collection, and this prep can influence microbial transfer from teat to milk. The scientific team tested this by collecting milk from treated and untreated teats in both indoor- and outdoor-housed animals. They were surprised to find that teat prep increased the amount of bacteria in milk from cows grazing outdoors and led to greater diversity in microbial composition. The authors suggest that the washing and drying process may loosen attached commensal bacteria, facilitating their movement into milk when it is collected.
Surprisingly, teat prep increased bacterial counts in raw milk Source
Most studies of raw milk focus only on the bulk tank milk, which is a mixture of all the milk from the herd. Here, the scientists looked at three individual cows as well as the bulk tank milk, and saw a difference in microbial composition between the individual milk and tank samples. The authors speculate this may be due to the milking machines and/or pipes used in collection, and highlight this as another important factor for future studies.
What does it mean that the environment in which a dairy herd is maintained is the primary factor in milk microbiota composition? This blog has previously discussed how seasonality affects raw milk microbiome, and now we know that where a cow lives is integral to this microbiome. Because of the industrial nature of our agricultural system, tracking sources of potential contamination plays a vital role in maintaining healthy, fresh foods. This new study highlights how environmental bacteria can enter the food system and demonstrates a valuable application of sequencing technologies to track that microbial movement.