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Microorganisms are ubiquitous in the environment. Microorganism can be found in soil, water, plants, decaying organic matter, exudates of animals or within contaminated treatment preparations on any dairy farm. These microorganisms can be isolated from an infected udder, respiratory, reproductive, and digestive tracts. The potential always exists for these microorganisms to gain access to the mammary gland during favorable conditions. When these isolates are present, it is possible that the samples were not collected in an aseptic technique. Care should be taken when deciding if these isolates were the true cause of an infection or if they are indicative of contamination. Therefore, it is important to evaluate the bulk tank milk (BTM) culture results using all available information.

Culturing bulk tank milk (BTM) to monitor milk quality has limitations based on the amount and frequency of sampling and the amount and types of microorganisms isolated. Samples taken over days or weeks are most helpful to understanding a problem. BTM culturing can provide information about the presence or absence of a bacterial group and the identity of predominate bacterial group(s). BTM results are most useful when examined along with other records such as somatic cell counts (SCC), clinical mastitis incidences, environmental conditions and other information. BTM cultures are not a substitute for quarter milk samples. Additionally, milk samples must be collected aseptically and immediately stored on ice in order to provide valid information upon culture.