In developing individual farm mastitis control and treatment strategies, it is often necessary to characterize the types of bacteria that are present on your farm. To answer this question, a microbiological analysis, or milk culture, must be performed on milk samples collected from cows showing clinical or subclinical signs of mastitis. Results of the milk cultures will help identify which bacteria are causing the mastitis. In turn, this information can be used to alter mastitis control, prevention, and treatment options to fit your herd’s conditions. During an investigation of a herd dealing with high somatic cell counts or a high incidence of clinical mastitis, milk culture results provide essential evidence for solving the problem.

When managing a contagious mastitis problem (Staphylococcus aureus or Streptococcus agalactiae), milk cultures are even more important to help make individual cow treatment and culling decisions.

Extra care and precaution are necessary during the collection process, using strict, clean, aseptic (without germs and bacteria) procedures to be sure that the bacteria originated from milk from the udder and not the teat end or hair, the sampler’s hands, or the barn environment.

If the samples are not collected, handled, and transported correctly, the bacteriological results will be of no diagnostic value.

Necessary supplies
Sterile glass or single-use disposable plastic vials with tight-fitting push-on or screw caps should be used to collect milk samples. Plastic sample bags are not an acceptable collection device.
To reduce potential contamination from the sampler’s hands, nitrile or latex gloves should be worn by the person collecting the samples.


Alcohol-soaked cotton balls or gauze pads should be used to sanitize the teat ends. If a large number of samples are being collected, a rack may be needed to handle sample vials.
A small cooler or ice chest may also prove useful to immediately chill the samples after collection.

Milk sample collection
Milk samples must be collected before the cow is treated with antibiotics. Samples for culture should be collected immediately before milking. When individual quarters show clinical signs of mastitis or positive California Mastitis Test results, individual samples should be collected from the affected quarters.

If the entire herd is being sampled, composite samples (all four quarters in one collection vial) will provide reasonable results. To minimize contamination and maximize the chances of receiving useful information from the milk culturing process, adhere to guidelines for the aseptic collection of clean milk samples. PD

References omitted due to space but are available upon request to

Excerpts from University of Kentucky College of Agriculture, Cooperative Extension Service ID-180

Jeffrey Bewley