MSU Extension

Dairy producers who do their own milk culturing say that it is an essential part of their mastitis control program, providing them with information not only about the individual cow, but also about what is happening in the herd. They are able to target treatment more effectively and reduce unneeded antibiotic use significantly.

Many other producers are interested in knowing what is involved in on-farm milk culturing. You can listen to an MSU Extension podcast of an interview with a dairy producer who does her own milk cultures.

If you make the decision to do on-farm milk culturing, there needs to be one person dedicated to this task who is willing to consistently follow protocol. That person must have a specific area that is clean and have access to the supplies they need. It is be best for that individual to make the decisions about treatment and be responsible for mastitis control on the farm, too. Subsequent training, time with the veterinarian and having authority in the parlor will all help ensure success beyond the short-term.

The steps of on-farm milk culturing are:

1. Identify animals to culture
2. Take a good sample
3. Plate it
4. Incubate the plate
5. Interpret the results
6. Make a decision based on results

Identify animals to culture. Certainly cows with clinical mastitis should be cultured. In addition, some producers will culture cows that show up on a report as new cases of high somatic cell count (SCC) cows, using a California Mastitis Test (CMT) paddle to identify the problem quarter. The results for every cow should be recorded along with her unique ID and date.

Take a good sample. You want to culture the microbes in the milk and not the microbes on the teat skin or your skin. Teat preparation and sampling technique are, therefore, extremely important. Wear gloves. Clean the teat as you would in preparing for milking, then use an alcohol pad to disinfect the teat end. Hold the teat at a 45 degree angle and squirt it into a sterile collection tube at an angle off to the side so that anything that falls from the teat does not enter the tube. Cap the tube, being careful not to touch the portion that will be inside the tube. Remember, 99.9% of contaminated samples result from mistakes during collection!  

Plate the milk sample. As soon after collection as possible, get the process started. Use a sterile loop to streak milk on each section of the plate, being very careful throughout the process not to contaminate the loop or the milk through contact with anything but the plate. Some producers use simple bi-plates that will distinguish Gram-positive from Gram-negative bacteria. Others use tri- or quad-plates enabling them to be more specific in their diagnosis. Whatever you choose, attention to detail and continued learning are important.  If you can’t plate out the sample within an hour, refrigerate. 

Incubate the plate. Most bacteria in the milk will grow in the incubator which is held at 37 °C (98.6° F). Culture results can be read in 24 hours. Often times, Gram-negative growth may be evident in 6-8 hours. If no growth, wait another 24 hours (48 total) to be sure that slow-growing bacteria have a chance to appear.

Interpret the results. Generally, only one type of microbe will be in an udder quarter. If a milk culture has more than one type of bacteria growing on the plate, it is likely a contaminated sample. You can get pictures that will show what the growth of various organisms looks like, however, we encourage you to work with your veterinarian and the milk quality professionals of your cooperative or company. The basic distinctions are simply growth or no growth and if there is growth, then are they bacteria that are Gram-positive or Gram-negative which you will know by the side of the plate on which they grow.

Make a decision based on results. Gram-negative and no-growth quarters will be left untreated because either the bacteria that caused the mastitis is dead already or it will not respond to most antibiotic therapy and is self-limiting. Quarters that have Gram-positive growth are microbes that are more likely to respond to antibiotics. However, if the sample yields Gram-negative organisms from a severe case of clinical mastitis antibiotic therapy may well be beneficial, and this protocol should be developed with your veterinarian.

Culture results should be recorded and those records should be retrievable so that you can pull a history on an individual and that you can tell what bugs are causing mastitis in your herd. After you read the plate, you may want to tape it shut and refrigerate it to have someone else look at it and interpret the results. However, if you are done with the plate, it is a good idea to flood it with bleach for 30 minutes to decontaminate it and then dispose of it.

The supplies and incubator are available from a variety of supply places. The cost of start-up and continuing is fairly low. It is best not to stock up on more plates than you will use in a month because they may become contaminated or dry out in the refrigerator.