LANCASTER, Pa. — Modern dairy farmers are not just farmers. They are to some extent nutritionists, veterinarians, even microbiologists.
A group of herd managers and dairy farmworkers met at the Farm and Home Center on Feb. 21 to improve their skills in that last role.
Amid stacks of Petri dishes, milk vials and sampling loops, Penn State Extension educators Amber Yutzy and Greg Strait taught the farmers how to grow bacterial cultures that can be used to identify the cause of mastitis, the most costly disease in the dairy industry.
Several types of bacteria can cause mastitis, the inflammation of the udder that leads to an astronomical somatic cell count and a displeased co-op.
Over half of the monetary losses from mastitis come from reduced production through the rest of a cow’s lactation, Yutzy said. “That’s the hidden money that you don’t know you’re losing.”
Mastitis is rarely the first reason for culling a cow, but it can be a factor contributing to major culling reasons like breeding difficulty, Yutzy said.
On-farm culturing helps identify the culprit, tailor a solution and save the farm vet’s time, she said.
Culturing is not necessary if a cow’s milk is orange or clear, Yutzy said. Treatment is obviously necessary in those cases.
A 2011 study presented in the team’s slides showed that on-farm culturing reduced antibiotic use by half and cut milk withholding time by one day.
By itself, “this culturing will not lower your somatic cell count,” Strait said. Instead, it gives more certainty about the treatment.
“You’ll have to make a change” through treatment or management to get the most out of the culture results, Strait said.
The California Mastitis Test, or CMT, is one of the most common tools to figure out which quarter or quarters of a cow’s udder are infected. The test uses a paddle with one container for each teat.
This simple test helps determine which quarter or quarters of the udder have mastitis, Strait said.
For that reason, a CMT should be used before every culturing, Yutzy said. The Penn State culturing method uses one Petri dish per teat, so there is no need to use Petri dishes for uninfected teats.
Still, the CMT will not indicate what type of bacteria is present, she said.
To find that out, a farmer needs to culture milk in a Petri dish with a specialized agar substrate.
The milk for that test should be collected in a special tube in an aseptic situation. Both sampling and culturing require extreme cleanliness to keep unrelated bacteria — from hands, coughing and even drafts — from contaminating the milk or the plate, Yutzy said.
The milk can then be applied to the Petri dish with either a single-use rod or a reusable metal loop, Yutzy said.
The metal tool should be heated until red hot and allowed to cool before applying the milk to the agar. A zigzag pattern is commonly used, though it does not really matter, she said.
It is important to only cover part of the agar surface so that the bacteria, which in many cases only grow where they are spread, can be distinguished from the agar, Yutzy said.
“There’s no need to press,” and there should be no ponding of milk, she said. The milk streak should be at most barely visible.
The plate then goes into an incubator at 98.6 degrees F for 24 hours. Plates should always be stored with the agar side on top. That prevents condensation from dripping onto the agar, causing mold and the wrong kind of bacteria to grow, Yutzy said.
Incubators, even expensive ones, are sensitive to the environment and should be placed out of the way of drafts and intense light, Yutzy said.
The milk sample tube and the Petri dish should both be labeled with the cow number, quarter, and date, Yutzy said.
A Penn State quad plate is a Petri dish split into four sections. The farmer should swab each compartment because each grows a different type of bacteria.
There will never be two bacteria in the same quarter of an udder, though different quarters on the same cows could have different infections, Yutzy said.
Blood agar, the red top left sector of the plate, grows almost any kind of bacteria and is used to check against the other quadrants. Bacteria should grow in the blood agar and only one of the other three compartments, she said.
If bacteria are growing in every quadrant of a plate, “it’s junk,” she said.
In the top right corner of the Penn State plate is MacConkey’s agar, a pale orange color. It is used to find gram-negative bacteria, which appear pink when dyed.
Bright pink colonies indicate coliform bacteria. Among these, E. coli is recognizable for its intensely pink colonies that stick closely to the milk swab lines, Yutzy said.
Klebsiella colonies are pink too, but they are larger and snotty-looking, Yutzy said.
Coliforms look snotty on the blood agar, she said.
Pseudomonas and Proteus grow as off-white colonies in the MacConkey’s quadrant, she said.
The bottom right quadrant of the plate contains a dark purplish agar used for identifying Streptococcus bacteria. The colonies will appear black, Yutzy said.
If there is no zone of clearing around the bacteria, the plate has Strep uberis. If the bacteria have cleared some of the agar away, they are considered esculin negative, a group that includes Strep agalactiae and Strep dysgalactiae, she said.
This clearing will also be apparent in the blood agar part of the plate.
The pale white or yellow quarter in the bottom left of the plate, made of Baird Parker agar, cultures Staphylococcus bacteria.
“You don’t want anything to grow here ever, ever, ever,” Yutzy said.
That’s because this is the quadrant where Staph aureus, the cause of contagious mastitis, shows up. Most other mastitis bacteria are environmental, she said.
“You can cure staph if you catch it early,” but it has a hard cell wall like Klebsiella that makes it more difficult to treat, she said.
Often, Staph aureus will go into remission and reappear later, she said.
Staph aureus shows up as black colonies with clearing around them on the Baird Parker section. On the blood plate, the colonies will be large and whitish with a clearing around them.
Farmers who see these colonies should send a sample to a lab for verification, Yutzy said.
“The blood clearing’s really easy to tell,” as the agar should be almost see-through, she said.
After use, quad plates should be flooded with bleach, sealed with a plastic bag, and discarded where children and animals cannot access them, Yutzy said.
Even if the plates have no growth, they are still designed to propagate bacteria — not a good thing to be putting into the environment, she said.
Farmers should work with their vets to develop a protocol for handling the different types of bacteria based on the likelihood of curing the mastitis they cause, she said.
Yutzy and Strait will hold three more milk culturing workshops around the state through the middle of April.