If you have cultured the milk of cows with mastitis you have probably found what are called “Coagulase-Negative Staphs.” or CNS. In fact, these bugs are becoming the most prevalent pathogen identified for mastitis but may be one of the least understood. However, some new research is opening the door to a better understanding of CNS.
Let’s talk about how it gets its name. In the process of identifying the bacteria that cause mastitis or any disease, various procedures are used to separate out different types of bacteria based in their response to lab procedures.
Whether culturing on-farm or in a lab, an investigator looks at the media on which growth occurs. On a bi-plate containing blood agar and MacConkey agar, a milk sample containing only CNS bacteria will yield growth on blood agar only. That narrows the possibilities of bacteria mainly to streptococcus and staphylococcus. Hydrogen peroxide is used on a glass slide to see if a colony bubbles. A bubbling reaction is called catalase-positive and indicates a staphylococcus species, whereas non-bubbling, or catalase-negative, indicates streptococcus bacteria.
To further differentiate a catalase-positive colony, colonies are placed in a test tube and rabbit serum is added. If the serum thickens to a gel (that is, it coagulates), the bacteria is called coagulase-positive and is most likely Staphylococcus aureus. Otherwise, the bacteria are labeled as a Coagulase-negative Staph. or CNS.
But CNS is not an individual type of bacteria – it is really a group of species of bacteria, and these do not all behave the same in the udder. A study by Fry published in the August 2014 edition of the Journal of Dairy Science showed 20 different CNS species bacteria could be identified in milk samples from 555 cows that had subclinical mastitis on 89 farms in Canada. Cows were sampled during late lactation, dry period, at calving, in the early post-partum period and during lactation. They used gene sequencing to determine the specific bacteria.
Some species occurred more commonly than others in the milk samples. Of the 20 different species differentiated, three species accounted for 77 percent of the CNS identified. Only nine species where identified in persistently-infected quarters, some for a period of greater than 200 days. The other 11 species appeared to be transient infections, or were cured. Some species of CNS were associated with significantly higher somatic cell count (SCC) than uninfected control mammary quarters. However, some milk samples from which CNS were isolated did not have a higher SCC than their uninfected quarters.