Discipline: mastitis; Keywords: antibiotic susceptibility test, Staphylococus aureus, oxacillin, zoonosis, milk testing, cytology, bacteriophages, selenium.

Antimicrobial resistance (AMR): The results of an antibiotic susceptibility testing trial of Staph. aureus isolates showed that 75-80% isolates were multidrug resistant. The isolates were found to be 100% resistant to amoxicillin, ampicillin and penicillin G, 95% resistant to cephalexin, 82% resistant to streptomycin, 75% resistant to oxacillin, 64% resistant to erythromycin, 50% resistant to tobramycin, 46% resistant to tetracycline and 39% resistant to vancomycin. None of the Staph. aureus isolates showed complete resistance to all antibiotics tested.

In a provincial trial, it was also shown that antibiotic resistance trends over time differed between maltose-negative and maltose-positive Staph. aureus. Resistance of maltose-negative Staph. aureus indicated no significant differences amongst provinces, with limited differences amongst seasons for ampicillin, penicillin G and cephalexin. Furthermore, the results indicated more resistance for maltose-negative than for the maltose-positive S. aureus isolates to most of the antibiotics used. As a concern, the findings also showed resistance of the maltose-negative strain isolated from milk samples to antibiotics that are only used in human medicine, which implies a possible transfer from humans to animals and requires further studies. The fact that there are differences in antibiotic resistance profiles between the maltose-positive and maltose-negative S. aureus is useful information for both producers and veterinarians. This should provide different management and treatment protocols for the maltose-negative S. aureus which seems not to be a chronic intra-mammary infection, therefore preventing unnecessary culling.

As a follow-up to the zoonotic argument above, several CNS/NAS strains from mastitis cases and from humans that were in contact with the infected dairy cows were isolated. The β-lactam encoding resistance gene was the most common resistance determinant detected and was identified in 29% of the bovine and 75% of the human Staph. aureus isolates respectively. Human Staph. aureus isolates from three sampling sites clustered together with bovine isolates recovered from milk samples at the sites. The three human isolates were genotypically indistinguishable from bovine Staph. aureus isolates present at the respective sites. The detection of genotypically indistinguishable Staph. aureus isolates from bovine and human hosts at three of the sampling sites is disturbing because it suggests bacterial transmission.

Treatment and prevention: As an alternative to indiscriminate antibiotic use, specific management plans have been introduced on farms involved in regular testing of milk samples. These include parlour hygiene, milker and supervisor education, routine microbiology and cytology examinations, and prudent treatment based on susceptibility testing. An overall trend of increased resistance to the nine antibiotics was seen for farms screened randomly, similar to current global trends. However, in twenty well managed herds, a trend of decreasing on-farm resistance was present.

Staphylococcus aureus is developing resistance towards antimicrobial agents faster than they are being developed. Bacteriophages, which are natural virus enemies of the pathogen, can serve as an alternative control measure for the disease. Phage therapy can provide a stable, effective and affordable system of mastitis control with little to no deleterious effect on the surrounding environment or the affected animal itself. Studies were designed to investigate the presence of Staph. aureus and its phages in a dairy environment, as well as the controlling ability of phage isolates against antibiotic-resistant strains of Staph. aureus. Twenty-eight phages were isolated and screened against four strains of Staph. aureus of which six showed potential for further testing, based on their wide host range, high titres and common growth requirements. At optimal titre the phages were able to reduce live bacterial cell counts by 64 to 95% in vitro. In addition, all six phages showed controlling activity towards another 18 Staph. aureus strains that were isolated from different milk-producing regions during a farm survey.

The phages were then applied in a pilot study to cows as a post-milking udder spray treatment. The phage suspension on the cow teats reduced the Staph. aureus cell count from 10.2 log CFU/ml to 8.8 log CFU/ml at day 16, whereas there was practically no reduction in the control cows. The results suggest that the phages were able to infect, multiply and inhibit the growth Staph. aureus. With respect to potency, the results showed that the phages were able to find their host (Staph. aureus) within one day and started to multiply immediately.

Selenium plays an important role in the immune system of animals and could therefore potentially be of value to support cows to resist udder and other infections, the benefits being less prone to mastitis pathogens and higher milk yields during the vulnerable periods. The data showed a significant negative correlation between serum selenium concentration and SCC. It was concluded that injectable Na-selenite, as selenium, can be important if supplemented at strategically physiological periods to improve immunity.


Meissner, H.H., 2021. Summary of Research Progress with Mastitis 2015 to 2021. Report to Milk SA Board, November 2021, Milk SA, Pretoria.