Efficacy of bacteriophage therapy against mastitis-causing Staph aureus.

Staphylococcus aureus is one of the most prevalent causal agents of mastitis. Apart from management procedures to prevent the disease, antibiotic therapy is still the most widely used procedure to control infection. However, resistance to antibiotic therapy, in particular the selection pressure it has created to favour resistant strains, is increasing and therefore alternative means of control have become imperative. One of the alternative ways is to consider natural enemies of the pathogen, such as bacteriophages which are viruses. Therefore, the focus of the study by the UKZN group referenced below was to use bacteriophages (phages) as biological control agents against S. aureus-induced mastitis.

From the initial screening of local farms in the Mooi River area, it was established that 85% of the herd animals that tested positive for mastitis, was infected by S. aureus. The S. aureus isolates sampled from the area showed resistance to penicillin, methicillin, vancomycin, amoxycillin/flucloxacillin, erythromycin, streptomycin, trimethoprim and ampicillin. In a second initiative, three phages were isolated in the laboratory (in vitro) and screened against four of the antibiotic-resistant S. aureus strains. All three phages showed the potential to control the strains.

Following the in vitro screening, the phages were applied to cows as a post-milking udder spray treatment. Approximately 74% of phage-treated cows showed a higher somatic cell count (SCC) compared to untreated cows, 87% of phage-treated cows showed a lower bacterial cell count and 96% of the untreated cows showed a higher bacterial cell count compared to treated cows. These results indicate the presence of internal (not introduced) phages which were able to inhibit and kill the targeted bacteria. Approximately 73% of untreated cows showed a lower phage count compared to the phage-treated cows. These results indicate that the phages were able to multiply and disperse to all mastitis infected areas within the treated cows.

To test the presence of S. aureus in the collected milk samples, a total of 272 milk samples were analysed and 70% tested positive for S. aureus. A total of 189 potential isolates were sub-cultured and further analysed of which 107 isolates were positively identified as being S. aureus. The phage combination was tested against each of the 107 isolates, to assess phage activity against the newly isolated S. aureus strains. The phages showed that they could control these strains.

In conclusion, the authors were able to show that phages imposed control over particular isolates of S. aureus. However, there are still many uncertainties and factors that need to be considered before phages can be applied on a large-scale in commercial dairy herds. In fact, this is the first report of a mastitis control programme funded by Milk SA and which will continue towards establishing a practical product for use on the farm.

Reference:

M. Ndlela, M.D. Laing, I.H. Basdew, 2016. Biological control of Staphylococcus aureus-induced bovine mastitis in dairy cows using bacteriophages. Proc. 6^th IDF Mastitis Conf., Nantes, France; 7-9 Sept. 2016.