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. Evaluation of the potential of bacteriophages (phages) is the topic of one of the studies in the mastitis-control R & D programme of Milk SA. This report by the authors referenced below provides an update.
The study investigated the prevalence of phages and S. aureus within the South African dairy environment, as well as infectivity of phage isolates against antibiotic-resistant S. aureus. The four selected S. aureus strains used in the study displayed resistance to representative antibiotics from the β-lactamases and non-β-lactamases, macrolides, aminoglycosides and glycopeptides. Susceptibility was only noted towards the tetracycline antibiotics.
Twenty-eight phages were isolated and screened against four strains of S. aureus. Only six phages showed superior bio-control potential based on their wide host range, high titres and common growth requirements. These six phages were able to reduce live bacterial cell counts of the pathogen by between 64% and 95%. In addition, they showed further infectivity towards S. aureus strains that were isolated from different milk-producing regions during on-farm surveys.
Morphological and preliminary genomic analysis was also carried out on the three best performing phages. Transmission electron microscopy showed that all three phages appeared structurally similar. Each possessed an icosahedral head, separated from a striated, contractile tail region by a constricted neck region. Rigid tail fibres were also present below the striated tail. Molecular analysis of the phages revealed that the genomes of phages SaPh1, SaPh2 and SaPh3 were all different from each other. Phages SaPh1 and SaPh3 showed sequence homology to a particular form of Pseudomonas phages, called “giant” phages. Phage SaPh3 showed sequence homology to a Clostridium perfringens phage. Major phage functional proteins (tail tape measure protein, virion structural proteins, head morphogenesis proteins, capsid proteins) were identified in all three phages. The level of sequence similarity between the screened phages and those classified in previous studies enabled preliminary classification of the phages into the order Caudovirales, family Myoviridae.
The isolated phages in terms of their effectivity are now in the process of being investigated in vivo.
I.H. Basdew & M.D. Laing. 2016. Investigations into the lytic ability of South African bacteriophages specific for Staphylococcus aureus, associated with bovine mastitis. In: Proc. of the IDF Mastitis Conf., Sept 2016, Nantes.