MRSA CC398 is wide spread among livestock and humans in contact with food-producing animals are at high risk for colonization and infection. Natural biocontrol of MRSA in livestock could be achieved using virulent phages. However, it is challenging to compose preparations consisting of a diverse and safe spectrum of phages that complement each other in terms of efficacy and host range and avoid bacterial phage-resistance development. In the JPIAMR project 'Phage-Ex', virulent phages are being tested for their suitability for S. aureus biocontrol.

Lytic phage activities were determined using spot assays on overlay agar supplemented with 20 representative S. aureus isolates from German pig farms. The suitability and safety of the recovered phages were evaluated using in vitro biocontrol assays and genomic analysis. The suitability of different S. aureus strains for phage propagation was tested to avoid cross-contamination of the preparations with spontaneously-induced prophages.

Although the sample material originated from a range of German pig farms and slaughterhouses, predominantly closely related podoviruses of the PSa-type were identified. These were highly efficient with similar host ranges covering ~70-75% of the 60 methicillin-resistance livestock MRSA isolates tested. Whole-genome sequencing and bioinformatics analysis show that the recovered phages are free of genes causing undesired effects. They are considered safe for application and S. aureus biocontrol. However, the selection of host bacteria needs further attention, as the propagation of high-titer phage lysates partially leads to the release of temperate phages encoding staphylococcal enterotoxins (SE).

Safe polyvalent S. aureus phages are ubiquitous in animal-associated sample materials, but are represented by a narrow spectrum of types. This makes it difficult to prepare compositions with broad bacterial receptor specificities to avoid the development of phage resistance in bacteria. Stress-induced prophage induction during propagation can lead to cross-contamination of the composition with temperate phages carrying SEs. This increases the likelihood of transduction of DNA from the bacterial host.