A link to the past: classical phage ISP infects the recently described Staphylococcus borealis species.

Background: Staphylococcal infections, caused by a large variety of species within the Staphylococcus genus, are a threat to human health. Although antibiotics are the current choice of treatment for these infections, bacteriophage (phage) therapy has been used with success against Staphylococcus since the dawn of phage therapy. In 2020 a new coagulase-negative species named Staphylococcus borealis was described in Norway. In this study, we focused on understanding phage infections in S. borealis.

Methods: First, we predict the presence of prophages and phage-defence mechanisms in the genomes of a collection of twelve S. borealis strains by bioinformatics. We also attempted to isolate S. borealis-infecting phages from Norwegian samples and tested the host-range of three known staphylococcal phages against a panel of fifty Norwegian staphylococcal strains.

Results: The presence of prophages and phage defence systems was verified in all tested S. borealis strains. No local Norwegian phages could be obtained in a phage isolation attempt targeted towards S. borealis. The host range analysis shows that phage ISP, originally isolated in the 1920s and still used for phage therapy to date, can infect S. capitis and the S. borealis Hus23 strain. Phage ISP was shown to limit S. borealis Hus23 growth in liquid cultures and lower the formation of biofilm by the bacterium. The efficiency of plating of phage ISP can be improved by repeated passages in the new S. borealis Hus23 host.

Conclusions: here we expand the known host range of the traditional phage ISP by showing that it also infects S. borealis and can be adapted to the new host by serial passages, showcasing the flexibility of phages as an antimicrobial strategy.