Bacteriophage therapy has received more attention recently in the West as an alternative approach to combat multidrug-resistant bacterial infections. Although there have been numerous anecdotal reports describing instances of successful phage therapy, there is still a need for controlled clinical trials to assess its effectiveness as well as for in-depth scientific studies to better understand its potential and to inform regulatory decisions. One frequently raised issue is the potential ability of bacteria to develop resistance to phage. This study characterizes Staphylococcus aureus mutants resistant to bacteriophage K.

Bacteriophage K is a virulent, polyvalent phage that lyses both coagulase-positive and -negative staphylococci. Bacteriophage K was shown to comprehensively lyse a panel of twenty-two methicillin-resistant S. aureus (MRSA) strains. Using S. aureus strain NRS384, nine genetically independent spontaneous bacteriophage K resistant mutants were isolated. Whole genome sequencing identified single-nucleotide polymorphisms (SNPs) in each of the nine strains. Interestingly, none of SNPs are predicted to be null mutations, but all resulted in amino acid substitutions in the affected gene products. These mutations provide insight into potential phage receptors, replicative pathways, and other cellular components that are crucial for phage growth. We also successfully screened a transposon library in strain JE2 for phage K-resistant phenotypes. A different spectrum of genes was identified than in the spontaneous mutants, which is not surprising, considering that this library comprises null mutations.

The results of these genetic screens lead to a number of testable hypotheses that we will pursue regarding interactions between phage K and host cells. In addition, they contradict a commonly-held belief that S. aureus does not become resistant to phage K.