Phage susceptibility of ten clinical and two laboratory S. aureus strains were determined in three different assays; the spot test, optical density assay and microcalorimetry.. The efficacy of on three commercially available phage cocktails against clinical Staphylococcus aureus strains

Phage susceptibility of twelve Staphylococcus aureus strains to three commercial phage cocktails and two single phages was assessed. These include ten clinical isolates, including five methicillin-resistant strains, belonging to varying clonal complexes thereby representing the vast variety in genetic backgrounds of S. aureus. In total three assays to test phage susceptibility were compared; the standard spot test, the optical density assay in bacterial culture media and microcalorimetry in human serum, of which the latter most closely mimics in vivo conditions. In both the spot test and the optical density assay all three cocktails were able to lyse several clinical strains. Microcalorimetry allowed the evaluation of phage efficacy in high concentrations of human serum, which completely abolished phage efficacy. Discrepancies between these assays, especially the lack of phage efficacy in human serum, emphasizes the need for assays mimicking physiological conditions to prevent overestimation phage susceptibility during phage selection for phage therapy.