P-1175. Discrepancies Between Bacteriophage Plaque Assays and Time-Kill Analyses in Daptomycin Non-Susceptible Methicillin-Resistant Staphylococcus aureus

Abstract Background Bacteriophage (phage) therapy is a promising adjunct to antibiotics in treating methicillin-resistant Staphylococcus aureus (MRSA), including daptomycin non-susceptible (DNS) strains. Phage activity against specific bacterial strains is most frequently determined using the double agar overlay plaque-forming assay (plaque assay). Although this assay is often utilized in laboratory settings for determining phage activity, the conditions under which it is performed may not correspond to those of other in vitro assays or models and thus may compromise its ability to reliably predict phage efficacy in these applications. In this study, we describe discrepancies observed between plaque assays and time-kill analyses (TKAs) across multiple phages and DNS MRSA strains. Activity of bacteriophages Intesti-13 (Kayvirus), Sb-1 (Kayvirus), and Romulus (Silviavirus) as determined by the double agar overlay plaque-forming assay against various daptomycin non-susceptible methicillin-resistant Staphylococcus aureus strains. Activity was classified as susceptible (S, >107 PFU/mL), intermediate (I, 103-107 PFU/mL), or resistant (R,