What question did this study set out to answer?

The aim is to explore phage defense mechanisms in MRSA affected by abiF-mediated resistance.

January 14, 2026Open Forum Infectious Diseases0 citationsOpen Access

P-1190. Targeting Phage Defense Mechanisms: Expanding Phage Activity in the Context of Suspected abiF-Mediated Resistance against MRSA

CBCallan Bleick SHSean R. Van Helden ABAndrew D. Berti

Key Points

The aim is to explore phage defense mechanisms in MRSA affected by abiF-mediated resistance.
Evaluated daptomycin non-susceptible MRSA isolates
Conducted time-kill assays with two-phage and three-phage cocktails
Measured bacterial burden reduction as log10 CFU/mL
Used statistical analysis with two-way ANOVA
Two-phage cocktail modestly reduced bacterial burden in most isolates
Three-phage combination significantly enhanced killing in select strains
Isolate C31 with abiF showed > 4.0 log10 reduction with three-phage combination

Abstract

Abstract Background Phage resistance mechanisms represent a critical challenge in the advancement of phage therapy. Staphylococcus aureus utilizes multiple defense strategies, including adsorption inhibition via wall teichoic acid modification, restriction-modification systems, CRISPR-Cas systems, and abortive infection (Abi) pathways. The Abi system has been proposed to trigger bacterial self-destruction to prevent Kayvirus phage propagation, although its functional role in S. aureus remains poorly characterized.Two versus three-phage combinations stratified by abiF and pdpSau genotypes Methods We evaluated four well-characterized daptomycin non-susceptible MRSA (DNS-MRSA) isolates from the Cubist repository. Time-kill assays (24h) were performed using either a two-phage cocktail (Intesti13 + Sb-1) or a three-phage cocktail (Intesti13 + Sb-1 + Romulus) at a multiplicity of infection (MOI) of 1.0. Change in bacterial burden was measured as log10 CFU/mL reduction. Statistical analysis was conducted using two-way ANOVA with Tukey's post hoc test (p 0.05). Whole genome sequences were used to assess the presence of known phage defense systems. Results While the two-phage cocktail modestly reduced bacterial burden in most DNS-MRSA isolates, adding Romulus significantly enhanced killing in select strains. The most pronounced effect was in C31, which harbors the abiF gene; this isolate was resistant to the two-phage cocktail (+1.14 ± 0.04 log10 CFU/mL) but exhibited a 4.0 log10 reduction with the three-phage combination (–2.75 ± 0.29, p 0.0001), suggesting Romulus may evade abiF-mediated defense. In contrast, abiF-negative isolates (C2, C4, C6) responded similarly to both cocktails, indicating a potential K-phage resistance mechanism. Conclusion These findings suggest that abiF may mediate resistance specific to Kayirus-like phages, with Silviavirus-like phages remaining unaffected. Incorporating phages with distinct infection mechanisms may help bypass such resistance and enhance therapeutic efficacy. A diverse phage cocktail could be key to overcoming intrinsic defenses and optimizing empiric treatment for multidrug-resistant S. aureus. Disclosures Michael J. Rybak, PharmD, PhD, MPH, Abbvie: Grant/Research Support|Innoviva: Grant/Research Support|Melina: Grant/Research Support|Merck: Grant/Research Support|Shionogi: Grant/Research Support

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