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

TL;DR

This study explores how adding a third phage can overcome bacterial resistance in MRSA, particularly when the bacteria have the abiF gene.

Contribution

The study identifies that the abiF gene may mediate resistance to certain phages and shows that Silviavirus-like phages can bypass this resistance.

Findings

Adding a third phage significantly enhanced bacterial killing in abiF-positive MRSA isolates.

abiF-negative isolates showed similar responses to two- and three-phage cocktails.

Phage cocktails with diverse infection mechanisms may improve treatment of multidrug-resistant S. aureus.

Abstract

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

Two versus three-phage combinations stratified by abiF and pdpSau genotypes

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.

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.

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.

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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