Phage resistance profiles of AB-phages treated Acinetobacter baumannii: implications for phage therapy strategies

Phage therapy is a promising alternative for combating multidrug-resistant bacteria, including Acinetobacter baumannii (AB). However, the development of phage-resistant variants after treatment, particularly when using phage cocktails, poses a significant challenge. This resistance can hinder the effectiveness of future phage-based treatments against pathogenic AB.

Three AB-specific phages-AB-phage 22, AB-phage 27, and AB-phage 32-susceptible to an AB isolate, designated ABU-3, were used as a model to study phage resistance development in AB following phage treatment. This study proposes a strategy to effectively eliminate pathogenic AB using an optimal multiplicity of infection (MOI), referred to as the MOI clearance value.

The MOI clearance values required for complete elimination of ABU-3 were determined to be 10 for AB-phages 22 and 32 and 100 for AB-phage 27. Surviving ABU-3 colonies from lower MOI treatments were analyzed for phage resistance. ABU-3 treated with AB-phage 27 developed resistance to AB-phage 27 but remained susceptible to AB-phages 22 and 32. ABU-3 treated with AB-phage 22 developed resistance to AB-phage 22 but retained partial susceptibility to the other phages at reduced MOI. In contrast, ABU-3 treated with AB-phage 32 displayed complete resistance to all three phages.

These findings highlight a key challenge in phage therapy: insufficient MOI ratio can promote phage resistance. The distinct resistance profiles observed emphasize the importance of optimizing phage combinations and dosages to prevent resistance development during treatment.