Extensive screening of ten bacteriophage cocktails revealed an optimal combination with potent therapeutic activity against Acinetobacter baumannii.

Multidrug-resistant (MDR) Acinetobacter baumannii poses a major clinical challenge due to its association with high morbidity and mortality, necessitating alternative treatment strategies. Bacteriophages offer a promising solution; however, their narrow host range limits efficacy against diverse A. baumannii strains. To address this limitation, we isolated five distinct phages (vB_AbaSI_1–5) of the class Caudoviricetes, each exhibiting a narrow host range (2.2%-29.6%) against 135 clinical A. baumannii isolates. Ten phage cocktails (A-J) were formulated and tested for host range expansion and bactericidal activity. Cocktails A, D, and E demonstrated enhanced efficacy, infecting 68 of 135 isolates (50.4%) and statistically significantly outperforming individual phages (p < 0.0001). Among these, cocktail A (comprising phages vB_AbaSI_1, vB_AbaSI_2, and vB_AbaSI_3) exhibited the highest killing efficiency against extensively drug-resistant A. baumannii strain DMST43250, along with superior biofilm inhibition and degradation capabilities. Application of cocktail A completely inhibited biofilm formation within 24 h. In vivo efficacy was evaluated using a Galleria mellonella infection model, in which cocktail A improved larval survival to 85.0% on day 1 and 60.0% by day 7, exceeding other cocktails (p < 0.0001). These results highlight cocktail A as a promising candidate for phage therapy targeting A. baumannii infections and associated biofilms.