Overcoming phage resistance: efficacy of sequential phage-colistin therapy against carbapenem-resistant Acinetobacter baumannii.

Carbapenem-resistant Acinetobacter baumannii (CRAB) is a major public health threat due to high mortality and limited treatment options. Bacteriophage (phage) therapy offers a promising alternative, but its long-term efficacy is challenged by the rapid emergence of phage-resistant bacterial populations. This study evaluates phage vB_AbaSt_W16 and whether sequential phage-antibiotic therapy enhances bacterial clearance and suppresses resistance. The optimal multiplicity of infection (MOI) for vB_AbaSt_W16 was determined across various CRAB strains. PAS was assessed in vitro using meropenem, colistin, tigecycline, rifampin, and ampicillin/sulbactam. An immunocompromised BALB/c mouse model was used to evaluate the therapeutic efficacy of phage monotherapy and combination therapy. Antibiotics were administered either concurrently with phage or sequentially 6 h after phage treatment. Bacterial clearance, survival rates, and the emergence of phage-resistant populations were monitored. Phage monotherapy inhibited bacterial growth at MOIs of 0.1-10, but high-dose treatment (MOI > 100) caused rapid regrowth within 12 h due to resistance emergence. Phage-antibiotic synergy (PAS) was observed only with tigecycline (FICI = 0.225), though the effect was transient. In the mouse model, phage monotherapy at MOIs of 0.1-1 achieved 100% survival, whereas MOI 10 reduced survival to 40%, correlating with increased bacterial burden and resistance emergence. Notably, sequential administration of colistin 6 h post-phage treatment improved survival to 100%, while colistin monotherapy resulted in survival below 40%. Sequential colistin treatment also effectively suppressed resistant bacterial populations, reducing them to nearly undetectable levels by day 4. High-dose phage treatment was limited by rapid resistance development. In contrast, sequential phage-antibiotic therapy, particularly when combined with colistin, significantly improved bacterial clearance, enhanced survival outcomes, and suppressed resistance emergence. These findings support sequential phage-antibiotic therapy as a strategy for CRAB infections.IMPORTANCEThe rise of carbapenem-resistant Acinetobacter baumannii (CRAB) presents a severe challenge in healthcare settings, where treatment options are increasingly limited. While bacteriophage therapy offers a novel antimicrobial approach, its effectiveness is often hindered by the rapid emergence of phage-resistant bacterial populations. This study demonstrates that sequential phage-antibiotic therapy, particularly phage followed by colistin, significantly improves survival rates and suppresses resistance emergence in a murine infection model. Unlike monotherapies, this strategy optimally combines the bacterial-killing mechanisms of phages and antibiotics, offering a clinically viable solution to combat multidrug-resistant infections. Our findings provide valuable translational insights, supporting the potential application of phage therapy in human medicine. By addressing the critical issue of phage resistance, this study advances the development of sustainable bacteriophage-antibiotic treatment regimens against CRAB and other drug-resistant pathogens.