Optimizing phage therapy for carbapenem-resistant <i>Enterobacter cloacae</i> bacteremia: insights into dose and timing

The increase in multidrug-resistant (MDR) <i>Enterobacter cloacae</i> complex (ECC) infections, particularly those resistant to carbapenems, underscores the urgent need for alternative therapies. Phage therapy, with its specific bactericidal action, offers a promising solution. However, there remains a shortage of well-characterized ECC-targeting phages, and dosing and timing optimization for ECC-specific phage cocktails is largely unexplored. In this study, we isolated and characterized three novel lytic phages with diverse genome sizes and host ranges. Notably, ФEBU8 demonstrated broad-spectrum activity, lysing both <i>Enterobacter</i> species and <i>Acinetobacter baumannii</i>. ФECL22 displayed stability across a wide temperature range (4-50°C), pH tolerance (6-10), and a burst size of 19 PFU/cell, with OmpA identified as its receptor. Our formulated phage cocktail, comprising ФEBU8, ФECL22, and ФECL30, effectively rescued mice with <i>E. cloacae</i> bacteremia in a dose-dependent manner, with a mid-dose regimen showing particularly strong efficacy. Immediate phage administration achieved full survival, whereas a combined prophylactic and therapeutic regimen ("-24 + 6") also resulted in 100% survival. These findings highlight the critical roles of dosing and timing in optimizing phage therapy for carbapenem-resistant <i>Enterobacter</i> infections, with prophylactic use providing a valuable window for delayed treatment and a promising strategy for combating severe bacterial infections.