Can phage–antibiotic combinations overcome uropathogenic Escherichia coli regrowth? evidence from in vitro and in vivo models

Phage therapy is currently gaining attention as a promising alternative for treating multi-drug resistant (MDR) bacterial infections, including urinary tract infections (UTIs). However, most studies have reported bacterial regrowth in vitro after hours of co-incubation with phage-host bacteria. In this study, we evaluated whether using a phage alone or combined with gentamicin could delay or prevent bacterial regrowth in vitro, in human urine, and in a rat model. The previously characterized lytic phage vB_Eco_ZCEC08 was combined with gentamicin to target clinical Uropathogenic Escherichia coli (UPEC) infection. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of gentamicin against the resistant EC08 clinical isolate were determined, revealing high gentamicin resistance (MIC = MBC, 500 µg/mL). Time-killing assays demonstrated that combining ½ MIC gentamicin (250 µg/mL) with the phage at different multiplicities of infection (MOIs) effectively controlled bacterial growth and prevented regrowth, even after 72 h, in both in vitro culture media and urine. Notably, the phage's growth exhibited distinct dynamics when used alone versus in combination with gentamicin in both in vitro and in vivo experiments. The combination showed higher replication rates in both urine and the rate model. For the in vivo experiments, bacterial counts showed significant reductions with both phage therapy and combination therapy compared to gentamicin monotherapy. Histopathological analysis of the tissues treated with the combination presented better tissue integrity than either monotherapy. These findings support the potential of phage-antibiotic combinations as an effective strategy against MDR-UPEC infections, highlighting the need for further studies to optimize treatment regimens for clinical applications.