In vitro and In vivo evaluation of phage-antibiotic synergy for the treatment of urinary tract infections.

The therapeutic challenges caused by multidrug-resistant (MDR) and extensively drug-resistant (XDR) bacteria have necessitated the development of alternative treatment strategies. Phage-antibiotic synergy (PAS) has recently emerged to replace or possibly supplement antibiotics. We isolated the lytic phage NTNC80A from hospital sewage in Chandigarh, India, and it belongs to the class Caudoviricetes of viruses. We tested multiple strategies to investigate the synergistic effects of phage and antibiotic combinations against highly drug-resistant uropathogenic Escherichia coli isolates. The maximum synergistic effects (ΣFIC ≤0.5) were observed when phage NTNC80A was combined with ciprofloxacin simultaneously. Cell filamentation, characterised by elongated and interconnected cells, was observed at the sub-inhibitory ciprofloxacin doses (1/4,1/10 of MIC). Ciprofloxacin could accelerate cell lysis by stimulating filamentation, disrupting the peptidoglycan layer and enhancing susceptibility to phage enzymes. Phenotypic changes in phage and antibiotic-resistant mutants indicate that bacteria become susceptible to antibiotics or phages when their growth rate, metabolism, or resistance gene expression is altered. The simultaneous strategy outperformed the sequential strategy by enhancing synergistic effects and minimising the risk of resistance development. A significant reduction (p < 0.01-0.001) in bacterial loads in urine, bladder, and kidneys was observed in mice treated with a combination of phage (106 PFU) and sub-inhibitory concentrations of ciprofloxacin. The optimized phage-antibiotic combination reduced tissue bacterial burden and enhanced animal survival rate, indicating improved PAS therapeutic effects. These findings have significant implications for the future of UTI treatment strategies, particularly in the context of growing antibiotic resistance.