Synergistic Effect of Bacteriophage vB_KpnS_Pey and Antibiotic Combinations against Clinical Carbapenem-Resistant Klebsiella pneumoniae Isolates.

Abstract

Background: The global rise of antimicrobial resistance, particularly among carbapenem-resistant Klebsiella pneumoniae (CRKP), presents a serious therapeutic challenge. This study aimed to isolate and characterize a bacteriophage effective against prophage-free CRKP and to evaluate its potential synergistic effects with ciprofloxacin, meropenem, gentamicin, and colistin.

Materials and methods: A total of 30 non-duplicate clinical bla_NDM-positive CRKP isolates were collected from hospitalized patients in Kerman, Iran. Prophage-free isolates were identified using mitomycin C induction assays. A bacteriophage was isolated from hospital wastewater using enrichment and double-layer agar methods. Phage morphology was identified by transmission electron microscopy (TEM). Host range and efficiency of plating (EOP) were determined using spot assays. The optimal multiplicity of infection (MOI) was assessed by optical density at 600 nm (OD₆₀₀), and phage replication dynamics, including latent period and burst size, were evaluated using a one-step growth curve. Antibiotic susceptibility profiles and minimum inhibitory concentrations (MICs) for ciprofloxacin, meropenem, gentamicin, and colistin were determined by disk diffusion and broth microdilution. Phage-time-kill experiments were used to evaluate antibiotic interactions.

Results: TEM revealed that phage vB_KpnS_Pey is a member of the class Caudoviricetes, with an icosahedral head and a long, non-contractile tail. This phage produced clear, halo-form plaques and exhibited lytic activity against 13.3% of CRKP isolates. It showed high EOP against most susceptible strains, a latent period of approximately 30 minutes, and a burst size of 315 plaque-forming units (PFU) per infected cell. The optimal MOI was determined to be 1,000 PFU/colony-forming unit. Combination assays demonstrated phage-antibiotic synergy between vB_KpnS_Pey and subinhibitory concentrations of ciprofloxacin and meropenem, resulting in enhanced bacterial clearance in time-kill experiments. No synergistic effect was observed with gentamicin and colistin.

Conclusion: These findings highlight the therapeutic potential of phage vB_KpnS_Pey, particularly when combined with ciprofloxacin and meropenem, as an alternative or combined strategy for treating infections caused by CRKP. Further research is required to fully understand the bacteriophage's mechanisms of action and its associated limitations.