Phage–meropenem synergy against multidrug-resistant Klebsiella pneumoniae biofilms: phenotypic and genomic characterisation of vB_KpnS_Tech

The increasing prevalence of multidrug-resistant (MDR) Klebsiella pneumoniae and its capacity to form persistent biofilms highlight the need for alternative antimicrobial strategies. This study characterised the lytic bacteriophage vB_KpnS_Tech (PQ859379.1) and evaluated its activity in combination with meropenem against established MDR K. pneumoniae biofilms of different maturities. vB_KpnS_Tech exhibited rapid adsorption, with 98.8% of phage particles adsorbed within 7.5 min, a latent period of approximately 30 min and a mean burst size of approximately 165 PFU per infected cell. The 49,562-bp genome had a GC content of 51.1% and contained 86 predicted coding sequences, including putative holin, endolysin and Rz-like spanin-associated lysis proteins. No recognised antimicrobial-resistance, virulence or lysogeny-associated determinants were detected by the applied genomic screens, and comparative analyses placed vB_KpnS_Tech within the family Drexlerviridae and genus Webervirus, within an existing species-level genomic cluster. Phage-meropenem checkerboard assays demonstrated a clear effect of biofilm maturity on treatment response. Maximum relative biomass reduction reached 66.0% in 24 h biofilms, while the selected combination of 1 × 10⁹ PFU/ml phage and 256 µg/ml meropenem produced reductions of 62.2% and 42.7% in 24 h and 48 h biofilms, respectively. Independent Bliss analysis of biological replicate checkerboards demonstrated synergy in 24 h biofilms, with a mean global score of 10.53 ± 0.53, whereas the interaction in 48 h biofilms was attenuated and remained within the additive/independent range (3.37 ± 0.88). The selected combination reduced recoverable viable bacterial burden by 2.60 and 1.70 log₁₀ CFU/ml relative to untreated endpoints in 24 h and 48 h biofilms, respectively, and reduced viable counts below corresponding pre-treatment baselines. Combination treatment was also associated with the greatest reductions in recoverable EPS-associated carbohydrate, extracellular protein and extracellular nucleic-acid material and retained activity against five additional phage-susceptible MDR K.pneumoniae isolates. Collectively, these findings demonstrate a concentration- and maturity-dependent phage-meropenem interaction and support further evaluation of vB_KpnS_Tech-meropenem combinations against established MDR K.pneumoniae biofilms.