Fosfomycin at sub-minimum inhibitory concentration impairs biofilm and efflux pump activity in multidrug-resistant Klebsiella pneumoniae isolates

Background Klebsiella pneumoniae plays a critical role in hospital-acquired infections, which pose a serious threat globally. Older antibiotics such as fosfomycin are being reconsidered to combat MDR K. pneumoniae . In healthcare settings, bacteria are exposed frequently to antibiotics at sub-inhibitory concentrations (sub-MICs), which may alter their virulent characteristics. The aim of this research was to investigate the impact of fosfomycin at sub-MICs on the virulence determinants of MDR K. pneumoniae . Methods The effect of sub-MIC fosfomycin treatment on biofilm formation was evaluated by crystal violet assay and confocal laser scanning microscopy (CLSM). The cartwheel technique was employed to study efflux pump activity phenotypically. The expression of genes of biofilm ( fimH , mrkD ) and efflux pump ( acrA , acrB ) was determined using reverse transcription quantitative PCR (RT-qPCR). Scanning electron microscopy (SEM) was also utilized to visualize the morphological changes. Results The MIC of fosfomycin against K. pneumoniae MDR isolates ranged from 256 to 1024 µg/mL. The biofilm formation ability of isolates ( n = 50) was reduced significantly ( P < 0.05) following the exposure to ¼ MIC of fosfomycin. CLSM analysis revealed disruption of biofilm structure, reduced thickness and density, and increased percentages of dead cells. RT-qPCR revealed 20–60% downregulation of fimH and mrkD and downexpression of acrA and acrB . SEM analysis showed pronounced morphological changes, including a characteristic cauliflower-like deformation. Conclusion Fosfomycin at sub-inhibitory levels is able to disrupt biofilm architecture and the efflux pump activity in MDR K. pneumoniae , pointing to its possible role as an adjunct agent in combating antibiotic resistance.