Rapid impedance-based Antimicrobial Susceptibility Testing (iFAST) of Enterobacterales in urinary tract infections

Objectives: Evidence-based antibiotic prescribing for urinary tract infections (UTIs) would increase treatment success and improve antibiotic stewardship. Current antimicrobial susceptibility tests (AST) are time-consuming. A novel phenotypic impedance-based Fast AST (iFAST) measures changes in the electrical phenotype of single bacteria in response to antibiotic exposure. Suitability of this technology for UTI causing bacteria was investigated. Methods: Fifty-eight strains of Escherichia coli and Klebsiella pneumoniae were exposed to EUCAST breakpoint concentrations of UTI antibiotics. Following a two-hour exposure, the % cell count compared to unexposed control populations were compared and susceptibility deduced. Results were compared to gold standard broth microdilution (BMD) AST results. Susceptibility thresholds were clinically evaluated. Strain-antibiotic combinations with a minimum inhibitory concentration (MIC) on or one doubling dilution above the breakpoint were exposed to doubling dilutions of antibiotics and measured on iFAST to determine an electrical MIC. Results: 100% correlation was obtained for all eight antibiotics against laboratory strains, when allowing for the inherent 2-fold variability of the BMD MIC measurement, within a five-hour test. Clinical evaluation showed concordance in at least 74 out of 80 tests. Electrical MICs showed broad equivalence with classical MICs. Conclusions: iFAST has potential as an accurate and rapid AST for UTI causing Enterobacterales.