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

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.

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.

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.

iFAST has potential as an accurate and rapid AST for UTI causing Enterobacterales.