Fig. 1: Principle of rapid impedance-based antimicrobial susceptibility testing.

a Multi-electrode microfluidic impedance chip. Cells flow one-by-one between sets of electrodes and are measured simultaneously at two frequencies using a lock-in-amplifier (LIA). b Equivalent electrical equivalent circuit model for a Gram-negative bacteria, and a simulated spectrum of the Real part of the impedance vs. frequency highlighting frequency windows where changes in cell properties become apparent. c Impedance scatter plot of bacteria (K. pneumoniae, 10,000 events) together with 1.5-μm diameter polystyrene beads (with doublets and triplets) that are used as reference particles. The x-axis is the cube root of the impedance (proportional to diameter) measured at a frequency of 5 MHz. The y-axis is the electrical opacity, a measure of membrane/cell wall properties normalised to cell volume. This is measured at 40 MHz where the electrical properties of the cell wall and membrane are most apparent (see b). Two data sets are pre- and post-exposure to Meropenem at the clinical breakpoint for 30 min at 37 °C. In the scatter plot, the red contour defines the initial cell population. The diagram illustrates the change in cell properties following exposure to a β-lactam antibiotic as the cell wall breaks down (reduction in opacity) and the bacteria swell (increase in volume). d Experimental methodology for the impedance-based Fast Antimicrobial Susceptibility Test (iFAST). An actively dividing culture is prepared and incubated for 30 min with antibiotics. Polystyrene beads are added and the sample is measured for 3 min to determine the electrical MIC (see Supplementary information for further details).