CONGRESS 2023 - Impedance-based Fast Antimicrobial Susceptibility Test (iFAST)

Antimicrobial resistance is a major concern with mortality rates growing exponentially. Current ASTs used clinically can take 24-48hrs to report results, ensuing in initial treatment with broad spectrum antibiotics. The novel iFAST method can report results within 2 hours of exposure to an antibiotic. The main objective of this study was to measure the impedance signal of resistant and sensitive isolates of Staphylococcus aureus that had been exposed to cefoxitin. Sequentially collected bacterial isolates were accessed from the clinical microbiology laboratory to determine susceptibility. 50 methicillin resistant/sensitive isolates of S. aureus were taken from the middle of the clinical workflow and tested on the iFAST. The isolates were streaked onto blood plates and incubated at 37 degrees for 2 hours. The bacteria were then exposed to cefoxitin for 2 hours at the EUCAST breakpoint concentration of 8mg/L. Following exposure, the samples were measured on the iFAST. The impedance cytometer measures the electrical signal of bacterial cells as they individually flow through a microfluidic channel, via electrodes driven by an AC current of multiple frequencies. This is interpreted as a read-out of cell volume and opacity. Exposure to antibiotics can change the electrical characteristics of the bacterial cell in size and opacity compared to the control sample. The number of exposed cells within the contour defined by the control sample can measure how the cells have altered in opacity and size following exposure. iFAST results showed 100% concordance with disk diffusion sensitivity testing carried out by the clinical laboratory. The data showed different electrical impedance changes for both resistant and sensitive strains of S. aureus. Sensitive strains showed a decrease in cell size and resistant strains showed an increase in cell size following exposure to cefoxitin. The bacterial impedance cytometer was able to rapidly differentiate between MRSA and MSSA isolates in concordance with current susceptibility testing in the clinical setting. The results help to show how the iFAST could reduce the time taken to provide critical and accurate antibiotic treatment to patients.

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