Biosensors & Bioelectronics

Novel Electrical Method for the Rapid Determination of Minimum Inhibitory Concentration (MIC) and Assay of Bactericidal/Bacteriostatic Activity

Abstract

Sachidevi Puttaswamy, Byung-Doo Lee, Banoo Amighi, Sounak Chakraborty and Shramik Sengupta

We present a rapid (4-hr) electrical method for Antibiotic Susceptibility Testing that not only yields the MIC of candidate antibiotics, but also simultaneously determines the antibiotics’ effect on the bacteria (bactericidal/bacteriostatic). Unlike conventional “impedance microbiology” methods that rely on measuring the effects of bacterial metabolism on the conductance/impedance of the suspension at a single chosen frequency, our method uses measurements at 500 frequencies between 1 KHz and 100 MHz to estimate the amount of electric charge stored due to charge-polarization at intact cell-membranes of living bacteria (the suspension “bulk capacitance”). By doing so, we are able to track the number of live bacteria in suspensions as the observations are taken (every 1 hour). It thus determines whether the numbers of viable bacteria present is increasing (bacteria proliferating in presence of antibiotic), decreasing (bacteria being killed) or holding steady (bacterial numbers held static). Three well-characterized bacterial strains (E. coli ATCC- 25922, S. aureus ATCC-29213 and P. aeruginosa ATCC-27853) were tested against a range of concentrations (0 to 128 mg/l) of known static and cidal antibiotics. For each sample (bacterial strain at a given concentration of antibiotic), statistical analysis of the “bulk capacitance” values, recorded over 4 hours was used to determine whether the bacteria were proliferating, being killed, or being held static. The minimum concentration of antibiotic for which the bacteria were killed or failed to proliferate is considered the Minimum Inhibitory Concentration (MIC). MICs obtained fell within the expected range for the strains tested, and “static” and “cidal” antibiotics were correctly identified. This method thus demonstrates the potential to provide in 4 hrs, clinically relevant information such as the MIC of bacterial strains (that currently take up to 2 days) and the mode of action (static/cidal) that currently takes an additional day.