(276b) Rapid Determination of the Minimum Inhibitory Concentrations (MIC values) of Antibiotics Using a Rapid, Culture-Based, Electrical Detection Method

For reasons both scientific (such as to study the development of antibiotic resistance in bacteria isolated from different sources) and clinical (such as to ascertain the efficacy of the medication and dose prescribed for a bacterial infection, and make changes if required), one often needs to evaluate the antibiotic susceptibility profile of various bacteria. The susceptibility of a given strain/isolate to a particular antibiotic is usually expressed as the Minimum Inhibitory Concentration (MIC) of the antibiotic.

In practice, a strain of interest is inoculated into liquid cultures with various concentrations of the candidate antibiotic (ranging from 0.5 to 128 ug/ml), and one seeks to determine the minimum antibiotic concentration for which bacterial growth is inhibited. This concentration is known as the Minimum Inhibitory Concentration. Current detection systems typically rely on optical density measurements to determine the occurrence of bacterial growth (or lack thereof) for the presence of given concentrations of antibiotic. While reliable, they take a long time to detect changes in bacterial numbers (and hence determine MIC values). Another major drawback of the existing system is that they cannot distinguish between bacteriostatic and bactericidal action of the antibiotic against specific bacteria.

This work presents a novel electrical method for rapid antibiotic susceptibility testing of bacterial strains of interest. The principle underlying our method of detection is the polarizability of viable bacterial cells. In the presence of an alternating electric field, there occurs a build-up of charge at the membrane, causing the cells to act like capacitors. If however, the cell membrane is compromised (as happens when cells die), this phenomenon no-longer occurs.

In our method, we set up a panel of cultures with different concentrations of the candidate antibiotic (as with current methods). However, each culture is monitored electrically. If the bacteria multiply in number, there will be a corresponding increase in the charge stored in the interior of the suspension (its “bulk capacitance”), and this increase in bulk capacitance indicates that the bacteria are proliferating despite the presence of the antibiotic. Similarly, as the bacterial number decreases due to the bactericidal effect of antibiotics, there will be a corresponding decrease in the value of the bulk capacitance. And finally, if the bacterial numbers are held static by the antibiotic, the bulk capacitance will also remain virtually unchanged.

Thus our detection method has the ability to differentiate between bactericidal and bacteriostatic action  of antibiotic for a given bacteria. Using our method we were able to measure the Minimum Inhibitory Concentrations (MICs) of various antibiotics for bacterial strains of interest in <4 hours (as opposed to more than 1 day using current methods).