Abstract: A mediated extracellular electron transfer as a rapid and direct method to classify antibiotic-susceptible and -resistant bacteria is disclosed herein. Particularly, devices and methodologies to interpret antibiotic susceptibility index of a known bacteria strain or uncharacterized bacteria by treating it with antibiotics of different mechanisms or different concentrations of an antibiotic is provided. In an example methodology, the process includes a number of current control signal responses and a number of inoculated current response signals and thereafter analyzing the inoculated current response signals over a range of concentrations of up to about 24 times a breakpoint of the antibiotic with respect to current control signal responses to provide a susceptibility index assessment indicative of the susceptibility or resistance of the pathogen to the antibiotic.