POTENTIOMETRIC MEASUREMENT OF LIPOIC ACID REDUCTION IN MICROBIAL CULTURES .2. EVALUATION OF THE BACTERICIDAL EFFICACITY OF DRUGS AFFECTING BACTERIAL-GROWTH

Multichannel potential-time measurements were carried out in cultures of Escherichia coli growing in the presence of increasing amounts of aminoglycoside antibiotics. The potentiometric lag times were an increasing function of the drug concentration. Theoretical apparent growth rates of organisms in the presence of the drug could be calculated from these potential-time data, and were related to the antibiotic concentration. For each antibiotic tested, it was possible to determine an inhibitation rate constant as a function of the drug concentration, which characterized the bactericidal efficacity of the drug. A potentiometric MIC (minimal inhibitory concentration) was defined as the concentration at which the inhibition rate constant would be equal to the specific growth rate of organisms with no drugs added to the culture broth: the potentiometric MIC values were consistent with the conventional MIC's determined by a standard diffusion technique.