In vitro evaluation of the determinants of bactericidal activity of ampicillin dosing regimens against Escherichia coli.

An in vitro flow model was used to examine the influence of peak concentration (Cmax), the area under the antibiotic concentration-time curve (AUC), the magnitude of AUC above the MIC, and the aggregate time the antibiotic concentration exceeds the MIC (TMIC) on the bactericidal effect of ampicillin against Escherichia coli ATCC 12407. Bacteria in the log phase were exposed to therapeutically realistic drug regimens. Ampicillin concentration and bacterial density (CFU per milliliter) were measured over time. Four parameters reflecting bactericidal activity were quantitated: difference between initial and minimum and initial and final bacterial densities, area under the bacterial density-time curve, and a fourth parameter, sigma, which is a function of these three. Multiple regression analysis confirmed AUC as the major factor in predicting bactericidal activity. An AUC of greater than 70 micrograms.h/ml correlated with the lack of emergence of resistance.