Influence of aminopenicillins on bacterial growth kinetics in vitro in comparison with the antibacterial effect in vivo.

To quantify the relative antibacterial efficacy of beta-lactam antibiotics in vitro, a mathematical description of bacterial growth was developed. Curves of growth in the presence of the antibiotic are expressed as quadratic functions of time, with initial growth rate (ko) and rate of change of growth (a) as concentration dependant variables. For these variables the concentration-effect slopes were not parallel; therefore the potency ratio varied between 1 (at about 2 mg/l) and 2 (at higher concentrations). The relative efficacy in vivo was expressed as a potency ratio with regard to the number of bacteria in an infected thigh muscle in mice. The potency ratio was 1.5, which is between the limits of the in vitro results.