The relative potency of amoxycillin and ampicillin in vitro and in vivo

The relative antibacterial efficacy of amoxycillin ampicillin was compared in vitro and in vivo against a strain of Escherichia coli in short-term experiments (hours). The quadratic fit to the growth curves in vitro in the presence of the antibiotic gave concentration dependent parameters. The concentration effect curves for both antibiotics showed that amoxycillin varied between equipotent (at 2 mg/l) and twice as potent (at higher concentrations) as ampicillin. The relative efficacy in vivo was determined in a thigh muscle infection in irradiated as well as normal mice. Numbers of colony forming units 3 h after infection were dose (or concentration) dependent, the slopes for amoxycillin and ampicillin being parallel. Slopes in irradiated and non-irradiated mice differed significantly. In both kinds of animals the average potency ratio was 1.5 in favour of amoxycillin. Thus, no difference was found between the in vitro and in vivo situation regarding relative efficacy.