Growth curve patterns and regrowth in post antibiotic period of Escherichia coli exposed to ampicillin and amoxycillin

The growth curve patterns of 8 strains of Escherichia coli exposed to amoxycillin or ampicillin were studied in the MS-2 system, which allows an accurate comparison over a wide range of concentrations. The early response (the first 2 h) of the growth curve showed residual growth (increase in OD before lysis) after the introduction of the antibiotics. The maximal value of this residual growth (MRVG) was measured for ampicillin and amoxycillin concentrations from 1 to 500 mg/l. The rate of regrowth in the post antibiotic period was studied by the use of beta-lactamase to eliminate the penicillins. In these experimental conditions the following observations were made: (a) the MVRG dropped more rapidly in amoxycillin-treated than in ampicillin-treated cultures. This effect was observed at concentrations from twice the MIC to 500 mg/l, despite the fact that some strains have a lower susceptibility to amoxycillin in terms of MIC values. (b) neutralization of antibiotic by beta-lactamase caused a longer lag of regrowth in amoxycillin-treated cultures than in ampicillin-exposed cultures.