Comparison of amoxicillin and ampicillin activities in a continuous culture model of the human urinary bladder

Sixteen isolates of Escherichia coli were obtained from women with significant acute urinary tract infections who were subsequently treated with amoxicillin. The activities of amoxicillin and ampicillin against these organisms were compared in urine in a continuous culture apparatus which partly simulated the milieu of the human urinary bladder. After introduction of amoxicillin into the bladder model, mean viable counts for 14 susceptible strains (minimum inhibitory concentration less than or equal to 32 microgram/ml) at 10, 20, 30, and 45 min fell to 34, 8, 0.4, and 0.2% of the original. Corresponding figures for ampicillin were 56, 24, 11, and 2.4%. Viable counts obtained at seven timed intervals up to 2 h were significantly (P = 0.025) lower with amoxicillin than ampicillin. Both antibiotics had a similar activity in conventional disk susceptibility and surface plate minimum inhibitory concentration tests. The realism of the model was confirmed by comparing response to amoxicillin in vivo and in vitro. A serious discrepancy was seen in only one of the 16 cases.