Double-blind study to compare the selection of antibiotic resistance by amoxycillin or cephradine in the commensal flora.

Elderly patients with acute urinary infections were treated in a double-blind study with either amoxycillin or cephradine. In 52 patients who had received amoxycillin for one week about a third of all intestinal Escherichia coli were highly resistant to amoxycillin, and many were resistant to tetracycline, trimethoprim, or chloramphenicol. Cephradine selected less resistance. At a week after completion of chemotherapy, cephradine-resistant E coli were replaced by sensitive cultures at a greater frequency than were amoxycillin-resistant E coli. Neither antibiotic altered the skin flora. Amoxycillin, but not cephradine, selected for Enterobacteriaceae in the saliva. The propensity of amoxycillin to select resistance in E coli will limit its usefulness in treating urinary infections.