The effect of doxycycline and tetracycline hydrochloride on the aerobic fecal flora, with special reference to Escherichia coli.

The effect on the aerobic faecal flora of a 10-day course of doxycycline or tetracycline hydrochloride was compared in 36 patients with acute infections mainly of respiratory origin. The patients were treated in hospital with a well-developed barrier nursing technique. The proportions of isolates of Escherichia coli resistant to tetracyclines were significantly lower after 10 days' treatment with doxycycline (80%) as well as one month later (24%), compared to the figures for tetracycline HCI (100% and 46%, respectively). The resistant strains selected during therapy probably represented the community flora. Only a small increase in multiresistant strains occurred and no identical strains were found in different patients. Thus, it seems possible to limit the biological side-effects of tetracyclines by a good barrier nursing technique. An increase in the number of E. coli resistant to tetracyclines cannot be avoided, but is more limited when doxycycline is used.