Effect of erythromycin and clindamycin on the indigenous human anaerobic flora and new colonization of the gastrointestinal tract.

Erythromycin and clindamycin were given orally to ten subjects in recommended doses for seven days in order to study the effects of these antibiotics on human flora. Saliva and faecal specimens were collected for up to 29 days after administration of the antibiotics. Erythromycin caused only minor changes in the saliva flora while the aerobic and anaerobic colon flora were considerably disturbed. Clindamycin depressed both the anaerobic saliva and colon flora. Both erythromycin and clindamycin induced new colonization of the oral cavity and colon. The levels of free volatile fatty acids sank in saliva and faeces when erythromycin and clindamycin were given. The ecological disturbances caused by antibiotics require further investigation and should be taken into consideration in therapy.