Influence of three antimicrobial agents--penicillin, metronidazole, and doxycyclin--on the intestinal microflora of healthy humans.

Antimicrobial drugs may affect the normal gut microflora in a potentially harmful manner. The purpose of the present study was to ascertain whether any of several tests could detect drug influence on the intestinal flora of healthy subjects. Jejunal secretions and feces were cultivated aerobically and anaerobically, with measurement of fermentation gas production in tubes supplemented with glucose or lactulose; bacterial bile acid deconjugation was measured with the 14C-GCA test; and pulmonary H2 and CH4 excretion were measured with gas-solid chromatography after lactulose and glucose ingestion in 18 healthy subjects before and after peroral treatment with either penicillin, metronidazole, or doxycyclin. Bacterial numbers and fermentation gas production were unchanged after treatment, as was the bacterial bile acid deconjugating activity. Pulmonary H2 excretion after lactulose ingestion was significantly reduced after penicillin and metronidazole but not after doxycyclin treatment. Pulmonary CH4 disappeared after metronidazole but remained unchanged after penicillin and doxycyclin treatment. It is concluded that pulmonary H2 and CH4 measurement after lactulose ingestion may serve as a sensitive determinant for drug effects on the normal gut microflora.