Influence of doxycycline on the normal human flora and colonization of the oral cavity and colon.

Doxycycline was given orally in doses of 100 mg every 24 h for 7 days to 10 subjects. Salivary and faecal specimens were taken up to 16 days for cultivation of aerobic and anaerobic microorganisms and for assay of doxycycline. Doxycycline was detected in both saliva and faeces. Only minor changes were observed regarding the number of aerobic and anaerobic microorganisms in the oral cavity. In the faecal flora a decrease in the number of aerobic microorganisms was observed in most subjects, while only minor changes in the number of anaerobic bacteria were noticed. A marked increase in doxycycline-resistant aerobic and anaerobic bacterial strains was noticed both in the oral cavity and the colon. Only a few patients were colonized with potentially pathogenic microorganisms.