Influence of amoxycillin on microbial colonization resistance in healthy volunteers. A methodological study.

The influence of amoxycillin 500 mg tid on microbial colonization resistance was investigated in 11 healthy volunteers. Analysis was performed in each volunteer individually. In the first five volunteers we investigated the influence of amoxycillin on the faecal concentration of Gram-negative bacilli, enterococci and yeasts and on spontaneously occurring secondary colonization. In the next six volunteers we also investigated the influence of amoxycillin on colonization resistance against amoxycillin-resistant challenge strains, in order to be independent of the accidental presence of resistant Gram-negative bacilli. In three volunteers all indicators employed did not show impairment of the anaerobic flora that provide colonization resistance. In five volunteers impairment of this flora was indicated both by increase of the faecal concentration of aerobic flora and by increase of spontaneously occurring secondary colonization or facilitation of colonization by the challenge strains. However, in the other three volunteers there was no concordance between the investigated indicators of the influence of amoxycillin on colonization resistance. Possible explanations are discussed. It is concluded that increase of the faecal concentration of aerobic flora is a more reliable indicator of impairment of the anaerobic flora that provides colonization resistance than increase of secondary colonization by strains acquired spontaneously or by challenge strains administered deliberately. In one volunteer, who was excluded from the trial, high-level faecal colonization occurred after challenge with Enterobacter cloacae in the pretreatment period.