Aim: To evaluate the influence of doxycycline therapy on the composition and antibiotic susceptibility of intestinal bifidobacteria.
Methods and Results: Faecal samples were collected from nine subjects receiving doxycycline therapy and ten control subjects, and analysed for bifidobacteria by culturing and PCR–DGGE (denaturing gradient gel electrophoresis). A marked decrease in the diversity (average number of amplicons detected by PCR–DGGE 0·8 in the antibiotic vs 4·3 in the control group) of Bifidobacterium populations was observed during doxycycline therapy. The proportion of a tetracycline‐resistant bifidobacterial population was higher in the antibiotic group than in the control group (83%vs 26%). Based on the tet gene PCR, resistance could be associated with the presence of tet(W). In two subjects, strains representing highly similar genetic fingerprints but different tetracycline susceptibilities were detected. A mutation causing lack of functionality in the tet(W) was observed in one of the susceptible strains.
Conclusions: Doxycycline therapy had a drastic effect on the diversity and tetracycline susceptibility of intestinal Bifidobacterium populations.
Significance and Impact of the Study: The use of broad‐spectrum antibiotics increased the pool of tetracycline‐resistant commensal bacteria in the intestine. The detection of resistance genes alone is not sufficient for the evaluation of bacterial antibiotic resistance.