Profiles from twelve volunteers have 100% similarity between day -6 and day 0, just before AMC exposure. Stability of microbiota was not absolute. In another study, monthly monitoring of Bifidobacterium species in 6 adults during 8 months, using specific qPCR, showed a very stable bifidobacterial microbiota for 5 volunteers and high variations in the population and composition for the last one. Using bifidobacterial DGGE on samples collected during a 6-week period, Vanhoutte and co-workers showed a close profile grouping for 3 subjects with similarity values ranging from 81.4% to 100%, and one subject with a mean similarity value of 70%. At day 5, in our study, mean similarity percentage of TTGE profiles versus the reference period was 55.8%67.6%. The microbiota were modified for the most, except for four volunteers, three resistant to AMC and the last perturbed only at day 8. The resistance of some microbiota has been clearly demonstrated. In the composition of their fecal microbiota than those observed in subjects without such activity. This was thought to be because the beta-lactamase activity destroyed the antibiotic residues that reach the colon during treatment, thus reducing alterations in the microbiota to a minimum. In spite of clavulanic acid presence which is a beta-lactamase inhibitor, remaining intestinal beta-lactamases from individual microbiota could influence the amount of beta-lactam present in the feces during AMC exposure and explain the resistance to changes of some microbiota. Similarity percentages of TTGE profiles at day 33 and day 64 were 59.6% and 62.3% respectively, showing that microbiota did not return to baseline. Cloning and sequencing were performed to identify bands of interest and to evaluate if the changes of bands corresponded to changes of species or of strains within the same species. The same identifications were obtained for bands with identical Rf. In agreement with previous studies, B. adolescentis, B. longum and the B. pseudocatenulatum/B. catenulatum group were the most frequent predominant bifidobacterial species present in adult microbiota followed by B. bifidum. The mean number of Bifidobacterium species per sample MLN4924 cost harbored in dominant microbiota is significantly lower at day 5 compared to reference period. In another study, the average number of species detected per individual were 2.861.2 in healthy adults. Furthermore, at day 5, significant alterations for some Bifidobacterium species were observed: for example, occurrence of B. adolescentis decreased significantly. In some cases, species not present at day 0 and probably belonging to the subdominant microbiota, became dominant, eg B. longum or B. breve. The occurrence of B. longum remained stable after the antibiotherapy. As enlightened in previous studies, the antimicrobial effect is dose-dependent and amoxicillin showed variable MIC depending on species or strains tested. Generally, B. adolescentis, B. bifidum and B. pseudocatenulatum seemed to be more susceptible in vitro than was B. longum. Thus, our results could be explained by MIC values, as well as intestinal beta-lactamases from individual microbiota. Similar results were previously obtained within microbiota of infants treated with a 7day-amoxicillin treatment, but long-term impact was not monitored. Jaccard’s similarity coefficients indicated that differences between TTGE profiles corresponded to species changes and not only to strains changes. B. bifidum was not entirely recovered at day 33 or day 64. In a previous study, a molecular monitoring of intestinal Bifidobacterium strains in four adults using RFLP and ribotyping, showed little variations 30 days and 90 days after an AMC exposure.