Background and aim: A role for the intestinal microbial community (microbiota) in the onset and chronicity of Crohn’s disease (CD) is strongly suspected. However, investigation of such a complex ecosystem is difficult, even with culture independent molecular approaches. Methods: We used, for the first time, a comprehensive metagenomic approach to investigate the full range of intestinal microbial diversity. We used a fosmid vector to construct two libraries of genomic DNA isolated directly from faecal samples of six healthy donors and six patients with CD. Bacterial diversity was analysed by screening the two DNA libraries, each composed of 25 000 clones, for the 16S rRNA gene by DNA hybridisation. Results: Among 1190 selected clones, we identified 125 non-redundant ribotypes mainly represented by the phyla Bacteroidetes and Firmicutes. Among the Firmicutes, 43 distinct ribotypes were identified in the healthy microbiota, compared with only 13 in CD (p<0.025). Fluorescent in situ hybridisation directly targeting 16S rRNA in faecal samples analysed individually (n = 12) confirmed the significant reduction in the proportion of bacteria belonging to this phylum in CD patients (p<0.02). Conclusion: The metagenomic approach allowed us to detect a reduced complexity of the bacterial phylum Firmicutes as a signature of the faecal microbiota in patients with CD. It also indicated the presence of new bacterial species.

Background and aim: The colonic microflora is involved in the pathogenesis of Crohn's disease (CD) but less than 30% of the microflora can be cultured. We investigated potential differences in the faecal microflora between patients with colonic CD in remission (n=9), patients with active colonic CD (n=8), and healthy volunteers (n=16) using culture independent techniques. Methods: Quantitative dot blot hybridisation with six radiolabelled 16S ribosomal ribonucleic acid (rRNA) targeting oligonucleotide probes was used to measure the proportions of rRNA corresponding to each phylogenetic group. Temporal temperature gradient gel electrophoresis (TTGE) of 16S rDNA was used to evaluate dominant species diversity. Results: Enterobacteria were significantly increased in active and quiescent CD. Probe additivity was significantly lower in patients (65 (11)% and 69 (6)% in active CD and quiescent CD) than in healthy controls (99 (7)%). TTGE profiles varied markedly between active and quiescent CD but were stable in healthy conditions. Conclusion: The biodiversity of the microflora remains high in patients with CD. Enterobacteria were observed significantly more frequently in CD than in health, and more than 30% of the dominant flora belonged to yet undefined phylogenetic groups.

The healthy intestine is characterized by a low level of oxygen and by the presence of large bacterial communities of obligate anaerobes. Dysbiosis of the gut microbiota has been reported in patients suffering from inflammatory bowel diseases (IBDs), but the mechanisms causing this imbalance remain unknown. Observations have included a decrease in obligate anaerobes of the phylum Firmicutes and an increase in facultative anaerobes, including members of the family Enterobacteriaceae. The shift of bacterial communities from obligate to facultative anaerobes strongly suggests a disruption in anaerobiosis and points to a role for oxygen in intestinal dysbiosis. Proposals to evaluate this hypothesis of a role for oxygen in IBD dysbiosis are provided. If this hypothesis is confirmed, decreasing oxygen in the intestine could open novel means to rebalance the microbiota and could provide novel preventative or therapeutic strategies for IBD patients in whom current treatments are ineffective.

Abnormalities have been described in the fecal microbiota of patients with IBD, but it is not known whether they are specific for inflammatory bowel disease (IBD) or to some extent common to other forms of colitis. The aim of this study was to compare the bacterial composition of the dominant fecal microbiota in patients with Crohn's disease (CD), ulcerative colitis (UC), infectious colitis (IC), and in healthy subjects (HS). Fluorescent in situ hybridization adapted to flow cytometry was used to analyze the bacterial composition of fecal samples from 13 patients with active CD, 13 patients with active UC, 5 patients with IC, and 13 HS. We used 6 group-specific probes targeting 16S rRNA and spanning the main phylogenetic groups of the fecal microbiota. A significantly higher proportion of the total fecal bacteria were recognized by the 6 probes in HS (86.6%+/-12.7) and in IC (84.0%+/-11.7) than in patients with IBD (70.9%+/-15 in CD and 60.1%+/-25.7 in UC). The Clostridium coccoides group was reduced in UC (20.0%+/-13.3 versus 42.0%+/-12.0 in HS; P<.001), whereas the C leptum group was reduced in CD (13.1%+/-11.9 versus 25.2%+/-14.2 in HS; P=.002). The Bacteroides group was more abundant in IC (36.4%+/-22.9) than in the other 3 groups (13.8%+/-11.8 in CD, 11.7%+/-11.7 in UC, 12.1%+/-7.0 in HS; P<.001 for all 3 comparisons). In IBD the dominant fecal microbiota comprises unusual bacterial species. Moreover, CD and UC fecal microbiota harbor specific discrepancies and differ from that of IC and healthy subjects.

The current classification of the rhizobia (root-nodule symbionts) assigns them to six genera. It is strongly influenced by the small subunit (16S, SSU) rRNA molecular phylogeny, but such single-gene phylogenies may not reflect the evolution of the genome as a whole. To test this, parts of the atpD and recA genes have been sequenced for 25 type strains within the alpha-Proteobacteria, representing species in Rhizobium, Sinorhizobium, Mesorhizobium, Bradyrhizobium, Azorhizobium, Agrobacterium, Phyllobacterium, Mycoplana and Brevundimonas. The current genera Sinorhizobium and Mesorhizobium are well supported by these genes, each forming a distinct phylogenetic clade with unequivocal bootstrap support. There is good support for a Rhizobium clade that includes Agrobacterium tumefaciens, and the very close relationship between Agrobacterium rhizogenes and Rhizobium tropici is confirmed. There is evidence for recombination within the genera Mesorhizobium and Sinorhizobium, but the congruence of the phylogenies at higher levels indicates that the genera are genetically isolated. rRNA provides a reliable distinction between genera, but genetic relationships within a genus may be disturbed by recombination.

The composition of the colonic microbiota of 91 northern Europeans was characterized by fluorescent in situ hybridization using 18 phylogenetic probes. On average 75% of the bacteria were identified, and large interindividual variations were observed. Clostridium coccoides and Clostridium leptum were the dominant groups (28.0% and 25.2%), followed by the Bacteroides (8.5%). According to principal component analysis, no significant grouping with respect to geographic origin, age, or gender was observed.