Phylogenetic distribution of genes encoding ß-glucuronidase activity in human colonic bacteria and the impact of diet on faecal glycosidase activities.

Bacterial ß-glucuronidase in the human colon plays an important role in cleaving liver conjugates of dietary compounds and xenobiotics, while other glycosidase activities are involved in the conversion of dietary plant glycosides. Here we detected an increase in ß-glucuronidase activity in faecal samples from obese volunteers following a high-protein moderate carbohydrate weight-loss diet, compared with a weight maintenance diet, but little or no changes were observed when the type of fermentable carbohydrate was varied. Other faecal glycosidase activities showed little or no change over a fivefold range of dietary NSP intake, although α-glucosidase increased on a resistant starch-enriched diet. Two distinct groups of gene, gus and BG, have been reported to encode ß-glucuronidase activity among human colonic bacteria. Degenerate primers were designed against these genes. Overall, Firmicutes were found to account for 96% of amplified gus sequences, with three operational taxonomic units particularly abundant, whereas 59% of amplified BG sequences belonged to Bacteroidetes and 41% to Firmicutes. A similar distribution of operational taxonomic units was found in a published metagenome dataset involving a larger number of volunteers. Seven cultured isolates of human colonic bacteria that carried only the BG gene gave relatively low ß-glucuronidase activity that was not induced by 4-nitrophenyl-ß-D-glucuronide. By comparison, in three of five isolates that possessed only the gus gene, ß-glucuronidase activity was induced.

Horizontal gene transfer from Bacteria to rumen Ciliates indicates adaptation to their anaerobic, carbohydrates-rich environment.

BACKGROUND: The horizontal transfer of expressed genes from Bacteria into Ciliates which live in close contact with each other in the rumen (the foregut of ruminants) was studied using ciliate Expressed Sequence Tags (ESTs). More than 4000 ESTs were sequenced from representatives of the two major groups of rumen Cilates: the order Entodiniomorphida (Entodinium simplex, Entodinium caudatum, Eudiplodinium maggii, Metadinium medium, Diploplastron affine, Polyplastron multivesiculatum and Epidinium ecaudatum) and the order Vestibuliferida, previously called Holotricha (Isotricha prostoma, Isotricha intestinalis and Dasytricha ruminantium). RESULTS: A comparison of the sequences with the completely sequenced genomes of Eukaryotes and Prokaryotes, followed by large-scale construction and analysis of phylogenies, identified 148 ciliate genes that specifically cluster with genes from the Bacteria and Archaea. The phylogenetic clustering with bacterial genes, coupled with the absence of close relatives of these genes in the Ciliate Tetrahymena thermophila, indicates that they have been acquired via Horizontal Gene Transfer (HGT) after the colonization of the gut by the rumen Ciliates. CONCLUSION: Among the HGT candidates, we found an over-representation (>75%) of genes involved in metabolism, specifically in the catabolism of complex carbohydrates, a rich food source in the rumen. We propose that the acquisition of these genes has greatly facilitated the Ciliates' colonization of the rumen providing evidence for the role of HGT in the adaptation to new niches.

An NAD(+)-dependent glutamate dehydrogenase cloned from the ruminal ciliate protozoan, Entodinium caudatum.

An NAD(+)-dependent glutamate dehydrogenase (GDH; EC 1.4.1.24) was cloned from the ruminal ciliate protozoan, Entodinium caudatum. The gene had high sequence similarity to GDH genes from the Bacteroides (class)--a class of bacteria which is highly represented in the rumen. When expressed in Escherichia coli the enzyme had a high affinity for ammonia and alpha-ketoglutarate (apparent K(m) of 2.33 and 0.71 mM, respectively) and a low affinity for glutamate (apparent K(m) of 98 mM). GDH activity and GDH mRNA concentration were increased by incubating washed E. caudatum cells with ammonia and antibiotics. These results suggest that the GDH is an anabolic enzyme catalysing the assimilation of ammonia by E. caudatum in the rumen and that the gene was probably acquired by lateral gene transfer from a ruminal bacterium.

Mechanism of conjugated linoleic acid and vaccenic acid formation in human faecal suspensions and pure cultures of intestinal bacteria.

Faecal bacteria from four human donors and six species of human intestinal bacteria known to metabolize linoleic acid (LA) were incubated with LA in deuterium oxide-enriched medium to investigate the mechanisms of conjugated linoleic acid (CLA) and vaccenic acid (VA) formation. The main CLA products in faecal suspensions, rumenic acid (cis-9,trans-11-CLA; RA) and trans-9,trans-11-CLA, were labelled at C-13, as were other 9,11 geometric isomers. Traces of trans-10,cis-12-CLA formed were labelled to a much lower extent. In pure culture, Bifidobacterium breve NCFB 2258 formed labelled RA and trans-9,trans-11-CLA, while Butyrivibrio fibrisolvens 16.4, Roseburia hominis A2-183T, Roseburia inulinivorans A2-192T and Ruminococcus obeum-like strain A2-162 converted LA to VA, labelled in a manner indicating that VA was formed via C-13-labelled RA. Propionibacterium freudenreichii subsp. shermanii DSM 4902T, a possible probiotic, formed mainly RA with smaller amounts of trans-10,cis-12-CLA and trans-9,trans-11-CLA, labelled the same as in the mixed microbiota. Ricinoleic acid (12-OH-cis-9-18 : 1) did not form CLA in the mixed microbiota, in contrast to CLA formation described for Lactobacillus plantarum. These results were similar to those reported for the mixed microbiota of the rumen. Thus, although the bacterial genera and species responsible for biohydrogenation in the rumen and the human intestine differ, and a second route of RA formation via a 10-OH-18 : 1 is present in the intestine, the overall labelling patterns of different CLA isomers formation are common to both gut ecosystems. A hydrogen-abstraction enzymic mechanism is proposed that may explain the role of a 10-OH-18 : 1 intermediate in 9,11-CLA formation in pure and mixed cultures.

Differences between human subjects in the composition of the faecal bacterial community and faecal metabolism of linoleic acid.

Conjugated linoleic acid (CLA) is formed from linoleic acid (LA; cis-9,cis-12-18:2) by intestinal bacteria. Different CLA isomers have different implications for human health. The aim of this study was to investigate LA metabolism and the CLA isomers formed in two individuals (V1 and V2) with different faecal metabolic characteristics, and to compare fatty acid metabolism with the microbial community composition. LA incubated with faecal samples was metabolized at similar rates with both subjects, but the products were different. LA was metabolized extensively to stearic acid (SA; 18:0) in V1, with minor accumulation of CLA and more rapid accumulation of vaccenic acid (VA; trans-11-18:1). CLA accumulation at 4 h was almost tenfold higher with V2, and little SA was formed. At least 12 different isomers of CLA were produced from LA by the colonic bacteria from the two individuals. The predominant (>75%) CLA isomer in V1 was rumenic acid (RA; cis-9,trans-11-18:2), whereas the concentrations of RA and trans-10,cis-12-18:2 were similar with V2. Propionate and butyrate proportions in short-chain fatty acids were higher in V1. A 16S rRNA clone library from V1 contained mainly Bacteroidetes (54% of clones), whereas Firmicutes (66% of clones) predominated in V2. Both samples were devoid of bacteria related to Clostridium proteoclasticum, the only gut bacterium known to metabolize VA to SA. Thus, the CLA formed in the intestine of different individuals may differ according to their resident microbiota, with possibly important implications with respect to gut health.

Metabolism of linoleic acid by human gut bacteria: different routes for biosynthesis of conjugated linoleic acid.

A survey of 30 representative strains of human gram-positive intestinal bacteria indicated that Roseburia species were among the most active in metabolizing linoleic acid (cis-9,cis-12-18:2). Different Roseburia spp. formed either vaccenic acid (trans-11-18:1) or a 10-hydroxy-18:1; these compounds are precursors of the health-promoting conjugated linoleic acid cis-9,trans-11-18:2 in human tissues and the intestine, respectively.

Rumen ciliate protozoa contain high concentrations of conjugated linoleic acids and vaccenic acid, yet do not hydrogenate linoleic acid or desaturate stearic acid.

Conjugated linoleic acids (CLA) have been shown to improve human health. They are derived from the microbial conversion of dietary linoleic acid (cis-9,cis-12-18 : 2 (LA)) in the rumen. An investigation was undertaken to determine the role of ruminal ciliate protozoa v. bacteria in the formation of CLA and its precursor in animal tissues, vaccenic acid (trans-11-18 : 1 (VA)). Mixed protozoa from the sheep rumen contained at least two to three times more unsaturated fatty acids, including CLA and VA, than bacteria. Different species had different composition, with larger fibrolytic species such as Epidinium ecaudatum caudatum containing more than ten times more CLA and VA than some small species, including Entodinium nanellum. In incubations with ruminal microbial fractions (bacterial fraction (BAC), protozoal fraction (PRO)), LA metabolism was very similar in strained ruminal fluid (SRF) and in the BAC, while the PRO had LA-metabolising activity an order of magnitude lower. Using PCR-based methods, no genes homologous to fatty acid desaturase genes were found in cDNA libraries from ruminal protozoa. The absence of an alternative route of VA/CLA formation via desaturation of stearate was confirmed by incubations of SRF, BAC or PRO with [14C]stearate. Thus, although protozoa are rich in CLA and VA, they appear to lack the ability to form these two fatty acids from LA or stearate. The most likely explanation is that protozoa preferentially incorporate CLA and VA formed by bacteria. The implication of the present findings is that the flow of unsaturated fatty acids, including CLA and VA, from the rumen could depend on the flow of protozoa rather than bacteria.