Juan J. Abellán

Metatranscriptomic Approach to Analyze the Functional Human Gut Microbiota

The human gut is the natural habitat for a large and dynamic bacterial community that has a great relevance for health. Metagenomics is increasing our knowledge of gene content as well as of functional and genetic variability in this microbiome. However, little is known about the active bacteria and their function(s) in the gastrointestinal tract. We performed a metatranscriptomic study on ten healthy volunteers to elucidate the active members of the gut microbiome and their functionality under conditions of health. First, the microbial cDNAs obtained from each sample were sequenced using 454 technology. The analysis of 16S transcripts showed the phylogenetic structure of the active microbial community. Lachnospiraceae, Ruminococcaceae, Bacteroidaceae, Prevotellaceae, and Rickenellaceae were the predominant families detected in the active microbiota. The characterization of mRNAs revealed a uniform functional pattern in healthy individuals. The main functional roles of the gut microbiota were carbohydrate metabolism, energy production and synthesis of cellular components. In contrast, housekeeping activities such as amino acid and lipid metabolism were underrepresented in the metatranscriptome. Our results provide new insights into the functionality of the complex gut microbiota in healthy individuals. In this RNA-based survey, we also detected small RNAs, which are important regulatory elements in prokaryotic physiology and pathogenicity.

Environmental distribution of prokaryotic taxa

BackgroundThe increasing availability of gene sequences of prokaryotic species in samples extracted from all kind of locations allows addressing the study of the influence of environmental patterns in prokaryotic biodiversity. We present a comprehensive study to address the potential existence of environmental preferences of prokaryotic taxa and the commonness of the specialist and generalist strategies. We also assessed the most significant environmental factors shaping the environmental distribution of taxa.ResultsWe used 16S rDNA sequences from 3,502 sampling experiments in natural and artificial sources. These sequences were taxonomically assigned, and the corresponding samples were also classified into a hierarchical classification of environments. We used several statistical methods to analyze the environmental distribution of taxa. Our results indicate that environmental specificity is not very common at the higher taxonomic levels (phylum to family), but emerges at lower taxonomic levels (genus and species). The most selective environmental characteristics are those of animal tissues and thermal locations. Salinity is another very important factor for constraining prokaryotic diversity. On the other hand, soil and freshwater habitats are the less restrictive environments, harboring the largest number of prokaryotic taxa. All information on taxa, samples and environments is provided at the envDB online database, http://metagenomics.uv.es/envDB.ConclusionsThis is, as far as we know, the most comprehensive assessment of the distribution and diversity of prokaryotic taxa and their associations with different environments. Our data indicate that we are still far from characterizing prokaryotic diversity in any environment, except, perhaps, for human tissues such as the oral cavity and the vagina.

Assessing Gut Microbial Diversity from Feces and Rectal Mucosa

Gut microbiota is the most complex bacterial community in the human body and its study may give important clues to the etiology of different intestinal diseases. Most studies carried out so far have used fecal samples, assuming that these samples have a similar distribution to the communities present throughout the colon. The present study was designed to test this assumption by comparing samples from the rectal mucosa and feces of nine healthy volunteers by sequencing libraries of 16S rRNA genes. At the family taxonomic level, where rarefaction curves indicate that the observed number of taxa is close to the expected one, we observe under different statistical analyses that fecal and mucosal samples cluster separately. The same is found at the level of species considering phylogenetic information. Consequently, it cannot be stated that both samples from a given individual are of similar composition. We believe that the evidence in support of this statement is strong and that it would not change by increasing the number of individuals and/or performing massive sequencing. We do not expect clinicians to stop using feces for research, but we think it is important to caution them on their potential lack of representativeness with respect to the bacterial biofilm on the rectal mucosa.

Microbial Succession in the Gut: Directional Trends of Taxonomic and Functional Change in a Birth Cohort of Spanish Infants

In spite of its major impact on life-long health, the process of microbial succession in the gut of infants remains poorly understood. Here, we analyze the patterns of taxonomic and functional change in the gut microbiota during the first year of life for a birth cohort of 13 infants. We detect that individual instances of gut colonization vary in the temporal dynamics of microbiota richness, diversity, and composition at both functional and taxonomic levels. Nevertheless, trends discernible in a majority of infants indicate that gut colonization occurs in two distinct phases of succession, separated by the introduction of solid foods to the diet. This change in resource availability causes a sharp decrease in the taxonomic richness of the microbiota due to the loss of rare taxa (pu200a=u200a2.06e-9), although the number of core genera shared by all infants increases substantially. Moreover, although the gut microbial succession is not strictly deterministic, we detect an overarching directionality of change through time towards the taxonomic and functional composition of the maternal microbiota. Succession is however not complete by the one year mark, as significant differences remain between one-year-olds and their mothers in terms of taxonomic (pu200a=u200a0.009) and functional (pu200a=u200a0.004) microbiota composition, and in taxonomic richness (pu200a=u200a2.76e-37) and diversity (pu200a=u200a0.016). Our results also indicate that the taxonomic composition of the microbiota shapes its functional capacities. Therefore, the observed inter-individual variability in taxonomic composition during succession is not fully compensated by functional equivalence among bacterial genera and may have important physiological consequences. Finally, network analyses suggest that positive interactions among core genera during community assembly contribute to ensure their permanence within the gut, and highlight an expansion of complexity in the interactions network as the core of taxa shared by all infants grows following the introduction of solid foods.

Structural alterations of faecal and mucosa-associated bacterial communities in irritable bowel syndrome.

Irritable bowel syndrome (IBS) is the most common functional gastrointestinal disorder in western countries. Previous studies on IBS, mostly based on faecal samples, suggest alterations in the intestinal microbiota. However, no consensus has been reached regarding the association between specific bacteria and IBS. We explore the alterations of intestinal bacterial communities in IBS using massive sequencing of amplified 16S rRNA genes. Mucosal biopsies of the ascending and descending colon and faeces from 16 IBS patients and 9 healthy controls were analysed. Strong inter-individual variation was observed in the composition of the bacterial communities in both patients and controls. These communities showed less diversity in IBS cases. There were larger differences in the microbiota composition between biopsies and faeces than between patients and controls. We found a few over-represented and under-represented taxa in IBS cases with respect to controls. The detected alterations varied by site, with no changes being consistent across sample types.

Instability of the faecal microbiota in diarrhoea-predominant irritable bowel syndrome.

The irritable bowel syndrome (IBS) is a functional gastrointestinal disorder with a largely unknown aetiology and a wide range of symptoms. Most cross-sectional studies carried out so far suggest subtle alterations in the structure of the intestinal microbiota that are barely reproduced, partly because of the high inter-subject variation in the community composition and disorder-specific features. We performed a longitudinal study to explore the within-subject variation in the faecal microbiota in two patients with IBS classified into the diarrhoea subtype and the healthy spouse of one of them. Faecal communities were monitored over 6-8xa0weeks and analysed through metagenomic and metatranscriptomic approaches. We found a higher temporal instability in the fraction of active microbiota related to the IBS condition and fluctuating symptoms. Strong and quick shifts in the distribution of the active microbiota and changes in the global pattern of gene expression were detected in association with acute diarrhoea, whereas microbial composition and encoded functions were more stable. The specific alterations in the microbiota were barely reproduced within and between patients. Further research is needed to assess whether these changes are a consequence of the abnormal gut function in acute diarrhoeic episodes and the potential usefulness of tackling them.

Metagenomics of human microbiome: beyond 16s rDNA

The gut microbiota presents a symbiotic relationship with the human host playing a beneficial role in human health. Since its establishment, the bacterial community is subjected to the influence of many different factors that shape its composition within each individual. However, an important convergence is observed at functional level in the gut microbiota. A metatranscriptomic study of healthy individuals showed homogeneity in the composition of the active microbiota that increased further at functional level.

Daily follow-up of bacterial communities in the human gut reveals stable composition and host-specific patterns of interaction

In the last decade, an extensive effort has been made to characterize the human intestinal microbiota by means of SSU rRNA gene sequence and metagenomic analysis. Relatively few studies have followed intestinal bacterial communities over time to assess their stability in the absence of perturbation. In this study, we have monitored the faecal bacteria of three healthy subjects during 15 consecutive days. The global community structure was analysed through SSU rRNA gene sequencing. In agreement with previous studies, we found that the between-subject variation in community structure was larger than within. The composition was fairly stable throughout, although daily fluctuations were detected for all genera and phylotypes at 97% of sequence identity. While the core shared between subjects was very small, each subject harboured a stable high-abundance core composed of a small number of bacterial groups (9% of the phylotypes accounted for between 74% and 93% of the sequences). This may suggest that studies aimed at linking the microbiota composition with disease risk should be limited to the numerically most dominant phylotypes, as the rest appears transient. Networks of potential interactions between co-occurring genera were also subject-specific, even for the same bacterial genus, which might be reflecting host-specific selective pressures and historic events.

Spatio-temporal patterns of bladder cancer incidence in Utah (1973-2004) and their association with the presence of toxic release inventory sites

BackgroundThe authors analyse the spatio-temporal variations of the incidence of bladder cancer between 1973 and 2004 in Utah at the census tract level (496 areas) to highlight areas of high and low relative risks that remained so throughout the 32 year period. Using these identified areas, a novel strategy is used to carry out a geographical case-control study of association between the risk of bladder cancer and presence of Toxic Release Inventory sites, where areas with stable high RRs are case areas and all remaining areas with stable non increased risks are control areas.ResultsThe time trend of bladder cancer risk fluctuated over the study period: A steady decrease was observed, followed by an abrupt increase from 1992 to 2004. Using a Bayesian space-time model, 93 census tracts were classified as having an excess relative risk and 81 a lower relative risk, sustained over the 32 years. We showed that these high relative risk areas for bladder cancer were associated with the presence of Toxic Release Inventory sites, after adjusting for the proportion of Latter-Day Saint Church members as an area level proxy for smoking habits.ConclusionsOur study has demonstrated that the modeling of data in time and space has additional benefits over a purely spatial analysis. In addition to highlighting the areas with high and low relative risks, this model also allows the simultaneous study of persistency of spatial patterns over time and detection of unusual time trends that may warrant further investigation.

Bayesian shared spatial-component models to combine and borrow strength across sparse disease surveillance sources

When analyzing the geographical variations of disease risk, one common problem is data sparseness. In such a setting, we investigate the possibility of using Bayesian shared spatial component models to strengthen inference and correct for any spatially structured sources of bias, when distinct data sources on one or more related diseases are available. Specifically, we apply our models to analyze the spatial variation of risk of two forms of scrapie infection affecting sheep in Wales (UK) using three surveillance sources on each disease. We first model each disease separately from the combined data sources and then extend our approach to jointly analyze diseases and data sources. We assess the predictive performances of several nested joint models through pseudo cross-validatory predictive model checks.