A new genomic blueprint of the human gut microbiota.

The composition of the human gut microbiota is linked to health and disease, but knowledge of individual microbial species is needed to decipher their biological roles. Despite extensive culturing and sequencing efforts, the complete bacterial repertoire of the human gut microbiota remains undefined. Here we identify 1,952 uncultured candidate bacterial species by reconstructing 92,143 metagenome-assembled genomes from 11,850 human gut microbiomes. These uncultured genomes substantially expand the known species repertoire of the collective human gut microbiota, with a 281% increase in phylogenetic diversity. Although the newly identified species are less prevalent in well-studied populations compared to reference isolate genomes, they improve classification of understudied African and South American samples by more than 200%. These candidate species encode hundreds of newly identified biosynthetic gene clusters and possess a distinctive functional capacity that might explain their elusive nature. Our work expands the known diversity of uncultured gut bacteria, which provides unprecedented resolution for taxonomic and functional characterization of the intestinal microbiota.

Metabolically-targeted dCas9 expression in bacteria.

The ability to restrict gene expression to a relevant bacterial species in a complex microbiome is an unsolved problem. In the context of the human microbiome, one desirable target metabolic activity are glucuronide-utilization enzymes (GUS) that are implicated in the toxic re-activation of glucuronidated compounds in the human gastrointestinal (GI) tract, including the chemotherapeutic drug irinotecan. Here, we take advantage of the variable distribution of GUS enzymes in bacteria as a means to distinguish between bacteria with GUS activity, and re-purpose the glucuronide-responsive GusR transcription factor as a biosensor to regulate dCas9 expression in response to glucuronide inducers. We fused the Escherichia coli gusA regulatory region to the dCas9 gene to create pGreg-dCas9, and showed that dCas9 expression is induced by glucuronides, but not other carbon sources. When conjugated from E. coli to Gammaproteobacteria derived from human stool, dCas9 expression from pGreg-dCas9 was restricted to GUS-positive bacteria. dCas9-sgRNAs targeted to gusA specifically down-regulated gus operon transcription in Gammaproteobacteria, with a resulting ∼100-fold decrease in GusA activity. Our data outline a general strategy to re-purpose bacterial transcription factors responsive to exogenous metabolites for precise ligand-dependent expression of genetic tools such as dCas9 in diverse bacterial species.

Preen gland microbiota of songbirds differ across populations but not sexes.

Metabolites produced by symbiotic microbes can affect the odour of their hosts, providing olfactory cues of identity, sex or other salient features. In birds, preen oil is a major source of body odour that differs between populations and sexes. We hypothesized that population and sex differences in preen oil chemistry reflect underlying differences in preen gland microbiota, predicting that these microbes also differ among populations and between the sexes. We further predicted that pairwise similarity in the community composition of preen gland microbiota would covary with that of preen oil chemical composition, consistent with the fermentation hypothesis for chemical recognition. We analysed preen oil chemistry and preen gland bacterial communities of song sparrows Melospiza melodia. Birds were sampled at sites for which population and sex differences in preen oil have been reported, and at a third site that has been less studied. Consistent with prior work in this system, we found population and sex differences in preen oil chemistry. By contrast, we found population differences but not sex differences in the community composition of preen gland microbes. Overall similarity in the community composition of preen gland microbiota did not significantly covary with that of preen oil chemistry. However, we identified a subset of six microbial genera that maximally correlated with preen oil composition. Although both preen gland microbiota and preen oil composition differ across populations, we did not observe an overall association between them that would implicate symbiotic microbes in mediating variation in olfactory cues associated with preen oil. Instead, certain subsets of microbes may be involved in mediating olfactory cues in birds, but experiments are required to test this.

Modifying a covarying protein-DNA interaction changes substrate preference of a site-specific endonuclease.

Identifying and validating intermolecular covariation between proteins and their DNA-binding sites can provide insights into mechanisms that regulate selectivity and starting points for engineering new specificity. LAGLIDADG homing endonucleases (meganucleases) can be engineered to bind non-native target sites for gene-editing applications, but not all redesigns successfully reprogram specificity. To gain a global overview of residues that influence meganuclease specificity, we used information theory to identify protein-DNA covariation. Directed evolution experiments of one predicted pair, 227/+3, revealed variants with surprising shifts in I-OnuI substrate preference at the central 4 bases where cleavage occurs. Structural studies showed significant remodeling distant from the covarying position, including restructuring of an inter-hairpin loop, DNA distortions near the scissile phosphates, and new base-specific contacts. Our findings are consistent with a model whereby the functional impacts of covariation can be indirectly propagated to neighboring residues outside of direct contact range, allowing meganucleases to adapt to target site variation and indirectly expand the sequence space accessible for cleavage. We suggest that some engineered meganucleases may have unexpected cleavage profiles that were not rationally incorporated during the design process.

Active site residue identity regulates cleavage preference of LAGLIDADG homing endonucleases.

LAGLIDADG homing endonucleases (meganucleases) are site-specific mobile endonucleases that can be adapted for genome-editing applications. However, one problem when reprogramming meganucleases on non-native substrates is indirect readout of DNA shape and flexibility at the central 4 bases where cleavage occurs. To understand how the meganuclease active site regulates DNA cleavage, we used functional selections and deep sequencing to profile the fitness landscape of 1600 I-LtrI and I-OnuI active site variants individually challenged with 67 substrates with central 4 base substitutions. The wild-type active site was not optimal for cleavage on many substrates, including the native I-LtrI and I-OnuI targets. Novel combinations of active site residues not observed in known meganucleases supported activity on substrates poorly cleaved by the wild-type enzymes. Strikingly, combinations of E or D substitutions in the two metal-binding residues greatly influenced cleavage activity, and E184D variants had a broadened cleavage profile. Analyses of I-LtrI E184D and the wild-type proteins co-crystallized with the non-cognate AACC central 4 sequence revealed structural differences that correlated with kinetic constants for cleavage of individual DNA strands. Optimizing meganuclease active sites to enhance cleavage of non-native central 4 target sites is a straightforward addition to engineering workflows that will expand genome-editing applications.

Unaccounted risk of cardiovascular disease: the role of the microbiome in lipid metabolism.

PURPOSE OF REVIEW: Not all of the risk of cardiovascular disease can be explained by diet and genetics, and the human microbiome, which lies at the interface of these two factors, may help explain some of the unaccounted risk. This review examines some of the well established links between the microbiome and cardiovascular health, and proposes relatively unexplored associations. RECENT FINDINGS: Byproducts of microbial metabolism are associated with health and disease: Trimethylamine N oxide is associated with atherosclerosis; whereas short-chain fatty acids are associated with decreased inflammation and increased energy expenditure. More broadly, a large number of association studies have been conducted to explore the connections between bacterial taxa and metabolic syndrome. In contrast, the relationship between the microbiome and triglycerides levels remains poorly understood. SUMMARY: We suggest that deeper understanding of the molecular mechanisms that drive linkages between the microbiome and disease can be determined by replacing 16S rRNA gene sequencing with shotgun metagenomic sequencing or other functional approaches. Furthermore, to ensure translatability and reproducibility of research findings, a combination of multiple different complementary '-omic' approaches should be employed.

omicplotR: visualizing omic datasets as compositions.

BACKGROUND: Differential abundance analysis is widely used with high-throughput sequencing data to compare gene abundance or expression between groups of samples. Many software packages exist for this purpose, but each uses a unique set of statistical assumptions to solve problems on a case-by-case basis. These software packages are typically difficult to use for researchers without command-line skills, and software that does offer a graphical user interface do not use a compositionally valid method. RESULTS: omicplotR facilitates visual exploration of omic datasets for researchers with and without prior scripting knowledge. Reproducible visualizations include principal component analysis, hierarchical clustering, MA plots and effect plots. We demonstrate the functionality of omicplotR using a publicly available metatranscriptome dataset. CONCLUSIONS: omicplotR provides a graphical user interface to explore sequence count data using generalizable compositional methods, facilitating visualization for investigators without command-line experience.

Targeted sequencing reveals expanded genetic diversity of human transfer RNAs.

Transfer RNAs are required to translate genetic information into proteins as well as regulate other cellular processes. Nucleotide changes in tRNAs can result in loss or gain of function that impact the composition and fidelity of the proteome. Despite links between tRNA variation and disease, the importance of cytoplasmic tRNA variation has been overlooked. Using a custom capture panel, we sequenced 605 human tRNA-encoding genes from 84 individuals. We developed a bioinformatic pipeline that allows more accurate tRNA read mapping and identifies multiple polymorphisms occurring within the same variant. Our analysis identified 522 unique tRNA-encoding sequences that differed from the reference genome from 84 individuals. Each individual had ~66 tRNA variants including nine variants found in less than 5% of our sample group. Variants were identified throughout the tRNA structure with 17% predicted to enhance function. Eighteen anticodon mutants were identified including potentially mistranslating tRNAs; e.g., a tRNASer that decodes Phe codons. Similar engineered tRNA variants were previously shown to inhibit cell growth, increase apoptosis and induce the unfolded protein response in mammalian cell cultures and chick embryos. Our analysis shows that human tRNA variation has been underestimated. We conclude that the large number of tRNA genes provides a buffer enabling the emergence of variants, some of which could contribute to disease.

Biasing genome-editing events toward precise length deletions with an RNA-guided TevCas9 dual nuclease.

The CRISPR/Cas9 nuclease is commonly used to make gene knockouts. The blunt DNA ends generated by cleavage can be efficiently ligated by the classical nonhomologous end-joining repair pathway (c-NHEJ), regenerating the target site. This repair creates a cycle of cleavage, ligation, and target site regeneration that persists until sufficient modification of the DNA break by alternative NHEJ prevents further Cas9 cutting, generating a heterogeneous population of insertions and deletions typical of gene knockouts. Here, we develop a strategy to escape this cycle and bias events toward defined length deletions by creating an RNA-guided dual active site nuclease that generates two noncompatible DNA breaks at a target site, effectively deleting the majority of the target site such that it cannot be regenerated. The TevCas9 nuclease, a fusion of the I-TevI nuclease domain to Cas9, functions robustly in HEK293 cells and generates 33- to 36-bp deletions at frequencies up to 40%. Deep sequencing revealed minimal processing of TevCas9 products, consistent with protection of the DNA ends from exonucleolytic degradation and repair by the c-NHEJ pathway. Directed evolution experiments identified I-TevI variants with broadened targeting range, making TevCas9 an easy-to-use reagent. Our results highlight how the sequence-tolerant cleavage properties of the I-TevI homing endonuclease can be harnessed to enhance Cas9 applications, circumventing the cleavage and ligation cycle and biasing genome-editing events toward defined length deletions.

Population Differences at MHC Do Not Explain Enhanced Resistance of Song Sparrows to Local Parasites.

Infectious disease represents an emerging threat to natural populations, particularly when hosts are more susceptible to novel parasites (allopatric) than to parasites from the local area (sympatric). This pattern could arise through evolutionary processes (host populations become adapted to their local parasites and genetically differentiated from other populations at immune-related loci) and/or through ecological interactions (host individuals develop resistance to local parasites through previous exposure). The relative importance of these candidate mechanisms remains unclear. In jawed vertebrates, genes of the major histocompatibility complex (MHC) play a fundamental role in immunity and are compelling candidates for spatially varying selection. We recently showed that song sparrows (Melospiza melodia) are more susceptible to allopatric than to sympatric strains of malaria (Plasmodium). In the current study, to determine whether population differences at MHC explain this pattern, we characterized the peptide-binding regions of MHC (classes I and II) of birds that did or did not become infected in the previous experiment. We recovered up to 4 alleles per individual at class I, implying at least 2 loci, and up to 26 alleles per individual at class II, implying at least 13 loci. Individuals with more class I alleles were less likely to become infected by Plasmodium, consistent with parasite-mediated balancing selection. However, we found no evidence for population genetic differentiation at either class of MHC, based on 36 individuals sequenced. Resistance to sympatric parasites previously described for this system likely stems from individuals' prior immune experience, not from population differentiation and locally protective alleles at MHC.

Control of catalytic efficiency by a coevolving network of catalytic and noncatalytic residues.

The active sites of enzymes consist of residues necessary for catalysis and structurally important noncatalytic residues that together maintain the architecture and function of the active site. Examples of evolutionary interactions between catalytic and noncatalytic residues have been difficult to define and experimentally validate due to a general intolerance of these residues to substitution. Here, using computational methods to predict coevolving residues, we identify a network of positions consisting of two catalytic metal-binding residues and two adjacent noncatalytic residues in LAGLIDADG homing endonucleases (LHEs). Distinct combinations of the four residues in the network map to distinct LHE subfamilies, with a striking distribution of the metal-binding Asp (D) and Glu (E) residues. Mutation of these four positions in three LHEs--I-LtrI, I-OnuI, and I-HjeMI--indicate that the combinations of residues tolerated are specific to each enzyme. Kinetic analyses under single-turnover conditions revealed that I-LtrI activity could be modulated over an ∼100-fold range by mutation of residues in the coevolving network. I-LtrI catalytic site variants with low activity could be rescued by compensatory mutations at adjacent noncatalytic sites that restore an optimal coevolving network and vice versa. Our results demonstrate that LHE activity is constrained by an evolutionary barrier of residues with strong context-dependent effects. Creation of optimal coevolving active-site networks is therefore an important consideration in engineering of LHEs and other enzymes.

The Microbiota of Breast Tissue and Its Association with Breast Cancer.

UNLABELLED: In the United States, 1 in 8 women will be diagnosed with breast cancer in her lifetime. Along with genetics, the environment contributes to disease development, but what these exact environmental factors are remains unknown. We have previously shown that breast tissue is not sterile but contains a diverse population of bacteria. We thus believe that the host's local microbiome could be modulating the risk of breast cancer development. Using 16S rRNA amplicon sequencing, we show that bacterial profiles differ between normal adjacent tissue from women with breast cancer and tissue from healthy controls. Women with breast cancer had higher relative abundances of Bacillus, Enterobacteriaceae and Staphylococcus Escherichia coli (a member of the Enterobacteriaceae family) and Staphylococcus epidermidis, isolated from breast cancer patients, were shown to induce DNA double-stranded breaks in HeLa cells using the histone-2AX (H2AX) phosphorylation (γ-H2AX) assay. We also found that microbial profiles are similar between normal adjacent tissue and tissue sampled directly from the tumor. This study raises important questions as to what role the breast microbiome plays in disease development or progression and how we can manipulate this for possible therapeutics or prevention. IMPORTANCE: This study shows that different bacterial profiles in breast tissue exist between healthy women and those with breast cancer. Higher relative abundances of bacteria that had the ability to cause DNA damage in vitro were detected in breast cancer patients, as was a decrease in some lactic acid bacteria, known for their beneficial health effects, including anticarcinogenic properties. This study raises important questions as to the role of the mammary microbiome in modulating the risk of breast cancer development.

Compositional analysis: a valid approach to analyze microbiome high-throughput sequencing data.

A workshop held at the 2015 annual meeting of the Canadian Society of Microbiologists highlighted compositional data analysis methods and the importance of exploratory data analysis for the analysis of microbiome data sets generated by high-throughput DNA sequencing. A summary of the content of that workshop, a review of new methods of analysis, and information on the importance of careful analyses are presented herein. The workshop focussed on explaining the rationale behind the use of compositional data analysis, and a demonstration of these methods for the examination of 2 microbiome data sets. A clear understanding of bioinformatics methodologies and the type of data being analyzed is essential, given the growing number of studies uncovering the critical role of the microbiome in health and disease and the need to understand alterations to its composition and function following intervention with fecal transplant, probiotics, diet, and pharmaceutical agents.

Analysis of neonatal brain lacking ATRX or MeCP2 reveals changes in nucleosome density, CTCF binding and chromatin looping.

ATRX and MeCP2 belong to an expanding group of chromatin-associated proteins implicated in human neurodevelopmental disorders, although their gene-regulatory activities are not fully resolved. Loss of ATRX prevents full repression of an imprinted gene network in the postnatal brain and in this study we address the mechanistic aspects of this regulation. We show that ATRX binds many imprinted domains individually but that transient co-localization between imprinted domains in the nuclei of neurons does not require ATRX. We demonstrate that MeCP2 is required for ATRX recruitment and that deficiency of either ATRX or MeCP2 causes decreased frequency of long-range chromatin interactions associated with altered nucleosome density at CTCF-binding sites and reduced CTCF occupancy. These findings indicate that MeCP2 and ATRX regulate gene expression at a subset of imprinted domains by maintaining a nucleosome configuration conducive to CTCF binding and to the maintenance of higher order chromatin structure.

Microbiota at Multiple Body Sites during Pregnancy in a Rural Tanzanian Population and Effects of Moringa-Supplemented Probiotic Yogurt.

The nutritional status of pregnant women is vital for healthy outcomes and is a concern for a large proportion of the world's population. The role of the microbiota in pregnancy and nutrition is a promising new area of study with potential health ramifications. In many African countries, maternal and infant death and morbidity are associated with malnutrition. Here, we assess the influence of probiotic yogurt containing Lactobacillus rhamnosus GR-1, supplemented with Moringa plant as a source of micronutrients, on the health and oral, gut, vaginal, and milk microbiotas of 56 pregnant women in Tanzania. In an open-label study design, 26 subjects received yogurt daily, and 30 were untreated during the last two trimesters and for 1 month after birth. Samples were analyzed using 16S rRNA gene sequencing, and dietary recalls were recorded. Women initially categorized as nourished or undernourished consumed similar calories and macronutrients, which may explain why there was no difference in the microbiota at any body site. Consumption of yogurt increased the relative abundance of Bifidobacterium and decreased Enterobacteriaceae in the newborn feces but had no effect on the mother's microbiota at any body site. The microbiota of the oral cavity and GI tract remained stable over pregnancy, but the vaginal microbiota showed a significant increase in diversity leading up to and after birth. In summary, daily micronutrient-supplemented probiotic yogurt provides a safe, affordable food for pregnant women in rural Tanzania, and the resultant improvement in the gut microbial profile of infants is worthy of further study.

Changes in vaginal microbiota following antimicrobial and probiotic therapy.

BACKGROUND: The composition of the vaginal microbiota is known to be important for health. When infections occur, antimicrobial therapy is often poorly efficacious. OBJECTIVE AND DESIGN: We used 16S rRNA gene sequencing to characterize changes in the bacterial microbiota following oral antimicrobial and probiotic interventions. RESULTS: While the bacterial vaginal profiles of women with vulvovaginal candidiasis were dominated by lactobacilli as in healthy women, and unchanged by therapy, Gardnerella vaginalis, Prevotella, Atopobium, Sneathia, and Megasphaera dominated the vagina of women with bacterial vaginosis (BV), and treatment with tinidazole plus Lactobacillus reuteri RC-14+L. rhamnosus GR-1 resulted in an increased relative abundance of indigenous L. iners or L. crispatus. CONCLUSIONS: The ability to restore homeostasis provides a rationale for conjoint use of probiotics with antibiotic treatment of BV.

Microbiota of human breast tissue.

In recent years, a greater appreciation for the microbes inhabiting human body sites has emerged. In the female mammary gland, milk has been shown to contain bacterial species, ostensibly reaching the ducts from the skin. We decided to investigate whether there is a microbiome within the mammary tissue. Using 16S rRNA sequencing and culture, we analyzed breast tissue from 81 women with and without cancer in Canada and Ireland. A diverse population of bacteria was detected within tissue collected from sites all around the breast in women aged 18 to 90, not all of whom had a history of lactation. The principal phylum was Proteobacteria. The most abundant taxa in the Canadian samples were Bacillus (11.4%), Acinetobacter (10.0%), Enterobacteriaceae (8.3%), Pseudomonas (6.5%), Staphylococcus (6.5%), Propionibacterium (5.8%), Comamonadaceae (5.7%), Gammaproteobacteria (5.0%), and Prevotella (5.0%). In the Irish samples the most abundant taxa were Enterobacteriaceae (30.8%), Staphylococcus (12.7%), Listeria welshimeri (12.1%), Propionibacterium (10.1%), and Pseudomonas (5.3%). None of the subjects had signs or symptoms of infection, but the presence of viable bacteria was confirmed in some samples by culture. The extent to which these organisms play a role in health or disease remains to be determined.

The distribution and functional properties of Pelizaeus-Merzbacher-like disease-linked Cx47 mutations on Cx47/Cx47 homotypic and Cx47/Cx43 heterotypic gap junctions.

GJs (gap junctions) allow direct intercellular communication, and consist of Cxs (connexins). In the mammalian central nervous system, oligodendrocytes express Cx47, Cx32 and Cx29, whereas astrocytes express Cx43, Cx30 and Cx26. Homotypic Cx47/Cx47 GJs couple oligodendrocytes, and heterotypic Cx47/Cx43 channels are the primary GJs at oligodendrocyte/astrocyte junctions. Interestingly, autosomal recessive mutations in the gene GJC2 encoding Cx47 have been linked to a central hypomyelinating disease termed PMLD (Pelizaeus-Merzbacher-like disease). The aim of the present study was to determine the cellular distribution and functional properties of PMLD-associated Cx47 mutants (I46M, G149S, G236R, G236S, M286T and T398I). Expressing GFP (green fluorescent protein)-tagged mutant versions of Cx47 in gap-junction-deficient model cells revealed that these mutants were detected at the cell-cell interface similar to that observed for wild-type Cx47. Furthermore, four of the six mutants showed no electrical coupling in both Cx47/Cx47 and Cx47/Cx43 GJ channels. These results suggest that most of the PMLD-linked Cx47 mutants disrupt Cx47/Cx47 and Cx47/Cx43 GJ function in the glial network, which may play a role in leading to PMLD symptoms.

At the crossroads of vaginal health and disease, the genome sequence of Lactobacillus iners AB-1.

Lactobacilli have long been regarded as important constituents of the healthy human vagina. Lactobacillus iners is the most frequently detected bacterial species in the vagina, but little is known about its characteristics. We report a description of the whole-genome sequence of L. iners AB-1 along with comparative analysis of published genomes of closely related strains of lactobacilli. The genome is the smallest Lactobacillus reported to date, with a 1.3-Mbp single chromosome. The genome seems to have undergone one or more rapid evolution events that resulted in large-scale gene loss and horizontal acquisition of a number of genes for survival in the vagina. L. iners may exhibit specialized adaptation mechanisms to the vaginal environment, such as an iron-sulfur cluster assembly system, and several unique σ factors to regulate gene transcription in this fluctuating environment. A potentially highly expressed homolog of a cholesterol-binding lysin may also contribute to host cell adhesion or act as a defense mechanism against other microbes. Notably, there is a lack of apparent adhesion proteins, but several cell-anchor proteins were identified and may be important for interaction with the host mucosal tissues. L. iners is widely present in healthy females as well as those suffering from bacterial vaginosis or who have undergone antimicrobial therapy, suggesting that it is an important indigenous species of the vagina.

Tapping natural reservoirs of homing endonucleases for targeted gene modification.

Homing endonucleases mobilize their own genes by generating double-strand breaks at individual target sites within potential host DNA. Because of their high specificity, these proteins are used for "genome editing" in higher eukaryotes. However, alteration of homing endonuclease specificity is quite challenging. Here we describe the identification and phylogenetic analysis of over 200 naturally occurring LAGLIDADG homing endonucleases (LHEs). Biochemical and structural characterization of endonucleases from one clade within the phylogenetic tree demonstrates strong conservation of protein structure contrasted against highly diverged DNA target sites and indicates that a significant fraction of these proteins are sufficiently stable and active to serve as engineering scaffolds. This information was exploited to create a targeting enzyme to disrupt the endogenous monoamine oxidase B gene in human cells. The ubiquitous presence and diversity of LHEs described in this study may facilitate the creation of many tailored nucleases for genome editing.