No abstract

SUMMARY In complex biological systems, small molecules often mediate microbe-microbe and microbe-host interactions. Using a systematic approach, we identified 3,118 small molecule biosynthetic gene clusters (BGCs) in genomes of human-associated bacteria and studied their representation in 752 metagenomic samples from the NIH Human Microbiome Project. Remarkably, we discovered that BGCs for a class of antibiotics in clinical trials, thiopeptides, are widely distributed in genomes and metagenomes of the human microbiota. We purified and solved the structure of a new thiopeptide antibiotic, lactocillin, from a prominent member of the vaginal microbiota. We demonstrate that lactocillin has potent antibacterial activity against a range of Gram-positive vaginal pathogens, and we show that lactocillin and other thiopeptide BGCs are expressed in vivo by analyzing human metatranscriptomic sequencing data. Our findings illustrate the widespread distribution of small-molecule-encoding BGCs in the human microbiome, and they demonstrate the bacterial production of drug-like molecules in humans.

Developments in the use of genomics to guide natural product discovery and a recent emphasis on understanding the molecular mechanisms of microbiota-host interactions have converged on the discovery of natural products from the human microbiome. Here, we review what is known about small molecules produced by the human microbiota. Numerous molecules representing each of the major metabolite classes have been found that have a variety of biological activities, including immune modulation and antibiosis. We discuss technologies that will affect how microbiota-derived molecules are discovered in the future, and consider the challenges inherent in finding specific molecules that are critical for driving microbe-host and microbe-microbe interactions and their biological relevance.

More than 100 cyclic peptides harboring heterocyclized residues are known from marine ascidians, sponges and different genera of cyanobacteria. Here, we report an assembly line responsible for the biosynthesis of these diverse peptides, now called cyanobactins, both in symbiotic and free-living cyanobacteria. By comparing five new cyanobactin biosynthetic clusters, we could produce the prenylated antitumor preclinical candidate, trunkamide, in E. coli culture using genetic engineering.

Summary Several recent studies describe the influence of the gut microbiota on host brain and behavior. However, the mechanisms responsible for microbiota-nervous system interactions are unknown. Using a combination of genetics, biochemistry, and crystallography, we identify and characterize two phylogenetically distinct enzymes found in the human microbiome that decarboxylate tryptophan to form the β-arylamine neurotransmitter tryptamine. Although this enzymatic activity is exceedingly rare among bacteria more broadly, analysis of the Human Microbiome Project data demonstrates that at least 10% of the human population harbors at least one bacterium encoding a tryptophan decarboxylase in their gut community. Our results uncover a previously unrecognized enzymatic activity that can give rise to host-modulatory compounds and suggests a potential direct mechanism by which gut microbiota can influence host physiology, including behavior.

A large family of cytotoxic cyclic peptides exemplified by the patellamides has been isolated from ascidians harboring the obligate cyanobacterial symbionts Prochloron spp.. Genome sequence analysis of these symbionts has revealed that Prochloron spp. synthesize patellamides by a ribosomal pathway. To understand how this pathway evolved to produce a suite of related metabolites, we analyzed 46 prochloron-containing ascidians from the tropical Pacific Ocean for the presence of patellamide biosynthetic genes and taxonomic markers. Here, we show that Prochloron spp. generate a diverse library of patellamides using small, hypervariable cassettes within a conserved genetic background. Each symbiont strain contains a single pathway, and mixtures of symbionts within ascidians lead to the accumulation of chemical libraries. We used this information to engineer the production of a new cyclic peptide in Escherichia coli, thereby demonstrating the power of comparative analysis of closely related symbiotic pathways to direct the genetic synthesis of new molecules.

The small intestine contains CD4+CD8αα+ double-positive intraepithelial T lymphocytes (DP IELs), which originate from intestinal CD4+ T cells through downregulation of the transcription factor ThpoK and have regulatory functions. DP IELs are absent in germ-free mice, suggesting that their differentiation depends on microbial factors. We found that DP IEL numbers in mice varied in different vivaria, correlating with the presence of Lactobacillus reuteri. This species induced DP IELs in germ-free mice and conventionally-raised mice lacking these cells. L. reuteri did not shape DP-IEL-TCR repertoire, but generated indole derivatives of tryptophan that activated the aryl-hydrocarbon receptor in CD4+ T cells, allowing ThPOK downregulation and differentiation into DP IELs. Thus, L. reuteri together with a tryptophan-rich diet can reprogram intraepithelial CD4+ T cells into immunoregulatory T cells.

Defining the functional status of host-associated microbial ecosystems has proven challenging owing to the vast number of predicted genes within the microbiome and relatively poor understanding of community dynamics and community-host interaction. Metabolomic approaches, in which a large number of small molecule metabolites can be defined in a biological sample, offer a promising avenue to 'fingerprint' microbiota functional status. Here, we examined the effects of the human gut microbiota on the fecal and urinary metabolome of a humanized (HUM) mouse using an optimized ultra performance liquid chromatography-mass spectrometry-based method. Differences between HUM and conventional mouse urine and fecal metabolomic profiles support host-specific aspects of the microbiota's metabolomic contribution, consistent with distinct microbial compositions. Comparison of microbiota composition and metabolome of mice humanized with different human donors revealed that the vast majority of metabolomic features observed in donor samples are produced in the corresponding HUM mice, and individual-specific features suggest 'personalized' aspects of functionality can be reconstituted in mice. Feeding the mice a defined, custom diet resulted in modification of the metabolite signatures, illustrating that host diet provides an avenue for altering gut microbiota functionality, which in turn can be monitored via metabolomics. Using a defined model microbiota consisting of one or two species, we show that simplified communities can drive major changes in the host metabolomic profile. Our results demonstrate that metabolomics constitutes a powerful avenue for functional characterization of the intestinal microbiota and its interaction with the host.