After Metagenomics: Drug Discovery from Microbial Genomes
This year’s Nobel prize in medicine went to William C. Campbell and Satoshi Omura for discovering new drug Avermectin from microbial organisms. They did their work long before the era of low-cost sequencing.
Hopefully, high-throughput sequencing will lead to many more success stories like theirs. The following two papers are excellent reviews on the status of the field.
Small molecules from the human microbiota
Two developments in distinct fields are converging to create interest in discovering small molecules from the human microbiome. First, the use of genomics to guide natural product discovery has led to the unexpected discovery of numerous biosynthetic gene clusters in genomes of the human microbiota. Second, the microbiome research community is moving from a focus on whos there? to what are they doing? with an accompanying emphasis on understanding microbiota-host interactions at the level of molecular mechanism. This merger has sparked a concerted hunt for the mediators of microbe-host and microbe-microbe interactions, including microbiota-derived small molecules.
ADVANCES
Numerous small molecules are known that are produced by the human microbiota. The microbiota-derived ribosomally synthesized, posttranslationally modified peptides (RiPPs) include widely distributed lantibiotics and microcins; these molecules have narrow-spectrum activity and are presumptive mediators of interactions among closely related species. Another notable RiPP is Escherichia coli heat-stable enterotoxin, a guanylate cyclase 2C agonist from which the recently approved gastrointestinal motility drug linaclotide was derived. Fewer amino acid metabolites are synthesized by the microbiota, but they are produced at very high levels that vary widely among individuals (e.g., indoxyl sulfate at 10 to 200 mg/day). Gut bacterial species convert common dietary amino acids into distinct end products, such as tryptophan to indoxyl sulfate, indole propionic acid, and tryptamineindicating that humans with the same diet but different gut colonists can have widely varying gut metabolic profiles. Microbially produced oligosaccharides differ from other natural products because they are cell-associated (i.e., nondiffusible) and because many more biosynthetic loci exist for them than for other small molecule classes. Well-characterized examples, such as Bacteroides polysaccharide A, show that oligosaccharides may not simply play a structural role or mediate adhesion; rather, they can be involved in highly specific ligand-receptor interactions that result in immune modulation. Similarly, the (glyco)lipids ?-galactosylceramide and mycolic acid can play roles in immune signaling. The most prominent microbiota-derived terpenoids are microbial conversion products of the cholic acid and chenodeoxycholic acid in host bile. These secondary bile acids can reach high concentration (mM) in the gut and vary widely in composition among individuals. Several canonical virulence factors from pathogens are derived from nonribosomal peptides (NRPs) and polyketides (PKs), but less is known about NRPs and PKs from the commensal microbiota. A recent computational effort has identified ~14,000 biosynthetic gene clusters in sequenced genomes from the human microbiota, 3118 of which were present in one or more of the 752 metagenomic sequence samples from the NIH Human Microbiome Project. Nearly all of the gene clusters that were present in >10% of the samples from the body site of origin are uncharacterized, highlighting the potential for identifying the molecules they encode and studying their biological activities.
OUTLOOK
There are two central challenges facing the field. The first is to distinguish, from among thousands of microbiota-derived molecules, which ones drive a key phenotype at physiologically relevant concentrations. Second, which experimental systems are appropriate for testing the activity of an individual molecule from a complex milieu? Meeting these challenges will require developing new computational and experimental technologies, including a capacity to identify biosynthetic genes and predict the structure and target of their biological activity, and systems in which germ-free mice are colonized by mock communities that differ only by the presence or absence of a biosynthetic gene cluster.
Commensal organisms of the human microbiota produce many diverse small molecules with an equally diverse array of targets that can exacerbate or modulate immune responses and other physiological functions in the host. Several act as antibacterials to remove competing organisms, but many other products have unknown targets and effects on commensals and the host.
Insights into secondary metabolism from a global analysis of prokaryotic biosynthetic gene clusters
Although biosynthetic gene clusters (BGCs) have been discovered for hundreds of bacterial metabolites, our knowledge of their diversity remains limited. Here, we used a novel algorithm to systematically identify BGCs in the extensive extant microbial sequencing data. Network analysis of the predicted BGCs revealed large gene cluster families, the vast majority uncharacterized. We experimentally characterized the most prominent family, consisting of two subfamilies of hundreds of BGCs distributed throughout the Proteobacteria; their products are aryl polyenes, lipids with an aryl head group conjugated to a polyene tail. We identified a distant relationship to a third subfamily of aryl polyene BGCs, and together the three subfamilies represent the largest known family of biosynthetic gene clusters, with more than 1,000 members. Although these clusters are widely divergent in sequence, their small molecule products are remarkably conserved, indicating for the first time the important roles these compounds play in Gram-negative cell biology.
