Gastrointestinal microbes respond to biochemical metabolites that coordinate their behaviors. Here, we demonstrate that bacterial indole functions as a multifactorial mitigator of Klebsiella grimontii and Klebsiella oxytoca pathogenicity. These closely related microbes produce the enterotoxins tilimycin and tilivalline; cytotoxin-producing strains are the causative agent of antibiotic-associated hemorrhagic colitis and have been associated with necrotizing enterocolitis of premature infants. We demonstrate that carbohydrates induce cytotoxin synthesis while concurrently repressing indole biosynthesis. Conversely, indole represses cytotoxin production. In both cases, the alterations stemmed from differential transcription of npsA and npsB, key genes involved in tilimycin biosynthesis. Indole also enhances conversion of tilimycin to tilivalline, an indole analog with reduced cytotoxicity. In this context, we established that tilivalline, but not tilimycin, is a strong agonist of pregnane X receptor (PXR), a master regulator of xenobiotic detoxification and intestinal inflammation. Tilivalline binding upregulated PXR-responsive detoxifying genes and inhibited tubulin-directed toxicity. Bacterial indole, therefore, acts in a multifunctional manner to mitigate cytotoxicity by Klebsiella spp.: suppression of toxin production, enhanced conversion of tilimycin to tilivalline, and activation of PXR. IMPORTANCE The human gut harbors a complex community of microbes, including several species and strains that could be commensals or pathogens depending on context. The specific environmental conditions under which a resident microbe changes its relationship with a host and adopts pathogenic behaviors, in many cases, remain poorly understood. Here, we describe a novel communication network involving the regulation of K. grimontii and K. oxytoca enterotoxicity. Bacterial indole was identified as a central modulator of these colitogenic microbes by suppressing bacterial toxin (tilimycin) synthesis and converting tilimycin to tilivalline while simultaneously activating a host receptor, PXR, as a means of mitigating tissue cytotoxicity. On the other hand, fermentable carbohydrates were found to inhibit indole biosynthesis and enhance toxin production. This integrated network involving microbial, host, and metabolic factors provides a contextual framework to better understand K. oxytoca complex pathogenicity.

Center for Environmental Sciences and Engineering University of Connecticut Storrs Connecticut USA

Department of Cell Biology and Genetics Faculty of Science Palacky University Olomouc Czech Republic

Department of Genetics and Genome Sciences UConn Health Farmington Connecticut USA

Department of Immunology UConn Health Farmington Connecticut USA

Department of Medicine Molecular Pharmacology and Genetics Albert Einstein College of Medicine Bronx New York USA

Department of Medicine UConn Health Farmington Connecticut USA

Department of Microbiology and Immunology Drexel University Philadelphia Pennsylvania USA

Department of Molecular Biology and Biophysics UConn Health Farmington Connecticut USA

Department of Pediatrics UConn Health Farmington Connecticut USA

Division of Neonatology Connecticut Children's Medical Center Hartford Connecticut USA

Institute of Molecular Biosciences University of Grazgrid 5110 5 BioTechMed Graz Graz Austria

The Jackson Laboratory for Genomic Medicine Farmington Connecticut USA

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Microbial Metabolites as Ligands to Xenobiotic Receptors: Chemical Mimicry as Potential Drugs of the Future