Abstract 3342: The colorectal cancer associated microbe Odoribacter splanchnicus produces genotoxic hydrogen sulfide via cysteine metabolism

The proinflammatory and genotoxic properties of hydrogen sulfide (H2S) have been implicated as an environmental trigger of colonic disease. Recent evidence links sulfidogenic bacteria capable of metabolizing organic sulfur substrates, like dietary sulfur amino acids and taurine conjugated bile acids, with increased colorectal cancer (CRC) and colitis risk as a consequence of a “western” type diet. Our previous work demonstrates that the sulfidogenic bacterium, Odoribacter splanchnicus, is a significant indicator of CRC in African Americans who are at higher risk of CRC incidence and death than other races and ethnicities in the United States. This observation has been corroborated by several recent studies that observed O. splanchnicus to be associated with CRC and colitis-induced CRC. While previous work has reported that O. splanchnicus may be sulfidogenic, its method of H2S production has yet to be characterized. Production of H2S by O. splanchnicus was confirmed in vitro using sulfide indole and motility media, and varying sulfur substrates. A predictive metabolic analysis of 74 O. splanchnicus metagenomes taken from publicly available CRC cohorts did not reveal complete pathways commonly associated with H2S production, however genes for a polar amino acid transporter were revealed. Therefore, a genomic analysis was performed revealing that O. splanchnicus harbors the sulfidogenic genes tryptophanase (tnaA) and cystathionine-β-synthase. The enzyme TnaA has been previously shown in Eschericia coli to produce indole or H2S from tryptophan or cysteine, respectively. However, the E. coli amino acid sequence shares only 44.85% identity with that from O. splanchnicus, thus heterologous protein expression of recombinant tnaA was performed to determine enzyme functionality. Enzyme kinetics and pH optimum were obtained using the spectrophotometric BiCl3 assay, which turns black in presence of H2S. Functional analysis of recombinant purified TnaA revealed bifunctionality of the enzyme with positive tryptophanase and cysteine desulfhydrase activity with differing substrates. Cysteine desulfhydrase activity resulted in consumption of L-cysteine with subsequent production of H2S and pyruvate. Thus, TnaA is a functional enzyme that warrants further evaluation as possibly relating to sulfidogenic O. splanchnicus being linked to CRC and colitis-associated CRC.

Citation Format: Patricia G. Wolf, Vladimir Kolosslov, Zhichao Zhou, Lindsey Ly, Heidi Doden, Saravanan Devendran, Adam M. Breister, Luke Lucio, Paige Polak, Sarah Matatov, Karthik Anantharaman, Jason M. Ridlon, Rex Gaskins. The colorectal cancer associated microbe Odoribacter splanchnicus produces genotoxic hydrogen sulfide via cysteine metabolism [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3342.

The 'in vivo lifestyle' of bile acid 7α-dehydroxylating bacteria: comparative genomics, metatranscriptomic, and bile acid metabolomics analysis of a defined microbial community in gnotobiotic mice

The formation of secondary bile acids by gut microbes is a current topic of considerable biomedical interest. However, a detailed understanding of the biology of anaerobic bacteria in the genus Clostridium that are capable of generating secondary bile acids is lacking. We therefore sought to determine the transcriptional responses of two prominent secondary bile acid producing bacteria, Clostridium hylemonae and Clostridium hiranonis to bile salts (in vitro) and the cecal environment of gnotobiotic mice. The genomes of C. hylemonae DSM 15053 and C. hiranonis DSM 13275 were closed, and found to encode 3,647 genes (3,584 protein-coding) and 2,363 predicted genes (of which 2,239 are protein-coding), respectively, and 1,035 orthologs were shared between C. hylemonae and C. hiranonis. RNA-Seq analysis was performed in growth medium alone, and in the presence of cholic acid (CA) and deoxycholic acid (DCA). Growth with CA resulted in differential expression (>0.58 log₂FC; FDR < 0.05) of 197 genes in C. hiranonis and 118 genes in C. hylemonae. The bile acid-inducible operons (bai) from each organism were highly upregulated in the presence of CA but not DCA. We then colonized germ-free mice with human gut bacterial isolates capable of metabolizing taurine-conjugated bile acids. This consortium included bile salt hydrolase-expressing Bacteroides uniformis ATCC 8492, Bacteroides vulgatus ATCC 8482, Parabacteroides distasonis DSM 20701, as well as taurine-respiring Bilophila wadsworthia DSM 11045, and deoxycholic/lithocholic acid generating Clostridium hylemonae DSM 15053 and Clostridium hiranonis DSM 13275. Butyrate and iso-bile acid-forming Blautia producta ATCC 27340 was also included. The Bacteroidetes made up 84.71% of 16S rDNA cecal reads, B. wadsworthia, constituted 14.7%, and the clostridia made up <.75% of 16S rDNA cecal reads. Bile acid metabolomics of the cecum, serum, and liver indicate that the synthetic community were capable of functional bile salt deconjugation, oxidation/isomerization, and 7α-dehydroxylation of bile acids. Cecal metatranscriptome analysis revealed expression of genes involved in metabolism of taurine-conjugated bile acids. The in vivo transcriptomes of C. hylemonae and C. hiranonis suggest fermentation of simple sugars and utilization of amino acids glycine and proline as electron acceptors. Genes predicted to be involved in trimethylamine (TMA) formation were also expressed.