(P40-06-26) Determining Associations Between Oral Cysteine Exposure and Bacteria That Produce Genotoxic Hydrogen Sulfide via Cysteine Metabolism

Objectives: In the United States, the sulfur containing amino acid cysteine is abundant in animal protein and cereal grains and is added to foods to increase shelf life and enhance texture. Cysteine metabolizing bacteria (CMB) have been previously associated with colorectal cancer (CRC) and produce genotoxic hydrogen sulfide (H₂S). Consumption of red and processed meat leads to proportional increases of H₂S in the stool. However, it is unknown how other dietary sources of cysteine impact the abundance of CMB and microbial composition of the gut and the oral cavity.

Methods: Fifty adults aged 18-80 were enrolled who had not taken antibiotics in the last 6 months, had normal GI transit, and no history of organ transplantation. Participants took part in two 24-hour dietary recalls, which were analyzed using the National Data System for Research (NDSR). Saliva and stool samples were collected, and oral samples were enriched for microbial DNA. DNA was extracted, sequenced using a NovaSeq X sequencing system, and reads were analyzed for pathway abundance using MetaPhlAn and HUMAnN. Reads were then assembled using MegaHit, contigs were binned using MetaBat2, and genes for cysteine metabolism were assessed using Hidden Markov Model databases. Stool functional analyses were performed in anaerobic SIM medium, which turns black in the presence of H₂S. 

Results: The mean age of the 40 participants who completed all parts of the study was 31, with a mean BMI of 23.8. Participants consumed an average of 1852 kcals daily. Total cystine intake averaged 1.18 total grams per day. At 6 hours, 50% of the stool samples undergoing functional analysis in anaerobic SIM media were black, highlighting differences in microbial H₂S production among participants.  Pathways for immunomodulatory isoprenoid biosynthesis in saliva and ceramide degradation in the stool were associated with cysteine intake prior to correction for multiple comparisons.

Conclusions: Future work will explore associations between dietary cysteine intake, genes for cysteine metabolism in oral and gut microbiomes, and functional H₂S production by participant microbiomes. If dietary cysteine leads to the production of H₂S or other immune-stimulating pathways, this could be a potential biomarker that could be used to assess the risk of CRC. 

Funding Sources: Ralph W. and Grace M. Showalter Research Trust