Dietary patterns, particularly the consumption of a red meat-rich or western-type diet, are associated with an increased risk of cardiovascular disease (CVD).1–5 L-Carnitine is a nutrient abundant in red meat that both associates with CVD event risks in humans with high TMAO, and enhances atherosclerosis in animal models.6 Dietary L-carnitine is converted by gut microbes into trimethylamine (TMA), and then absorbed and converted in the host to TMAO by liver flavin-containing monooxygenases (FMOs), especially FMO3.7 Interestingly, chronic dietary patterns greatly influence the capacity of the gut microbial community to convert L-carnitine  TMA, with vegetarians and vegans (in contrast to omnivores) showing a minimal capacity to generate TMA (and thus TMAO) from dietary carnitine. The microbial gbu gene cluster links microbial L-carnitine catabolism to red meat diet-enhanced cardiovascular disease risk

L-Carnitine, a nutrient abundant in red meat, is metabolized by gut microbiota into trimethylamine (TMA), a precursor of the atherothrombotic metabolite trimethylamine N-oxide (TMAO). Gut microbiota conversion of carnitine into TMA occurs in omnivores, but far less so in vegetarians or vegans. Previous studies revealed the L-carnitineTMA microbial transformation occurs via -butyrobetaine (BB), and while multiple microbes convert dietary L-carnitine into BB, relatively few have capacity to transform BB into TMA. The heightened risk for cardiovascular disease (CVD) observed with omnivores compared to vegetarians/vegans is thought to be linked, in part, to gut microbiota-dependent TMAO generation from carnitine. Despite the central role of BB in this transformation, neither the relationship of BB to CVD risks, nor the gut microbial genes responsible for BBTMA transformation in omnivores are known. Herein we show circulating BB is strongly associated with incident risks of heart attack, stroke or death amongst patients (n=2,918) undergoing elective cardiac evaluations, but only amongst those with elevated TMAO, suggesting the heightened risks associated with BB may correspond to its ability to be metabolized into TMAO. In animal studies, ingestion of BB, like carnitine, raised TMAO levels and enhanced thrombus formation in vivo. In vitro human fecal culturing studies, and parallel studies in germ-free mice with defined synthetic communities, show the introduction of Emergencia timonensis, which can metabolize BB into TMA, is sufficient to impart polymicrobial communities with the capacity to complete the carnitineBBTMA transformation, elevate TMAO levels, and enhance thrombosis potential in recipients following arterial injury. RNAseq analyses of E. timonensis revealed a 6 gene cluster, herein named gamma-butyrobetaine utilization gene cluster (gbu), which is uniquely upregulated in response to BB. Subsequent combinatorial cloning and functional expression studies identified a minimum contingent of 4 genes (gbuA, gbuB, gbuC, and gbuE) that are necessary and sufficient to recapitulate the conversion of BB TMA when co-expressed in E. coli. Finally, metagenomics analysis of human fecal communities from subjects (n=50) enrolled in a randomized cross-over design diet intervention study comparing red meat versus alternative dietary protein sources revealed that the abundance of gbuA is both significantly correlated with plasma TMAO and a red-meat rich diet. The present studies reveal a novel bacterial gene cluster whose abundance is regulated by consumption of red-meat, critical to the transformation of the dietary carnitine intermediate BB into TMAO in hosts, and contributes to both heightened thrombosis potential in vivo and CVD risk.