ABSTRACT
The dietary methylamines choline, carnitine and phosphatidylcholine are used by the gut microbiota to produce a range of metabolites, including trimethylamine (TMA). However, little is known about the use of trimethylamine N-oxide (TMAO) by this consortium of microbes. A feeding study using deuterated TMAO in C57BL6/J mice demonstrated microbial conversion of TMAO to TMA, with uptake of TMA into the bloodstream and its conversion to TMAO. Antibiotic-treated mice lacked microbial activity necessary to convert TMAO to TMA, with deuterated TMAO being taken up directly into the bloodstream. In batch-culture fermentation systems inoculated with human faeces, growth of Enterobacteriaceae was stimulated in the presence of TMAO. Human-derived faecal and caecal bacteria (n = 66 isolates) were screened on solid and liquid media for their ability to use TMAO, with metabolites in spent media analysed by 1H-NMR. As with the in vitro fermentation experiments, TMAO stimulated the growth of Enterobacteriaceae; these bacteria produced most TMA from TMAO. Caecal/small intestinal isolates of Escherichia coli produced more TMA from TMAO than their faecal counterparts. Lactic acid bacteria produced increased amounts of lactate when grown in the presence of TMAO, but did not produce large amounts of TMA. In summary, TMA can be produced by the gut microbiota (predominantly Enterobacteriaceae) from TMAO. This TMA is then taken up by the host and converted back to TMAO. That is, metabolic retroconversion occurs. In addition, TMAO influences microbial metabolism depending on isolation source and taxon of gut bacterium.
Footnotes
Abbreviations: AUC, area under the curve; DMA, dimethylamine; FISH, fluorescence in situ hybridization; FMO, flavin mono-oxygenase; MMA, monomethylamine; PC, phosphatidylcholine; TMA, trimethylamine; TMAO, trimethylamine N-oxide; UPLC–MS/MS, ultra-performance liquid chromatography–tandem mass spectrometry