Bacterial strains from human feces that reduce CO2 to acetic acid

We used dilutions of fecal suspensions from a human volunteer to enrich cultures for bacteria that reduce CO2 to acetate in the colon. The soluble enrichment substrates used were glucose, methanol, formate, and vanillate, which were used with a gas phase that contained 80% N2 and 20% CO2. The gaseous enrichment substrates used were 80% H2-20% CO2 and 50% CO-50% CO2. We isolated three different strains that produced acetate from CO2. One strain produced acetate from methanol, vanillate, H2-CO2, glucose, and other sugars. The other two strains did not form acetate from methanol or vanillate. Both of the latter strains formed acetate from glucose and other sugars, but only one of these strains formed acetate from H2-CO2. Both of these strains cometabolized formate. However, none of the enrichment cultures or pure cultures used CO or formate as a substrate for growth. The two strains that produced acetate from H2 and CO2 grew slowly when the gases alone were used as substrates, but they rapidly cometabolized H2 and CO2 when they were grown with organic substrates. The ability of all of the strains to produce acetate from CO2 and/or other one-carbon precursors was verified by determining the radioactivity of the methyl and carboxyl groups of the acetate formed after growth with 14CO2 or other radioactively labeled one-carbon precursors.