H2/CO2 metabolism in acetogenic bacteria isolated from the human colon

The present work reports on autotrophic metabolism in four H2/CO2-utilizing acetogenic bacteria isolated from the human colon (two Clostridium species, one Streptococcus species, and Ruminococcus hydrogenotrophicus). H2/CO2-utilization by these human acetogenic strains occurred during both exponential and stationary phases of growth. Acetate was the major metabolite produced by all isolates following the stoichiometric equation of reductive acetogenesis. Furthermore, the ability of these acetogenic bacteria to incorporate 13CO2 into acetate in the presence of H2 in the gas phase demonstrated the utilization of the reductive pathway of acetate formation from a one-carbon compound. Energy conservation during the autotrophic metabolism in colonic acetogens might involve sodium- or proton-chemiosmotic mechanisms. A sodium-dependent ATP generation was only demonstrated in one Clostridium species, whereas sodium could be replaced by potassium in other strains. The minimal thresholds of hydrogen uptake were determined and varied from 1100 to 3680 ppm depending on the acetogenic strain. These values appeared higher than those measured for the colonic methanogen,Methanobrevibacter smithii.