Most probable number enumeration of H2-utilizing acetogenic bacteria from the digestive tract of animals and man

A method is proposed that allows the enrichment and most probable number estimation of H2/CO2-utilizing acetogenic bacteria. It is based on the difference in acetate production for serial dilutions incubated under either a test H2/CO2 (4:1), or a control N2/CO2 (4:1) headspace atmosphere. A nutritionally non-selective medium was used, containing bromoethane-sulfonic acid as inhibitor of methanogenic archaea and 10% pre-incubated clarified rumen fluid. Acetogenic bacteria were enumerated in rumen and hindgut contents of animals and in human feces. They ranged from below 102 to above 108 per gram wet weight gut content and their population levels were the highest in the absence of methanogenesis. The method described therein should prove useful to better understand the diversity and ecological importance of dominant gut acetogens.