Increase in colonic methanogens and total anaerobes in aging rats.

Methanogens are present in the colons of our local Wistar rat colony. We studied the changes in concentrations of their fecal methanogenic and nonmethanogenic bacteria with age as a model of the development of these communities in humans. We found that the predominant methanogen in the rats is a Methanobrevibacter species. The log of the concentration of total anaerobes increased from 9.8/g (dry weight) at 3.0 weeks of age (shortly after weaning) to 10.7/g (dry weight) at 96 weeks (shortly before the end of the life span). In contrast, the log concentration of methanogens increased from 5.5 to 9/g (dry weight) during the same time period. Therefore, methanogens increased as a percentage of the total anaerobes from 0.005% at 3.0 weeks to 2.0% at 96 weeks. About 12 doublings of the methanogenic population and 3.3 doublings of the nonmethanogenic population took place from weaning until death. The slow increase in the ratio of methanogens to total anaerobes with age followed the same pattern in cecal contents as found in feces. There were no relationships between animal weights or fecal outputs and the increase in total anaerobe and methanogen concentrations in feces. A possible explanation for the slow increase in the Methanobrevibacter species in Wistar rats with age is a gradual shifting of the use of electrons from the reduction of CO2 to acetate by acetogens to the reduction of CO2 to CH4. The results provide the first evidence for an age-related change in the nonmethanogenic bacteria of the colon and supporting microbiological evidence for physiological studies that have shown age-related increases in colonic methane production in humans.