The mean microbial concentration after 45 d of supplementation of humic acids increased 14% in the essential groups (P < 0.01), 28% (NS) in the individual pioneer groups and 41% (P < 0.002) in the individual substantial groups. The accidental bacterial groups with initially marginal concentrations demonstrated no response to humic acids.

The response to humic acids of single bacterial groups was principally the same as in all functional sets of substantial bacteria. The concentrations of most bacterial groups within essential (2 of 3) pioneer (3 of 4) and individual substantial groups (19 of 28) increased in rates of 20% to 60%. In most cases, the increase was observed already at day 10 and continued to day 45. In groups with comparatively low initial mean concentrations (Ebac1790, Cdif198, Chis150, Eram997, Lab158, Veil223) an increase could be higher than 70% and up to 96%, but the contribution of these groups to the overall bacterial numbers was relatively low. Only the concentrations of bacteria detected with Bac303 (Bacteroides) and Myc657 (mycolic acid-containing Actinomycetes) FISH probes decreased under humic acids supplementation, but was statistically not significant, because of the high variance and low number of probands.

The increase in concentrations of microbiota was caused by preexisting groups, and not due to emerging new microorganisms. The individual microbial profile remained constant. In none of the test persons did the ratio of positive/negative individual groups change more than 5%.

Humic acid supplementation did not affect microbial diversity. Mean percent of substantial individual bacterial groups positive for bacteria for each person was nearly the same over time with 72%; 74%; 76%; 72% at the control days accordingly.

The patterns in distribution of single bacterial groups over the stool cylinder differed depending on the species but remained the same in the mucus close transient zone and in the center of the fecal cylinder regardless of humic acids supplementation.