Others, facultative anaerobes such as E. coli , Streptococcus , etc., are less common and are thus sub-dominant. Only the dominant bacteria can play a role in the host's physiology. In man, the sub-dominant population of facultative anaerobic bacteria has no effect as long as the population level is kept in check. A bacterial strain does not develop and proliferate unchecked in the intestinal ecosystem as it does in a pure culture. Interactions between the various species in the lumen can result in two strains mutually promoting one another, a given bacterial population being totally or partially inhibited, and genetic information being exchanged by DNA transfer.
Interactions are of vital importance to keeping the ecosystem stable.
Along with larger size there is the secretion of a considerable surplus of mucin in the faeces. According to some authors, the excess secretion is due to the absence of intestinal microflora breaking down the mucin. In the conventional animal, mucin synthesis and secretion is balanced by loss due to lubrication and bacterial degradation. Disagreeing with this, in 1990 Szentkuti et al. demonstrated that the absence of intestinal microflora in the axenic rat reduced the thickness and density of the pre-epithelial mucous layer. Given the absence of mucolytic enzymes breaking down the mucous layer, it should, at a constant rate of secretion, be thicker and more compact.