Probiotics modulate the composition of the intestinal microflora. The survival of ingested probiotics in different parts of the gastrointestinal tract differs between strains. As a result of their concentration in the lumen, they contribute to transient modulation of the microflora ecology, at least during the period of intake. This specific change may be seen in the GI tract for a few days after the start of consumption of the probiotic preparation, depending on the dosage of the strain in question. Results show that with regular consumption, the bacteria temporarily colonise the lower intestine. Once consumption stops, the number of probiotic microorganisms quickly falls. This applies to all probiotic supplements available in the market today.
Many studies have demonstrated significant shifts in bacterial counts in human faeces following consumption of specific probiotic strains, generally resulting in increased numbers of health-promoting genera (Lactobacillus and Bifidobacterium) and decreased numbers of potentially harmful ones (such as several strains of Clostridum, Enterococcus and Candida). These studies, however, reflect the bacteriological situation in faecal matter only and do not provide an accurate picture of the situation in different parts of the gastrointestinal tract or in the mucosal layer of the gut. Furthermore, many species of intestinal bacteria from faecal samples cannot be cultured on specific plates.
Probiotic bacteria modulate the metabolic activity of the gut flora. Probiotics, being able to lower the pH in the intestinal tract, may thus be able to interfere with the enzymatic activity of the flora.