Bifidobacteria mainly inhabit the intestines of humans and animals. It is almost undetectable in the upper part of the small intestine, up to 103-105/g in the lower part of the small intestine, and up to 108-1012/g in the feces of the large intestine. in addition, bifidobacteria also reside in the oral cavity and the vagina. Bifidobacteria are found in the intestinal tracts of many animals such as chickens, pigs, dogs, mice, rats, gophers, wolves, monkeys, rabbits, cows, sheep, horses, and even honeybees, and most of them are found in dominant flora.
Fetal feces of newborns are sterile, but enterobacteria, fine cocci, streptococci, etc., can be detected a few hours after birth, and their numbers are increasing. 24 hours later, E. coli predominates, reaching 108-1011/g (wet stool). The number of bifidobacteria detected on the second postnatal day was not high, but increased very rapidly; they began to predominate on the fourth to fifth day, when the number of Enterobacteriaceae, which were the first bacilli to appear, declined, decreasing to 106-107/g on about the sixth day (Yoshioka 1983), and a colony in which bifidobacteria predominated absolutely was established on the sixth to eighth day. In the feces of breast-fed children, bifidobacteria accounted for 98% of the total bacterial population and could reach 109 to 1011/g.
The general explanation for this phenomenon is that, soon after birth, a dominant flora of aerobic and partially anaerobic bacteria led by Escherichia coli first appeared in the intestines of newborn infants, which consumed oxygen in the intestinal tract and lowered the local redox potential (Eh) of the intestinal tract, which promoted the development of the bifidobacteria. colonization of the intestine by anaerobic bacteria such as bifidobacteria. Generally, in the first 6-8 days after birth, the intestinal flora with bifidobacteria as the absolute dominant group is established in the intestinal tract. They ferment sugars, produce large amounts of acetic acid and lactic acid, inhibit the growth and reproduction of potentially pathogenic E. coli, etc., and achieve a balance of intestinal microecology. This balanced intestinal flora is important for the nutritional metabolism, infection resistance and growth and development of infants and young children. Bifidobacterium bifidum still plays an important physiological role in the adult intestinal tract, and can play an important role in adjusting intestinal flora imbalance, treating intestinal dysfunction, anti-tumor and so on, and can also play the effect of prolonging life.