Bacteria in our colons
It has been demonstrated that there are common patterns of microbiome composition evolution during life. In general, the diversity of microbiota composition of fecal samples is significantly higher in adults than in children, although interpersonal differences are higher in children than in adults. Much of the maturation of microbiota into an adult-like configuration happens during the three first years of life.

As the microbiome composition changes, so does the composition of bacterial proteins produced in the gut. In adult microbiomes, a high prevalence of enzymes involved in fermentation, methanogenesis and the metabolism of arginine, glutamate, aspartate and lysine have been found. In contrast, in infant microbiomes the dominant enzymes are involved in cysteine metabolism and fermentation pathways



evolution of the vertebrate gut microbiota :

Multi-cellular eukaryotes have existed for at least a quarter of Earth's history, or 1.2 billion years. Thus an already long history of interaction between multi-cellular lifeforms and microbial communities preceded, and likely shaped, the evolution of the vertebrates. The legacy of ancient associations between hosts and their epibiotic microbial communities is evident in the present day effects that the gut microbiota exerts on host biology, ranging from the structure and functions of the gut and the innate and adaptive immune systems, to host energy metabolism. Host responses to microbial colonization are evolutionary conserved among diverse vertebrates including zebra fish, mice and humans

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2664199/



"the human body is made up of about 10 times more microbial cells than human cells . Further, there may be millions more microbial genes than human genes in this human+microbiome system (which is often thought of as a human ‘superorganism’), and it is the ways in which these microbial genes interact with the human host that describe their ultimate role in our health. Scientists now believe that infants are sterile (meaning free of microbes) in the womb and receive their first inoculum of microbes from the mother during natural childbirth. This inoculum goes on to colonize the newborn and initiate a succession of events leading to the development of the child’s own microbiome. The newborn relies on this maternal vaginal microbial inoculum and the additional inoculum of microbes from mother’s breast milk for microbial colonization of all exposed surfaces in and on the infant’s body (e.g., oral, nasal/airways, gut, urogenital, skin).2 This is a dynamic process in which microbial abundances increase from effectively zero at birth to over six orders of magnitude (that’s more than a million times!) within just the first few weeks of life, with wide swings in the microbial membership of these communities until the microbiota largely stabilize in composition and numbers after approximately three years of life."

http://www.actionbioscience.org/genomics/the_human_microbiome.html



you can read about Gastrointestinal Tract Microflora here:

https://www.ncbi.nlm.nih.gov/books/NBK7670/

and normal flora :
https://www.ncbi.nlm.nih.gov/books/NBK7617/