During my education in Iran, until now, which I must say that I'm graduating high school this year and will go to college next year, our biology book has said that there's some bacteria in our colons and those little cells live there and use the celloluse of our food and because mostly they are anaerobic they produce methane or hydrogen and hydrogen sulfide and so on and sometimes they are the reason of the smell of our farts. Also they produce little amounts of B and K vitamins which is good for us and also, by using cellulose they make the rejection of feces better and easier. Anyway I wonder that where do these bacterias come from? I mean are they with us since birth? Then it means they come from our mother's body but, wait how our mothers have them? But maybe they are not there since birth and they come to colon from the food we eat during our life, but why don't they die in stomach. Even if we think that they go there by eating food, passing stomach easily, what if we don't eat the "food that can have those bacteria", then could it be possible to some humans don't have those bacteria? But they can't have a good release of feces.

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    $\begingroup$ Please read about your microbiota. The organisms both come from your mother and are acquired from the environment, including via food consumption. $\endgroup$ – MattDMo Jan 4 '17 at 17:11

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


"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."


you can read about Gastrointestinal Tract Microflora here:


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


It's true - you are given a starter colony by your mother during a vaginal birth, covering your skin surface and intestinal tract. The population of your gut is likely to change throughout life, depending on the environment you provide (food, exercise, antibiotics, etc).

It's also unlikely all bacteria are killed in your stomach when you eat, and only one cell is necessary to start a colony.

we've seen correlation of bacterial populations in the gut with certain disease states. I won't go beyond that since it's not my field. try checking out an amazing paper my prof gave me - The Gut Brain Axis

have fun with your studies.

  • $\begingroup$ Which is a problem for caesarean births. $\endgroup$ – Graham Chiu Jan 4 '17 at 22:55

The birth canal origin of the bacteria is not the whole story. It appear that most of the bacteria are acquired from the environment, which in naturally born infants does include the birth canal, but also included the mother's skin and dozens of other environmental contacts. The mature population of bacteria in the gut seems to be the same in both natural born and cesarean infants however the cesarean infants take significantly longer to develop this population indicating the birth canal is not the only factor.

It also appears that certain factors in breast milk encouraging the growth of the most desirable bacteria, and may have a stronger effect then form of birth.


I wonder that where do these bacterias come from?

I'll answer this question since your post revolves around this question and the rest are follow-ups.

The infant gets its first bacteria (commensal or mutualist) from the mother's vagina as the amniotic membrane ruptures during parturition and gradually the diversity increases from its contact with other organisms and environment.

Nolte (1977) suggested that the body acquires its first microflora of the skin, nose, mouth and conjuctiva from the mother’s birth canal during normal vaginal delivery. The local environment is also important as a source from which neonates acquire bacteria (Mortimer et al., 1966; Nolte, 1977; Harvey, 1979; Shinebaum, Cooke and Brayson, 1979). The colonisation of the skin surface starts at the umbilical stump (Mortimer et al., 1966)

The normal commensal flora of the oral cavity is more complex. The mouth is sterile at birth but is rapidly colonised by bacteria derived from attendants and the local environment. By the second day of life, aerobes and facultative species can be detected, but strict anaerobes do not appear until after the eruption of the deciduous teeth (Hurst, 1957; McCarthy, Snyder and Parker, 1965).


Also (updated)

A research published in 2013 has found that colostrum contain more than 700 species of bacteria! It being the only source of food for an infant also contributes to the infant's normal flora, though the composition of the bacteria colonising an infant's oral cavity is inconsistent with the composition of the colostrum.


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