<P> In humans , a gut flora similar to an adult 's is formed within one to two years of birth . The gastrointestinal tract of a normal fetus is considered sterile , but microbial colonisation may occur in the fetus and Lactobacillus and Bifidobacterium species were present in placental biopsies in one study . During birth and rapidly thereafter , bacteria from the mother and the surrounding environment colonize the infant 's gut . As of 2013 , it was unclear whether most of colonizing arise from the mother or not . Infants born by caesarean section may also be exposed to their mothers ' microflora , but the initial exposure is most likely to be from the surrounding environment such as the air , other infants , and the nursing staff , which serve as vectors for transfer . During the first year of life , the composition of the gut flora is generally simple and it changes a great deal with time and is not the same across individuals . </P>
<P> In humans , a gut flora similar to an adult 's is formed within one to two years of birth . The gastrointestinal tract of a normal fetus is considered sterile , but microbial colonisation may occur in the fetus and Lactobacillus and Bifidobacterium species were present in placental biopsies in one study . During birth and rapidly thereafter , bacteria from the mother and the surrounding environment colonize the infant 's gut . As of 2013 , it was unclear whether most of colonizing arise from the mother or not . Infants born by caesarean section may also be exposed to their mothers ' microflora , but the initial exposure is most likely to be from the surrounding environment such as the air , other infants , and the nursing staff , which serve as vectors for transfer . During the first year of life , the composition of the gut flora is generally simple and it changes a great deal with time and is not the same across individuals . </P>
where does our first normal flora come from
where does our first normal flora come from
the mother and the surrounding environment
the mother and the surrounding environment