Table 1. Oxidative pathways of glycolysis employed by various bacteria. Bacterium Embden-Meyerhof pathway Phosphoketolase (heterolactic) pathway Entner Doudoroff pathway Acetobacter aceti - + - Agrobacterium tumefaciens - - + Azotobacter vinelandii - - + Bacillus subtilis major minor - Escherichia coli + - - Lactobacillus acidophilus + - - Leuconostoc mesenteroides - + - Pseudomonas aeruginosa - - + Vibrio cholerae minor - major Zymomonas mobilis - - +
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.