Although probiotic bacteria grow naturally in the human gut, acquisition of the gut microbiome varies in each individual depending on various factors, including birth method and early diet. Research suggests that newborns' first exposure to bacteria and acquisition of the first components of their microbiome takes place during birth. Infants acquire their mothers' bacteria as they pass through the vaginal canal during childbirth. Research indicates that, for this reason, children born through cesarean section have a less diverse microbiome (Dominguez-Bello, Costello, Contreras, Magris, Hidalgo, Fierer, & Knight, 2010). Research also suggests that following birth, infants continue to acquire their gut microbiome in the first two years of life under the influence of dietary nutrition. For example, in a study of six breast-fed and six formula fed infants, those who consumed a specially enriched formula that contained both prebiotics (substances that feed probiotics) and a variety of probiotic bacteria had a much more diverse intestinal flora than those who received breast milk (Harmsen, Wildeboer-Veloo, Raangs, Wagendorp, Klijn, Bindels, & Welling, 2000). Please note that this study does not negate the benefits of breast milk and was intended to assess the effect of heightened prebiotic and probiotic concentrations in a baby formula that was chemically similar to breast milk.