I found this passage in "Lactation; Physiology, Nutrition, and Breast-
Feeding," by Neville and Neifert:

'The gut flora of exclusively breast-fed babies is composed mainly of
lactobacilli and bifidobacteria. Their feces have a pH of 5 to 6 due to the
fermentation of sugars and the production of acetic acid by the
bifidobacteria. Infants fed on formula, either entirely or as a supplement
to breast milk, have a higher fecal pH with a predominant flora of
Streptococcus faecalis, Bacteroides sp. and E. coli. The low colonic pH of
breast-fed babies inhibits the growth of the latter bacteria. In addition,
milk contains a carbohydrate growh factor for lactobacilli which may
contribute to their predominance.

It seems likely that newborns are exposed to bifidobacteria and
lactobacilli during their passage through the birth canal. For these
bacterial species to become established, it appears to be necessary that
the newborn is fed a milk with a low buffering capacity like human milk.
The effect of breastfeeding on the infants' gut flora was abolished by
cow's milk supplementation.'

Ellen

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