Human milk encourages implantation of Bifidobacteria ( Yoshioka et al 1983)
The mechanisms by which this occurs are not fully understood but appear to relate to a number of different components. Ducluzeau (1993) identified a large number of factors in breast milk capable of inhibiting or permitting in vitro growth of various bacterial species. These factors include the following:
Oligosaccharides - oligosaccharides in breast milk include N-acetylglucossamine, glucose, galactose and fucose oligomers (Collins & Gibson 1999). The large oligosaccharide fraction of human milk may be of special significance because it provides growth factors for Bifidobacteria and Lactobacilli (Stahl et al 1994, Kunz & Rudolf 1993). Moreover, Newberg et al (1990) found from studies on sucking mice that oligosaccharides also act as receptor analogues that inhibit the binding of certain bacterial pathogens and their toxins.
Immunoglobulins - Secretory IgA is an antibody that binds microbes and other antigenic materials to prevent them from reaching mucosal membranes where they might cause infections or damage. Most human milk antibody is secretory IgA - almost 4 g of which is ingested by the newborn during the first day of breast-feeding.
Although IgA is produced locally in the mammary glands, it results from antigen exposure in the gut. Lymphoid cells migrate from maternal Peyer's patches in the gut and from lymphoid centers in the respiratory tract, to the breast - this is known as the enterobronchomammary pathway for the migration of lymphocyte( Roux et al 1977). Thus, secretory IgA antibodies from mother's milk are a reflection of the mother's enteric and respiratory antigens, and will be directed to all the bacteria and other antigenic substances to which the mother has recently been exposed.
Lactoferrin - A biologically active glycoprotein found in high concentrations (about 20% of total protein) in human milk, that has been found by Brock (1980) to inhibit bacterial growth in vitro. Lactoferrin has a strong influence on microbial flora by binding to excess iron in the body. Bifidobacteria and Lactobacilli do not need iron, whereas Bacteroides species and Enterobacteria need iron to proliferate.
Hormones & Growth Factors - in contract to artificial milk, human milk contains a number of hormones (cortisol, insulin, thyroxine) and growth factors (epidermal growth factor, nerve growth factors, transforming growth factor) that promote gastrointestinal maturation and also contribute to the neonate's own defense system (Sheard & Walker 1988).
PH - may allow elevated growth of Bifidobacteria (Collins & Gibson 1999). Zetterstrom et al (1994) noted that the pH level of the stool in breast-fed infants was 5.0-5.5, whereas it was as high as 7.0 in formula-fed infants. The low pH level promotes growth of Bifidobacteria and Lactobacilli but inhibits other bacteria.