Why the Newborn and Young Infant Needs Mother’s Milk by Lars Hanson, MD, PhD

At delivery, the newborn is sterile, but from then on it meets many microbes. Most of these microbes are totally harmless and even protective, but others can cause infections in the infant - some very dangerous.

For quite a while we did not understand that the baby needed to pick up the right bacteria from delivery on. We also did not understand that these bacteria best came from the mother at delivery and afterwards from her and her family. For some time, very strict hygiene was instituted at delivery preventing the baby from meeting any of the mother’s bacteria. Thus the baby had to pick up bacteria from the hospital staff, the hospital environment, etc. But we should have given this a second thought and realized that man, like all mammals, delivers their offspring next to the mother’s anus. This is because the mother’s gut bacteria are the best source for normal bacterial colonization of the baby and the most harmless to the infant.

There are two major reasons for this. First, more than 99.9 % of the bacteria in the mother’s gut and stool are harmless, consisting of anaerobic bacteria (they cannot live in the presence of oxygen). They take up almost all the space in the big bowel (colon), where most gut bacteria are found, and consume just about all the food that can be utilized by bacteria, making it very difficult for other, potentially dangerous, bacteria to settle in the gut. Thus the anaerobes keep down the number of the potentially dangerous bacteria, like Escherichia coli, Klebsiella, and others. This is important because if these bacteria meet little competition with anaerobes in the gut, they can increase to high numbers and may cause infections in the infant.
The second reason that the mother’s gut bacteria are harmless to the breastfed baby is that white blood cells (lymphocytes) migrate from the lactating mother’s gut to her mammary glands. There, they produce the special secretory IgA (SIgA) antibodies, which are plentiful in milk. As a consequence of this “entero-mammaric link” many of these antibodies are directed against the bacteria the mother carries in her gut. Thus the mother’s milk antibodies make sure that bacteria from the mother that colonizes her newborn cannot get into the baby’s tissues and cause infections. These special SIgA antibodies prevent the bacteria from attaching to the infant’s gut tissues so infection cannot occur. You can actually see the bacteria in the gut of a breastfed baby covered by the SIgA antibodies from the mother’s milk.

There are further circumstances that make the milk SIgA antibodies especially useful for the young infant. The IgG antibodies from the mother’s blood, which reach the offspring via the placenta during pregnanc