Bacteria are found in large numbers all over the human body where there is a channel to the outside world, for example in the gut, lungs, and surface of the skin. I've always thought that actually inside the human body was a bacteria-free environment unless an infection was raging so I was very excited to find that bacteria are present in breast milk. As a product which is made within the human body, breast milk was traditionally thought to be sterile.
Several recent studies have found that breast milk contains a healthy dose of commensal bacteria; all the staphylococci, streptococci, and lactic acid bacteria that are found in the infant gut. This isn't just bacteria from the skin which have contaminated the samples, but bacteria that have come from inside the breast as an integral component of the milk.
In a study of 16 women it was found that while each milk sample contained hundreds of different bacterial species around half of the microbiotic community was made up of nine species present in all samples. The other half varied from person to person. This pattern is also found in human gut microbes; a core set present in all individuals along with a large diversity of separate species to make up a unique individual microbiome.
So how do bacteria get into breast milk? Some of them may come from the mouth of the baby. During feeding the skin of both the mother and baby will be in contact with the baby's open mouth and a certain amount of flow-back can occur between the mouth and nipple. More excitingly it's been suggested that immune cells in the mothers gut may be able to pick up bacteria and carry them around the body using the lymphatic system. The lymphatic system is a network of vessels used to transport blood plasma. It's a main highway for immune cells inside the body and is also involved in the absorption and transports of fats.
The cells thought to be responsible for picking up the bacteria and transporting them to the breast are called dendritic cells. In non-breastfeeding people they are used to transport bacteria and bits of bacteria to the rest of the immune system to prepare a defence, and can sometimes carry entire live bacteria. Recent studies have shown that in mice bacteria from the gut translocate into lymph nodes and mammary glands during the late stages of pregnancy and the beginning of lactation. It's also been shown that if breastfeeding women are given a capsule of live (and safe!) bacteria to swallow, these bacteria can be isolated later from the breast milk.
This particular model, bacteria picked up by immune cells and ferried to the breast to be added to breast milk, is looking promising although there is still plenty of research to be done. It's not yet understood how the bacteria travel with the dendritic cells, either attached to the surface or completely engulfed by the human cell. It might even be other immune cells, such as macrophages, that are responsible for transport. It's also not certain why the dendritic cells don't just kill the bacteria once they've got hold of them (as is normal with pathogenic bacteria). A further question is exactly when the bacteria colonise the breast - are they continually ferried up from the gut or is there a specific window of opportunity for them to be introduced?
Like all humans, infants have a range of bacteria within their gut. It looks like these bacteria are initially supplied from the mother's vaginal and skin bacteria, before being replaced by bacteria from the breast milk. Researchers also found that when babies started eating solid food a whole new range of bacteria was introduced, forming the gut microbiome that persisted into adulthood.
1: Fernández L, et al. The human milk microbiota: Origin and potential roles in health and disease. PharmacolRes (2012), http://dx.doi.org/10.1016/j.phrs.2012.09.001
2: P.V. Jeurink, J. van Bergenhenegouwen, E. Jiménez, L.M.J. Knippels, L. Fernández, J. Garssen, J. Knol, J.M. Rodríguez and R. Martín. Beneficial Microbes, March 2013; 4(1): 17-30
Word of warning: Dealing with an infant is no easy task, and breastfeeding for many women can be both painful and difficult. While breast-milk is fascinating and marvellous stuff, formula milk can be a literal lifesaver in many circumstances, and has no major adverse effects on the child's growth and development. Comments discussing the properties and/or benefits of breast milk are fine but any that shame, ridicule or mock women for not breastfeeding will be deleted. As will any that shame, ridicule or mock women for breastfeeding.