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.