The breast milk that a mother feeds her baby is laden with bacteria. Does that sound bad? It shouldn’t! While there are some pathogenic bacteria, most bacteria are incredibly beneficial to the life that exists on this planet. That’s especially true of bacteria that live in and on people. It turns out that most people live in a relationship with more than 150 different species of bacteria, and the individual bacteria that participate in this relationship far outnumber the human cells that make up a person’s body. In one sense, then, a person is not an individual. Instead, he or she is a walking ecosystem!
Scientists now call the collection of bacteria that lives in a person’s body the microbiome, and as the article linked above indicates, each person seems to have his or her own special mix of bacteria in that microbiome. Indeed, some researchers think that analyzing the DNA of the bacteria a criminal leaves behind can aid in identifying that criminal in cases where his or her own DNA is not available at the crime scene or too degraded to analyze properly.1
So where does an infant start collecting the bacteria that will make up his or her own microbiome? One of the sources is the breast milk that the infant drinks. It has been known for quite some time that breast milk contains bacteria, but the details have not been well studied. However, a group of Spanish researchers have begun to shed some light on those details. They studied the breast milk of 18 mothers who varied in weight, weight gain during pregnancy, and the mode in which the baby was delivered. They sampled the milk these mothers produced at three different times: the first secretions of milk produced after giving birth (called colostrum), the milk that was produced one month after giving birth, and the milk that was produced six months after giving birth. They sequenced the DNA of the bacteria found in these samples of milk, and they came up with some amazing results.2