It is well-known that breastfeeding gives infants nutrients. However, for the first time Spanish researchers created a map of the bacterial microbiota in new mothers' breast milk. They found that, on average, women's breast milk contains about 700 bacterial species.

Researchers had previously been well-aware that mothers' breast milk is instrumental in creating a baby's bacterial flora, or the unique bacterial community that exists in each person. However, until this study, it was not known what types of species existed in breast milk and what exactly their role was. This study examined the breast milk of 18 different mothers in an effort to answer the questions during three different time periods: six months after the baby's birth, one month after the baby's birth and the colostrum, or the first excretion of the mammary glands following the baby's birth.

"This is one of the first studies to document such diversity using the pyrosequencing technique (a large scale DNA sequencing determination technique) on colostrum samples on the one hand, and breast milk on the other, the latter being collected after one and six months of breastfeeding,"  the researchers said in a statement.

Researchers found 700 species residing in the colostrum, most commonly Weissella, Leuconostoc, Staphylococcus, Streptococcus and Lactococcus. As the babies became older, the bacterial species present changed, with Veillonella, Leptotrichia and Prevotella becoming increasingly common.

Pre-birth variables affected the bacterial diversity in breast milk as well. For example, overweight mothers tended to have less bacterial diversity, as did mothers who had put on a lot of weight during pregnancy or who had undergone elective caesarean sections. However, mothers who had emergency caesarean sections had bacterial diversity that rivaled mothers who had given birth vaginally. Researchers also suggested that hormonal states of mothers could affect the biodiversity in their breast milk.

Now, this finding has spurred researchers to wonder whether the bacteria in breast milk play a metabolic role or an immune one. Do the bacteria help infants digest breast milk or do they help babies distinguish between beneficial and foreign invaders? Subsequent studies should shed light on the matter, and the findings would certainly bear importance.

"If the breast milk bacteria discovered in this study were important for the development of the immune system, its addition to infant formula could decrease the risk of allergies, asthma and autoimmune diseases," the researchers say.

The study was published in the American Journal of Clinical Nutrition.