It has long been regarded that human breast milk is sterile, much like everywhere else inside our body. With the exception of our digestive tract which is technically not inside our body (it’s like a tube of porous membrane that is inside us like a hole of a doughnut is "inside" the doughnut), traditional medical wisdom has always suggested that bacteria is quickly destroyed by a healthy immune system should it happen to find its way into our general blood circulation. As such, breast milk has long been thought to be unquestionably sterile as it is produced from nutrients found in general circulation – where there would be no bacteria. Much like urine, any bacteria found in breast milk could only have been the result of infection (mastitis or acute cystitis for instance), OR contamination with the fluids coming into contact with bacteria on the surface of the skin. Indeed, this has been microbiology 101 for decades.

Last year, a small study identified several species of bacteria in samples of breast milk, and that these species contribute to the health of the infant by providing gut bacteria and resulting immune health. Various species of bifidobacterium, lactobacillus, staphylococci, and streptococci were isolated from human breast milk samples to much less (if any) fanfare than I would have expected from the scientific community. Surely this finding would have turned a long-held notion completely on its head.

Mom and Daughter by Donnie Ray Jones licensed under CC BY 2.0

Currently, there are 2 theories as to why samples of bacteria were found in human breast milk. The simplest assumption would be cross-contamination with the saliva from infants. It’s likely that breast fed infants will transmit bacteria from their saliva via back-flow into their mother’s breast. In this case, the bacteria did not originate from the mother, but was rather transmitted by the baby. This makes a lot of sense quite frankly.

The second suggestion is totally up for grabs. Various theories have proposed that types of immune cells in our body such as specific monocytes or dendritic cells that carry bacteria from one part of the body to another in attempts to generate better immunity (kind of like how people would share books to spread knowledge) might be involved in the process. While the research is very preliminary, we personally feel as though there are still too many hurdles in the way to fully embrace this theory as live bacteria in the blood stream (and ultimately in milk) is called bacteremia, and is always medically considered abnormal. If species such as staphylococcus and streptococcus actually travel from the mother’s gut (where most of our bacteria is located) in significant quantities through the mother’s bloodstream and into breast milk without causing mom to develop an infection; then call me impressively stumped.

Until more research is done, it would seem that most medical professionals are taking the wait-and-see approach to see if such studies are reproducible and can prove that bacteria actually moved from a mother’s gut, through general circulation to breast milk. Until then, we’ll be giving our kids probiotics and letting them play in the dirt once and awhile.