Bacteria in Breastmilk
by Anne Eglash MD, IBCLC, FABM
Despite our compulsive use of hand sanitizers, research and the media tell us that bacteria are not so bad after all, and that our gut bacteria play a major role in our body fat, systemic illnesses, and emotions. Breastmilk is not sterile, and there has been a great deal of research into what determines the bacterial balance in breastmilk. This matters, because the bacteria in breastmilk helps to establish the infant gut microbiota, hence affecting short and long term health issues such as obesity, asthma and allergies.
There are two main pathways for bacteria to establish themselves in the lactating breast. One way is through the enteromammary pathway, defined by maternal gut bacteria traveling through her lymphatic system and blood stream into her breasts. It is theorized that this is how bacteria colonizes colostrum before the infant is born. The second pathway is retrograde movement of bacteria from the infant’s mouth into mom’s breasts. What factors determine the type of bacteria in the maternal gut and infant mouth?
The researchers of the current study examined the bacterial balance in breastmilk samples of 393 breastfeeding dyads, and examined the association between maternal, infant, early life, and milk factors with breastmilk bacterial composition.
- A non-lactating breast is colonized by bacteria.
- The bacterial load in breastmilk decreases over the course of a feeding.
- Expressed breastmilk has a different bacterial balance and more pathogens than breastmilk directly fed from the breast to the infant.
- Infants fed expressed breastmilk may have a higher risk of pediatric asthma than infants fed exclusively at the breast.
- Disruption of the infant gut microbiota in the first few months of life is associated with allergies and asthma later in childhood.
- Mothers with male infants have a different bacterial balance in their milk as compared to mothers with female infants.
See the Answer
Breastmilk contains a complex community of bacteria that may help seed the infant gut microbiota. The composition and determinants of milk microbiota are poorly understood. Among 393 mother-infant dyads from the CHILD cohort, we found that milk microbiota at 3–4 months postpartum was dominated by inversely correlated Proteobacteria and Firmicutes, and exhibited discrete compositional patterns. Milk microbiota composition and diversity were associated with maternal factors (BMI, parity, and mode of delivery), breastfeeding practices, and other milk components in a sex-specific manner. Causal modeling identified mode of breastfeeding as a key determinant of milk microbiota composition. Specifically, providing pumped breastmilk was consistently associated with multiple microbiota parameters including enrichment of potential pathogens and depletion of bifidobacteria. Further, these data support the retrograde inoculation hypothesis, whereby the infant oral cavity impacts the milk microbiota. Collectively, these results identify features and determinants of human milk microbiota composition, with potential implications for infant health and development.
There is more to come on this topic. Mom’s diet partially determines her gut bacteria, which in turn colonizes her breastmilk. I expect that we are going to see a barrage of probiotics marketed for mothers, with the goal of optimizing bacterial colonization of her breastmilk.
Another factor here is how we advise moms who exclusively or predominantly pump milk. The researchers indicate that having the baby at the breast might benefit the balance of bacteria in the breast, but using a pump may lead to more pathogens in breastmilk. One clinical take-away for me is to encourage moms to keep their pump parts clean to limit the burden of pathogens in the lactating breast.