Good bacteria for my baby?? The prebiotic properties of breastmilk

When I first started breastfeeding, I remember being worried about any contamination of my milk before it got to my baby, whether because of the breast pump not being perfectly clean or simply from the breast itself – I mean, my skin isn’t sterile, so I assumed it could be a source of problematic contamination for my newborn daughter. And these concerns didn’t come out of nowhere: there are all sorts of problems that can arise from poor maternal hygiene or unsanitary pumping tools. So while pathogenic bacteria and other contamination of breastmilk is a legitimate concern, there is a small, but significant plot twist here: breast milk actually contains lots of bacteria and it’s not by accident or because of contamination- it’s on purpose. Allow me to explain… 

Originally thought to be sterile, only in the past two decades or so scientists have come to find that human milk is a naturally occurring source of gut bacteria – this means, women’s (and I assume other mammals but I didn’t do that research yet) milk production mechanism allows for growth of certain types of bacteria, leading to the transfer and subsequent colonization of these strains of bacteria in the newborn baby when it nurses. You might be asking yourself why on earth would this be beneficial to anyone and while the answer is still under lots of scientific investigation, there’s evidence that this process leads to the development of a healthy gut microbiome in the baby.  

Let’s start at the beginning though: what is a microbiome and why is it important? 

There is considerable debate about the exact definition of the human microbiome – some would even go so far as to call our microbiome one of our organs – but simply put, a microbiome is essentially a fancy term to describe a collection of different species of bacteria and other microorganisms that live in a particular part of our body. I know what you might be thinking – in this age of COVID and fear of disease – you might be questioning the benefits of any microbes in our lives. It’s a legitimate concern. First of all, we’re not talking about pathogenic microorganisms; we’re talking about microbes that live in harmony with us. Sure, some bacteria and viruses cause terrible infections and are genuinely dangerous. It turns out, however, pathogenic bacteria only make up less than 1 percent of known bacterial species and there are vast arrays of bacteria that colonize our bodies, be it our skin, the inside of our mouth, or line our intensitines, that actually have shown to protect us from bad bacteria. There is a lot of evidence that a “healthy” microbiome is linked with all sorts of positive health outcomes (need more specifics). While there is growing evidence now that the uterine environment in which the fetus grows is in fact not completely sterile, it seems newborn babies are born with an underdeveloped skin, gut, and other microbiomes. Therefore, it’s precisely this early stage of life when some of these key colonies are established – there’s evidence to indicate that they might last through adulthood (need more specifics and citation or previous). And it’s not just gut microbiomes that are important, a 2016 study showed that newborn babies born via Cesarean section missed a key microbiome inoculation point as they passed through the vagina at delivery, potentially putting them at greater risk for some immune diseases such as asthma and other inflammatory diseases. While currently not recommended by the American College of Obstetrics and Gynecology (ACOG) outside of research settings, vaginal seeding (the process of swabbing a baby with vaginal fluid post-delivery by C-section) might be a way to avoid missing out on the benefits of these microbes. The ACOG does recommend, however, lots of skin-to-skin contact for mother and newborn, as well as exclusive breast feeding when physical and medical conditions allow it, with one of the key purposes being the healthy transfer of maternal microbiota to the  newborn.  

So now that you know more about the immense benefits of a healthy microbiome, it might not be a surprise that there are actual mechanisms for transferring healthy gut microbiome bacteria from mother to child. Initially, people though the breast milk had bacteria in it because of contamination from the infant to the mother during nursing, since studied showed there is some reverse flow during feeding. Turns out that while backflow certainly is significant, studies of precolostrum milk, meaning the infant had not yet had the chance to feed from the first-time mother, show the presence of bacterial species. Wondering how this bacteria gets to the milk? There’s still lots of research to be done in this field but scientists have evidence to indicate an entero-mammary pathway that uses dendritic cells, a specific kind of immune cell, to bring bacteria from the maternal intestine to the mammary glands where milk is produced (see Figure 1 below from the Rodríguez 2014 paper). I know, wild! Women’s bodies are so amazing! 

So what does this all mean? Well, I guess sterility isn’t a real possibility or goal for my breast milk. But more importantly, going down this rabbit hole has taught me that breast milk carries even more health benefits than I thought before! It also raised questions about the effects of freezing and heating milk – which likely compromise the stability of these important bacteria colonies. Finally, it reminded me how important it is to take care of my own body and microbiome because it’s what my daughter is getting too. Giving her a healthy microbiome is one of the many ways I can shape her future life right now. 

Breastfeeding my first child over the past few months has been a journey and I certainly considered giving up on a number of occasions but circumstance and luck allowed me to continue. Most recently, with her teeth coming in, I was ready for the beginning of the end of this stage of our relationship, but after learning about the microbial benefits of breastfeeding, I think I’ll hang on a little longer. 

At the end of the day, moms are superheroes who do the best they can and I’m a supporter of “fed is best” but if I have a choice about how to feed my daughter, I’ll go for the nursing when possible because I want to give her as much of these good bacteria as I can.