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Artwork by Alba Llach Pou


Breast milk, friend or foe?

Updated: Oct 14

Did you know that breast milk is not sterile? Breast milk is newborn babies’ reference diet. It is known for its nutritional, hormonal and immune properties which ensure infant babies an optimal growth. Moreover, it represents a biological fluid that is colonised by millions of bacteria that can be the source of some infections, notably in preterm infants.

Since the end of 2021, I have been working in a research team working on metagenomics at the National Centre of Human Genomics Research. Metagenomics is the methodological process allowing us to study the human microbiota. A microbiota represents all the microorganisms (bacteria, yeasts, plankton…) that live in a specific environment (gut, oceans, soils, air…). Researchers from Pasteur Institute and Inserm also study the development of newborn babies’ microbiota and immune system.

Our project is focused on the analysis of the bacterial composition of breast milk. In my opinion, it is important to improve our knowledge on this subject in order to better understand the positive and negative impacts of its consumption. In the end, we seek to isolate bacteria which could be beneficial to us, for instance those who could allow us to cure diseases. This is why I would like to share this aspect of my research with you.

Breast milk and microbiota

The microbiota is composed of all the microorganisms that interact in a specific environment within a host (e.g a human being) which could be beneficial to them (protection against diseases) and/or deleterious (a source of infections).

Nowadays, we mainly know gut microbiota, however there are other microbiota such as skin, vaginal or lung microbiota. During the last few years, several studies have shown that breast milk also harbours its own microbiota. However, it is still understudied.

Breast milk has long been considered as a sterile biological fluid, which means it is devoid of any microorganism such as bacteria. This is false! Thanks to the advent of more precise technologies, it has been shown that breast milk is a source of numerous so-called beneficial bacteria that improve the health of newborn babies by favouring their colonisation of the digestive tract. These bacteria are part of the commensal intestinal flora, which favours the development of the intestinal barrier and helps the development of the immune system.

Breast milk microbiota begins to form during the last trimester of pregnancy and constantly diversifies all along the breastfeeding period until the mother stops it. Many similarities exist between maternal microbiota including large bacterial families. However, each mother’s milk has a unique bacterial composition in terms of species.

Breast milk undergoes variations according to several factors such as diet, age, drug intake, hormonal variations, pregnancy weight gain and even the method of childbirth [1]. Moreover, breast milk harbours a transient and dynamic microbiota that evolves in a number of steps. Indeed, from the day of birth and during the following two months the bacterial composition of breast milk changes in just a few weeks. It is also enriched in potassium and becomes depleted in sodium and magnesium. Two weeks after birth, the milk is considered mature in terms of nutrients, however its bacterial composition keeps changing [2].

Breast milk, the best food for newborn babies

Breast milk contains carbohydrates, proteins and lipids in abundance. Within the carbohydrates, oligosaccharides are highly important molecules that are the third main component of the milk, after lactose and lipids. They feed the bacterial communities of the newborn’s gastrointestinal tract. In addition, some oligosaccharides play an important role in the prevention of gut infections in newborns and also display antibacterial properties [2].

Breast milk microbiota contains elements that are important for the development of the newborn’s intestinal barrier. One example of this importance is its capacity to regulate the “weaning reaction”. The weaning reaction is a physiological immune response to the colonisation by the microbiota during the newborn’s development. Breast milk plays an important role in the establishment of this reaction through several proteins such as the growth factor EGF [3-4].

Researchers from Pasteur Institute and Inserm have shown that in newborn babies, gut microbiota is first conditioned by breast milk’s component. When the diet becomes more diverse the microbiota develops and numerous bacteria proliferate. Afterwards, the incorporation of solid food to the diet induces the maturation of the gut microbiota and an important immune response. This immune response is essential as it is a part of the immune system’s maturation. It also lowers the chances of developing inflammatory diseases in adults (auto-immune diseases, cancer) [4].

The advantages associated with breastfeeding encompass the protection against pathogens, a better immune development, a comprehensive nutrition and a lower risk of gastrointestinal diseases and diarrhoea in some developing countries. Studies have proven that breastfed children have a lower risk of chronic diseases such as allergies, asthma, diabetes and obesity during childhood and adulthood [5].

Breast milk, a potential unrecognized risk?

Now you could be saying that breast milk is perfect. But beware, it is more complex than it seems! We just covered that breast milk has a microbiota and therefore it contains microorganisms such as bacteria that are usually beneficial. A three-month old infant drinks about 800mL of milk daily, which corresponds to 104 - 106 bacteria [6]. You have to imagine a baby that weighs 5kg and receives all this milk and therefore all these bacteria daily.

Infants have an immature immune system. Breast milk can also occasionally become a source of infections for them. For instance, one study uncovered a nosocomial outbreak (from a hospital) of a potentially dangerous bacteria called Escerichia coli in six infants, all receiving milk from the same mother [7].

Breast milk as a medical treatment

Breast milk, friend or foe? It is a question of balance!

Many uncertainties remain in terms of the exact composition of Breast milk. Just like the very promising gut microbiota, breast milk microbiota could represent a source for new treatments.

Nowadays, bacteria are already used as food supplements for their probiotic properties. Probiotics were defined by the Food and Agriculture Organization of the United Nations and the World Health Organization in July 2002 as “living microorganisms that, when consumed in adequate quantity, produce a benefit to the host’s health”. The more frequent ones are bacteria called lactobacilli (from the Lactobacillus genus) that are common in yogurt, cheese and wine.

Research teams have managed to isolate the bacteria Lactobacillus reuteri from breast milk and have shown its health benefits, such as its antibacterial activity, its capacity to inhibit the growth of pathogens in the infant’s gut, the improvement of food tolerance and the absorption of nutrients, minerals and vitamins, which could potentially make it a new medical treatment. One of the best examples is the efficiency of L. reuteri as a probiotic agent against infections and its utilisation for treating Helicobacter pylori stomach infections. H. pylori infections are a major cause of chronic gastritis and gastric ulcers as well as a risk factor for developing malignant gastric tumours. Many studies have explored the utilisation of L. reuteri against H. pylori: L. reuteri acts as competitor to H. pylori and prevents its binding to receptors. This competition reduces the bacterial load of H. pylori and relieves the associated symptoms. L. reuteri is advantageous in the treatment against H. pylori because the supplementation eradicates the pathogen without provoking the usual side effects associated with antibiotic treatments [8]. The bacteria L. reuteri has also proven itself to be efficient when administered to children. Indeed, it has decreased the duration and frequency of infectious diarrhoea and also respiratory infections [9].

The role of breast milk microbiota remains to be further explored but our research teams’ first results are a step toward understanding its action on our development and our long term health.


1. Cabrera-Rubio, R et al. (2012). The human milk microbiome changes over lactation and is shaped by maternal weight and mode of delivery. The American Journal of Clinical Nutrition.

2. Katríona, E. Lyons et al. (2020). Breast Milk, a Source of Beneficial Microbes and Associated Benefits for Infant Health. Nutrients.

3. Kalbermatter, C et al. (2021). Maternal Microbiota, Early Life Colonization and Breast Milk Drive Immune Development in the Newborn. Front. Immunol.

4. Al Nabhani, Z et al. (2019). A Weaning Reaction to Microbiota Is Required for Resistance to Immunopathologies in the Adult. Immunity.

5. Klopp, A et al. (2017). Modes of infant feeding and the risk of childhood asthma: A prospective birth cohort study. The Journal of Pediatrics.

6. Heikkilä, MP et al. (2003). Inhibition of Staphylococcus aureus by the commensal bacteria of human milk. Journal of Applied Microbiology.

7. Nakamura, K et al. (2016). Outbreak of extended-spectrum β-lactamase-producing Escherichia coli transmitted through breast milk sharing in a neonatal intensive care unit. Journal of Hospital Infection.

8. Mu, Q et al. (2018). Role of Lactobacillus reuteri in Human Health and Diseases. Frontiers in Microbiology.

9. Gutiérrez-Castrellón, P et al. (2014). Diarrhea in preschool children and Lactobacillus reuteri: A randomized controlled trial. Pediatrics.

This article was specialist edited by Dr. Fabien Guendel Rojas and copy edited by Emile Auria.


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