The Human Milk Dynamic Duo: Bacteria and Sugar

Evi Dewhurst


Human milk is a wonder of engineering resulting in straightforward nourishment… but behind the scenes it is actually much, much more. In fact, some scientists believe it evolved as a protective, and not nutritional, adaptation.3 This coveted natural resource contributes to a healthy balance of gut microbes and boosted immunity for infants, which is especially necessary for those who are immunocompromised to begin with.

How does this happen? One way is the combination of bacteria and sugar molecule super powers. Look closer, and discover their secrets.

The human milk bacteria bounty

Bacteria can be construed as a negative word, associated with illness. But human milk defies this perception, with colostrum containing a whopping 700 different forms of bacteria, such as Weissella, Leuconostoc, Staphylococcus, Streptococcus and Lactococcus. 1-6oz of milk contained even more, with more typical inhabitants of the oral cavity, such as Veillonella, Leptotrichia, and Prevotella.2 How could a healthy resource meant for newborn nourishment contain so much risk? What at first seems a negative actually contributes to an incredible boost in gut health and maturity in infants. Premature infants are especially in need of these benefits.

The sterile newborn gut environment
is often first exposed to bacteria from human milk.

Research into this human milk phenomenon seems to indicate “milk bacteria are not contaminants,” as stated by Cabrera-Rubio et al.2 Rather, they boost immunity and contribute to the development of gut microbiota and even metabolic development. These bacteria are often the first external microbes introduced into the body, with the important job of “boosting immunity by cultivating a balance of microbes in the gut and the rest of the body”3 – in fact, the whole internal ecosystem. This phenomenon seems so effective that many illnesses are significantly reduced when infants are fed breast milk. In particular, NICU infants who received breast milk had a 50% reduction in the risk of necrotizing enterocolitis (NEC) as compared to their formula-fed peers.3

Interestingly, researchers have discovered that a mother’s hormonal status contributes to the bacteria present in her milk. The “lack of physiological stress and hormonal signals of labor may influence composition and diversity of microbes in breast milk,” state Cabrera-Rubio et al. They studied the range of microbes found in human milk and determined human milk bacterial composition varied between women who had elective cesarean sections, vaginal delivery or intrapartum cesarean, with the least amount of breast milk bacteria composition in the elective cesarean group. An additional factor also proved to be maternal weight. Women who gained more than the recommended weight during pregnancy had reduced bacteria in comparison to the women who remained within the range of recommended weight gain. In the end, however, no matter what method of birth or the weight of mom, bacteria is always present in breast milk, much to the benefit of an infant’s overall development and health.

I’ll take that with sugar, please

Human milk contains sugar molecules, which at first glance may seem to offer limited value to an infant. However, to preterm, full term and older infants, breast milk sugars actually serve a higher purpose. Call them the defenders of the immune system: They are the oligosaccharides army.

Oligosaccharides are sugar molecules. They are among the most common solid components of milk.3 They are, in fact, the third most common biomolecule in breast milk, right behind lactose and lipids.4 There are more than 150 different known human milk oligosaccharides, and possibly up to 200. Bruce German, a food chemist at U.C Davis, took a particular interest in the benefit of oligosaccharides for newborns. His initial attraction to study them was the mysterious reason for their existence.

Human milk oligosaccharides are present in breast milk,
yet they are not digestible.

While oligosaccharides were abundantly present in breast milk, they were not digestible. They were ingested, then expelled through natural body functions without being digested. What was their purpose? Why would the five hundred calories invested each day by mom to produce milk have ten percent wasted on creating unusable nutrition? He then considered their role tied to bacteria. Could it be they worked together for the benefit of the body’s internal ecosystem?

Bruce German teamed with Microbiologist David Mills to explore oligosaccharides. Mills tested them in specific environmental conditions, namely in oxygen-deprived chambers to mimic human gut conditions. After using fecal bacteria from infant stool samples in his testing, Mills discovered something very intriguing. A dominant bacteria present in healthy infant poop (Bifidobacterium infantis) was gobbling up specific oligosaccharides called fucosylated oligosaccharides. As it did so, the other, more dangerous gut bacteria were starved out, all while the host benefitted overall. In other words, this oligosaccharide served one purpose: attract specific bacteria while causing deterioration in others. “The oligosaccharides help B. infantis nurture the integrity of the lining of the babies’ intestines, thus playing a vital role in protecting them from infection and inflammation.”4

What could the implications of this research be on a devastating disease such as necrotizing enterocolitis (NEC)? Extremely premature infants are at high risk for this disease, with ten percent afflicted and forty percent of those enduring the most devastating form resulting in death.4 Lars Bode, a nutrition scientist, found another go-getter in the breast milk oligosaccharides army named Disialyllacto-N-tetraose. This sugar was shown to reduce NEC mortality rate in rats from twenty-five percent to five percent.3  Bode believes these oligosaccharides sugars make the journey to the colon fully intact. There, they mimic molecules on the surface of the gut epithelial cells and act as a decoy for potentially lethal pathogens such as Entamoeba histolytica. E. hystolytica then latches onto them, and eventually the sugars are flushed from the body along with the pathogen. Bode also discovered during his research that oligosaccharides inhibit Streptococcus pneumoniae, which contributes to respiratory and ear infections.

Yet another identified oligosaccharide called 2’-fucosyllactose shows protective properties by helping eliminate campylobacter, cholera, and enteropathogenic E. coli in animal models. Boston College Biochemist David Newburg and his colleagues, who did the research, believe it could also be positively impactful for humans. This news only reinforces the intrinsic value of human milk to infants, no matter what their size or development.

These research studies point to the wonderful and hidden properties of human milk that make a real difference in infant health. Clinicians are familiar with the positive impacts of human milk for infants and these research studies have only added to the value of this “liquid gold.” Breast milk has a more important job than ever: protect and nurture microbiota gut growth and health, especially for at-risk infants in the NICU. This goes beyond simple nourishment, and leads to potentially healthier patient outcomes and a future of lifelong improved immunological strength. Encouraging moms with your knowledge and experience and providing the tools they need for optimal breast milk production will go a long way to helping future generations experience the health benefits they deserve.


1More Than 700 Bacteria Live in Breast milk, Gut Microbiota Worldwatch, February 20, 2013.

2Cabrera-Rubio et al, The Human Milk Microbiome Changes Over Lactation and is Shaped By Maternal Weight and Mode of Delivery, The American Journal of Clinical Nutrition, September 2012 vol. 96.

3Florence Williams, The Impressive Power of Breast Milk, Discover Magazine, June 2012.

4Decoding Breast Milk Secret Reveals Clues to Lasting Health: UC Davis News and Information