Bacteria in Breastmilk


“30% of the beneficial bacteria in a baby’s gut come directly from mother’s milk and an additional 10% from skin on the mother’s breast… there are at least 700 bacterial species present in breast milk.”

From “Meet your Bacteria” by Nicola Temple and Catherine Whitlock

In his book "I Contain Multitudes" Ed Yong discusses breast milk as “the perfect source of nutrition” it contains specific ingredients that have evolved over time to selectively feed gut bacteria. This is a summary:

Milk is specific to mammals. It has been perfected in over 200 million years of evolution. Breast milk has the following 3 main ingredients:

  1. Lactose
  2. Fats
  3. Complex sugars (oligosaccharides). Humans churn out 200 different types of these oligosaccharides named Human Milk Oligosaccharides (HMOs).

Guess what? Babies can’t digest HMOs at all! So why would a mother expend an enormous amount of energy creating 200 different types of complex sugars that her baby can’t even digest? Why would evolution favour this model? The clue is that HMOs pass through the stomach undigested and arrive in the colon not to feed the baby but to feed the microbes! The HMOs in breast milk selectively nourish a particular bacterial strain called Bifidobacterium infantis, this particular strain can outmanoeuvre all other bacteria.

You could think of Bifidobacterium infantis as “baby BIF” – your baby’s Best Intestinal Friend! This bacterial strain releases compounds called Short Chain Fatty Acids (SCFA) that feed the gut cells directly. It also encourages these gut cells to make adhesive proteins that seal the gaps between gut cells and make anti-inflammatory molecules – a form of Immune system training. This cascade of changes only happens when Bifidobacterium infantis grows on HMOs in breast milk.