Mother’s Milk, Microbes, and a 100,000-Year-Old Munchkin

It’s National Breastfeeding Month. Party like it’s, uh, August.

Just as it has been every year since 2011, August is National Breastfeeding Month, brought into being (we were going to say “conceived,” but obviously thought better of it) through a proclamation from the United States Breastfeeding Committee.

You’re right, there really is a committee for everything.

It seemed appropriate this week, therefore, to take a brief look at some breast milk/bacteria connections, but before we do, we’d really like to stress how much we think every mom is a superhero, whether she breastfeeds or not.

Some moms choose not to, others would like to but can’t, and there are those who do.

But, moms of the world – every single one of you – we salute you.

For many women, lactation is a process that begins automatically after delivery, just as it does in other mammals.

When researchers used a laser to examine the tooth of a Neanderthal child born 100,000 years ago, and found in a cave in Belgium, they determined that this particular child was breastfed.

Now, ignoring the fact that this is perhaps yet another indication that your dentist almost certainly knows far more about you than you let on, it does indicate that breastfeeding has been going on for one heck of a time.

For most of the comparatively brief part of this that microbiological research has been going on, it was generally believed that unless a woman was suffering from an infection such as mastitis, human breast milk was sterile and contained no bacteria.

The (now outdated) theory was that it was only the parts of a body that got exposed to environmental microbes which became colonized by them.

On this basis, bacteria could get into the gut when they were present in food, but you wouldn’t normally expect to find bacteria in the blood, say, or in breast milk.

Relatively recently, however, studies have shown that breast milk does indeed contain bacteria, largely of a commensal nature.

A commensal relationship is one between two organisms whereby one benefits from the other, without affecting it.

A study of 16 women showed that each individual’s milk sample contained hundreds of different species, but there were nine species that were found in all women, including staphylococci, streptococci, and lactic acid bacteria.

Despite their slightly intimidating names, these are all largely good guys.

But how do they find their way into the breast in the first place?

What’s going on?

We should begin by acknowledging that some microbes may indeed travel in from the outside, originating from a suckling infant’s mouth in a process indelicately labeled “flow back.”

Most, though, don’t take this route.

Instead, a recently proposed model suggests that immune cells in a mother’s gut operate a kind of Lyft scheme, where they pick up bacteria and ferry them around the body using the lymphatic system, usually used for transporting blood plasma.

These Lyft-like organisms are known as dendritic cells, but their role as rides for some kinds of bacteria raises questions about why they’re happy to carry certain microorganisms while killing those they view as pathogens.

Just as Lyft drivers presumably don’t pass moral judgment on prospective passengers, it seems odd – but admittedly amazing – that dendritic cells only pick up microbes that are going to prove beneficial, not necessarily to their host, but to their host’s munchkin.

Whatever the precise mechanism, maternal microbes can definitely be transferred from gut to breast milk.

We know this thanks to an experiment that involved asking breastfeeding women to swallow capsules of live bacteria (of a non-pathogenic type, we hasten to add).

These “marker microbes” were then isolated in the women’s breast milk samples.

Studies involving mice have also demonstrated that bacteria from the gut transfer into the lymph and mammary glands during late pregnancy and the start of lactation.

In addition, we know that breast milk benefits infants, sometimes in quite unexpected ways.

Scientists have identified no fewer than 200 different complex sugars, called Human Milk Oligosaccharides (HMOs), in human breast milk.

However, despite being the third most common ingredient in breast milk, babies cannot actually digest them.

In theory, natural selection should have done something about this – unless the HMOs are actually there are for some other clever reason.

And indeed they are.

Thanks to two unconnected groups of researchers coming together in the 1950s, we now know that HMOs pass intact through the stomach and small intestine, to end up in a baby’s large intestine, which is where his or her bacteria hang out.

And guess what?

HMOs then actually provide nourishment, not for the baby, but for their bacteria.

The whole thing gets even more ingenious because HMOs don’t just feed any old species.

Oh no.

In fact, it turns out that they’re targeted at one very specific sub species – Bifidobacterium longum infantis – which one group of researchers termed a “champion colonizer of the infant gut.”

B. longum infantis is regarded as delivering big health benefits to a baby, with lifelong consequences.

The role of breast milk in a baby’s development can clearly be substantial, so let’s hear it for National Breastfeeding Month.

Do you think they sell celebratory cards for it at Hallmark?

We’ll leave that one with you.


More reading

A History of Infant Feeding

Breast-Feeding the Microbiome

How Long Did Neandertals Breastfeed?

Human milk: a source of more life than we imagine.

Mastitis While Breast-Feeding

National Breastfeeding Month 2017

The bacteria in breast milk

The human milk microbiota: origin and potential roles in health and disease.