Bad bugs and biofilms don't cause breast inflammation when you're lactating

The female breast or mammary gland evolved from ancient mammalian immune systems

Two million years ago, a primordial ancestor secreted immune fluids from her body to protect her young.

The collection of cells which secreted this powerful fluid became what we call her mammary gland. She, this ancient grandmother, became the first mammal. Evolutionary biologists think that the special sheltering fluid she made for her baby only developed into the caloric-rich nutrition which we call milk much later on down the evolutionary journey.

So before all else, from the dawn of time, our lactating Homo sapiens breasts has brought the human baby into the shelter of the maternal body's immune ecosystem, washing every cell of that nascent little human in a pulsing network of chemical and cellular protection. This mammary immune ecosystem includes interactions between four key elements

1. The microbes in a mother's milk and their chemical secretions (metabolites)

2. The bacteria or microbes in a mother's breast tissue (stroma) and their chemical secretions (metabolites)

3. The protective immune cells in a mother's milk (white cells)

4. A mother's overall immune system, the many elements of which are brought into her breast by the blood stream (capillaries) as required.

Human milk is alive with microorganisms which are foundational to the Earth's biosphere and which help keep both a woman and her baby safe

By the time I was born, in 1960, scientists knew that human milk was a living tissue, full of powerfully protective goodness. But human milk was widely believed to be sterile, even 'pure'. Any bacteria found in it were thought to be contaminants or backwash from the nipple.

The first brave Argentinian researchers who published the finding of a milk microbiome were ignored by the scientific community. But in the last couple of decades it's become clear that human milk definitely isn't sterile!

The discovery of a teaming microbiome in a woman's milk should not surprise us. Microorganisms were the first living things on our Earth. Bacteria are foundational to our planet's biosphere, which could not have supported the development of more complex life forms without the changes the bacteria created. Plants and animals evolved from microbial ancestors. This means that humans, too, evolved from microbial ancestors. Microbes continue to live in us, and on us, and our essential for life. Human bodies are made up of more bacterial cells than human cells. Bacteria, with a few important exceptions, take care of us and protect us.

Human milk is alive not just with two hundred or more kinds of bacteria, but with fungi including thrush or Candida, viruses, protozoa and other organisms. This life-giving microbiome of your milk has a smaller load of microbes than is found in other sites of the body, but remains richly diverse.

The make-up of the human milk microbiome varies from woman to woman and throughout her lactation in ways that are highly dynamic and variable. Scientists increasingly recognise that they are not able to be meaningfully document the categories of microorganisms in human milk for this reason.

The milk microbiome interacts with the cells and other elements of human milk to provide immune protection. Fluctuating and dynamic microbiome diversity promotes the resilience of the host, always acting to keep populations in balance so that no specific microbe species becomes so dominant that it turns destructive.

Human milk is a living tissue which contains living (and dead) somatic cells

As well as a microbiome, human milk also contains what we collectively call somatic cells. There are about 4000 of these living cells (which don’t include microorganisms) in each millimetre of your milk. The cell populations in human milk are highly dynamic and variable.

The population of somatic cells is made up of

• Protective white cells. There are high numbers of leucocytes in colostrum, but they fall by four-fifths to comprise just 2% of cells in mature milk. But when the breast is stressed with a ‘’mastitis’, the white cells increase to 95% of the milk cells. They increase but less so with painful nipples or with blocked ducts

• Luminal cells (including lactocytes), the kind of mammary epithelial cells which line the alvoli and lactiferous ducts, which are regularly shed. Some of these cells still secrete milk proteins once they've fallen off their basement membranes and are floating freely in the milk! 98% of the somatic cells are mature lactocytes and myoepithelial cells shed from the duct linings. This because the duct linings constantly renew. (The mammary epithelial cells in a lactating breast are highly dynamic and adaptive.)

• Myoepithelial cells, the kind of mammary epithelial cells which hug the basement membrane and wrap star-like round the luminal cells.

• Up to 6% of these cells are stem cells, which have the capacity to repair tissue by becoming lactocytes and myoepithelial cells.

What is a biofilm?

Biofilms are a normal part of human microbiomes. It might be a community of just a few dozen microorganisms, or it might be made up of hundreds of thousands or more. Your skin, for example, has normal biofilms living on it, and if there isn’t a skin biofilm, you might develop a disease like hidroadenitis suppuritiva.

A biofilm gives members of its community adhere together, communicating amongst one another, and this which protects them from environmental stresses. But there is no reason to think that pathological biofilm forms inside human milk ducts to cause inflammation. In fact, a new study shows that the organism that some people hypothesise causes pathological biofilm inside milk ducts, Staphylococcus epidermiditis, isn’t particularly associated with mastitis.

It is most likely that any biofilm that forms in human milk is at the end-stage of the inflammatory disease process, not causative.

Inflammation in your breast can be thought of as your breast’s protective response to the tissue being stressed. Breast ducts, glands and stromal tissue are stressed by mechanical forces and rising milk pressures – which can be caused by all sorts of things. (SEE MECHANISMS)

We can trust your breasts’ two million year history.

The breast stroma or tissue itself also contains bacteria

Why the theory that bad bugs and biofilm initiate breast inflammation when you're lactating don't make sense

Lately it’s been widely claimed that inflammation in your breast during breastfeeding is caused by biofilm. This theory proposes that bacteria create biofilms to line all your tiny milk ducts, which thicken up sometimes (without clear reasons) to form into pathological biofilms which obstruct the duct and cause pain and inflammation.
Now, it's been claimed that bacterial biofilm (including Staphylococcus epidermidis) normally lines all milk ducts right throughout each breast. The problem, this theory claims, occurs when they thicken up.

The biofilm theory was previously used to justify long courses of antifungal medications, claiming that the Candida had formed biofilms in the ducts. You can find out about thrush or Candida and breast pain here.

Research into pathological biofilm has occurred in the context of burns and vascular or diabetic ulcers and should not be extrapolated into the incredibly immune-rich environment of the lactating breast.

But this idea that it’s biofilm causing the inflammation is old-fashioned concept, arising out of an old-fashioned understanding of the nature of bacteria. It's a theory which doesn’t trust the way your body has evolved in a two million years history to self-regulate and protect your breasts. Biofilms are a normal part of healthy human microbiomes.16

The milk microbiome contains bacterial species also found on nipple skin and in the infant oral microbiome, regardless of whether or not there is nipple damage, and some have been identified in colostrum prior to the first breastfeed, perhaps due to translocation from the maternal gut. The human milk microbiome is best understood as an extension of, and in complex stabilising interactions with, the mammary immune system.17-19

This theory lacks supporting evidence, and confuses normal skin biofilms (composed of aggregates of dozens or hundreds of cells) with the complex, extensive and microbial resistant biofilms which form when medical devices are inserted in the human body or when vascular insufficiency or diabetes cause chronic wounds.

It has been hypothesised that most bacteria in human milk is planktonic, that is, floating freely within the fluid, though it’s possible that some bacteria are associated with milk immune cells in vivo. There is no evidence to support the hypothesis that pathogenic biofilm causes sticky milk, duct blockage, or breast inflammation.

Biofilm formation is a potential property of the various Staphylococcus strains which have been isolated from human milk but pathologic biofilm formation in a lactating breast is likely to be a late-stage manifestation of severe inflammation or tissue necrosis, not causative.6, 8, 10

Breastfeeding women sometimes express stringy or solid milk substance in the context of blocked ducts or mastitis. This is most accurately understood as a very late-stage bi-product of inflammatory cascades, not causative.

There is no reason to think that changes in the milk itself causes breast inflammation, although lots of changes will occur in the milk in response to breast inflammation! The concept of milk stasis is not helpful because milk secretion is never in stasis.

The bacteria in your milk and breast stroma change in response to inflammatory processes, as part of your mammary immune system's mechanisms for downregulating inflammation - but don't initiate breast inflammation in lactation

A woman’s milk, and her lactating breast tissue, and also any breast inflammation, will never be sterile.

WHO: “Many lactating women who have potentially pathogenic bacteria on their skin or in their milk do not develop mastitis. But many women who do develop mastitis do not have pathogenic organisms.” (In Ingam).

As we increasingly grasp the complexity of human microbiomes, scientists now tell us that the old distinction between pathogenic and commensal bacteria is simplistic and often unhelpful. That's not to say that there aren't bacteria, such as certain strains (but not all strains) of Vibrio cholerae, which cause potentially fatal infections in humans. But what matters very often is context. Many bacteria normally exist in our microbiomes but may on occasions take advantage of biological disruption to dominate, and cause destruction to the host. An example of this may be Staphylococcus aureus and its dominance in end-stage breast inflammation.

The belief that mastitis is triggered by external infection which enters the breast arose during a time when human milk was believed to be sterile. A biological systems approach to breast inflammation challenges the assumed central role of pathogenic bacteria.

Now, researchers hypothesise that severity is associated with worsening inflammation, to which bacterial overgrowth including bacteria with pathogenic potential may contribute.

There is evidence that certain bacteria, in particular Staphylococcus aureus, are more common in women with mastitis than those without (Wilson systematic review).6 In mastitis, cultures show fewer species at higher concentration.

The most important thing when you have a breast inflammation is to remove

• The underlying backpressure of milk in the alveoli, and

• Remove all the obstacles that can worsen breast inflammation.

Then, your two-million-year-old breasts know what to do, and I would expect the number of women who go on to develop worsening mastitis to decrease.