Bad bugs and biofilms don't cause breast inflammation when you're lactating
What's in your milk's microbiome?
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.
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 have hypothesised to cause pathological biofilm inside milk ducts, Staphylococcus epidermiditis, isn’t linked 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. We can trust the million's of years of your breasts’ immune history.
Why the theory that bad bugs and biofilm cause breast inflammation when you're lactating doesn't make sense
Lately some have 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.
This 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.
The belief that biofilm causes breast inflammation arises out of an old-fashioned understanding of the nature of bacteria. It's a theory which doesn’t trust the way your body and immune system has evolved over millions of years to self-regulate and protect your breasts.
This theory not only lacks supporting evidence, it mixes up findings about 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. And then applies these findings to the finely branching network of milk ducts inside your breasts, even though this is a completely different kind of environment.
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.
There is no reason to think that changes in the milk itself causes breast inflammation, although lots of changes do occur in your milk in response to breast inflammation! The concept of milk stasis is not helpful because milk secretion is never in stasis.
Recommended resources
Is the concept of milk dysbiosis helpful?
Selected references
Douglas PS. Does the Academy of Breastfeeding Medicine Clinical Protocol #36 'The Mastitis Spectrum' promote overtreatment and risk worsened outcomes for breastfeeding families? Commentary. International Breastfeeding Journal. 2023;18:Article no. 51 https://doi.org/10.1186/s13006-13023-00588-13008.
Douglas P. Re-thinking benign inflammation of the lactating breast: a mechanobiological model. Women's Health. 2022;18:17455065221075907.
Rodriguez JM, Fernandez L. Infectious mastitis during lactation: a mammary dysbiosis model. In: McGuire M, Bode L, editors. Prebiotics and probiotics in human milk: Academic Press; 2017. p. 401-428.
Mitchell K, Eglash A, Bamberger E. Mammary dysbiosis and nipple blebs treated with intravenous daptomycin and dalbavancin. Journal of Human Lactation. 2020;36(2):365-368.
Mitchell KB, Johnson HM, Rodriguez JM, Eglash A, Scherzinger C, Cash KW, et al. Academy of Breastfeeding Medicine Clinical Protocol #36: The Mastitis Spectrum, Revised 2022. Breastfeeding Medicine. 2022;17(5):360-375.