Is the concept of human milk dysbiosis helpful?

The term dysbiosis is now in widespread use

Dysbiosis is now frequently identified as a significant aspect of disease, and some researchers are interested in "whole body dysbiosis". A reductionist lens is typically applied, so that laboratory findings of dysbiosis trigger, for instance, recommendations for prebiotic and probiotic treatments.

Some human microbiome researchers are increasingly concerned that the concept of dysbiosis is based on an outdated assumption of a normative, categoriseable eubiotic state. Human microbiomes are not able to be definitively taxonomically categorized due to their astonishing complexity, and are highly variable between individuals and within the one individual over time.

• Some researchers argue that a focus on cataloguing microbiomes will yield little extra clinical advantage.

• Microbial diversity is not always associated with improved health, as is currently assumed.

• Researchers have hypothesised that a fluctuating and dynamic microbiome reduces the probability of a specific organism assuming control and makes it easier for the immune system to control the environment. For example, chronic diabetic foot ulcers with a fluctuating microbiome had a higher probably of healing compared to those with a less varied and less fluctuating microbiome. If the microbiome stopped changing, the wound remained non-healing.

• The pathway from dysbiosis to pathology typically involves multiple host vulnerabilities and environmental triggers, complicating causal connections derived from observational data. The wide variety of factors associated with clinically relevant microbiome imbalance (or dysbiosis, or infection) — like virus-triggered chronic allergic inflammation in chronic rhinosinusitis, or host predispositions and obstetric risks linked to vaginal microbiota changes — argues against simplistic, dysbiosis-centric etiological models.

NDC advocates for a biological systems perspective on dysbiosis

For these reasons, NDC advocates for a more systems-oriented perspective on how microbial balances (or imbalances) interact with host biology. The NDC Clinical Guidelines for Breast Inflammation draw upon the work of microbiome researchers who argue that 'dysbiosis' is most accurately used to refer to a host-microbiome interaction which is consistently linked with suboptimal clinical presentations and outcomes (rather than to lists and comparisons between the myriad kinds of micro-organisms comprising a microbiome).

This theoretical re-framing acknowledges that critical host factors control or modulate microbiome populations, and that the human microbiome is really a DNA fusion or 'chimera' made up of microbiome DNA and human cell DNA, blended together. The focus then, clinically, is on the environmental factors which optimise microbiomes and health in general, without attempting to quantify microbiome change.

It's true, however, that powerful transnational prebiotic and probiotic industries remain eager to benefit commercially from diagnoses of dysbiosis! The global probiotics market was valued in 2023 at USD $87.70 billion, and is projected to reach USD $220.14 billion by 2030. (By way of comparison, the global infant commercial milk formula (CMF) market size was valued at USD $81.72 billion in 2024, and the CMF market is projected to grow from USD $90.91 billion in 2025 to USD $178.83 billion by 2032.)

The diagnosis 'mammary dysbiosis' drives antibiotic overuse

The pathogenicity of most bacterial species depends

1. On the state of the host microenvironment, and

2. On the strain of the bacteria.

Unfortunately, the terms commensal and pathogenic are unhelpful in discussions of milk microbiome and breast inflammation, because pathogenicity in the context of human milk is on a spectrum, and context specific. A potentially pathogenic microbe which exists quite normally within the human milk microbiome is only pathogenic when regulatory feedback loops have been overwhelmed, resulting in prolonged or severe clinical illness and the need for antibiotics.

Milk dysbiosis should not be a prescriptive, etiologic diagnosis, which is the case, for instance, in popular use of the diagnoses 'mammary dysbiosis' and 'subclinical mastitis'. Contemporary milk-focused research documents associations between certain outcomes (e.g. mastitis) and specific microbial signatures (e.g. dominant presence of Staphylococcus aureus), but evidence is correlative.

The causal chain from a milk microbial pattern to health or disease is unclear and mediated by multiple host factors (genetics, immunity, maternal metabolism), feeding patterns, antibiotic exposures, and environmental exposures. This correlational nature aligns with broader critiques of dysbiosis as an unhelpfully simplistic framing.

Conceptually, there is no universal, site-neutral definition of dysbiosis, and applying a broad, infection-centric notion to human milk risks mislabeling normal biological variation or beneficial immune responses as pathological. We discuss the link between S. aureus and mastitis elsewhere.

The image below is of a lactobacillus probiotic.

Selected references

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