Skin Cancer

Why Gut Bacteria Affect Skin Cancer Treatment

The evidence here is strong for a specific question, and it was published simultaneously by independent groups. Melanoma is the skin cancer that kills most people. Immunotherapy transformed its treatment by releasing the brakes on the immune system so it can attack tumour cells. It works remarkably well in some people and not at all in others, and gut bacteria turn out to be part of the difference.

A treatment that depends on the immune system working

Checkpoint inhibitors don't attack cancer directly.

Tumours display molecules that switch off approaching immune cells. These drugs block that switch, so the immune system can do what it was already trying to do.

That makes the treatment entirely dependent on how well the immune system is functioning to begin with. If the response is weak, releasing the brake achieves little.

Between 20% and 40% of patients respond. Predicting who has been a central problem, and tumour characteristics alone don't explain it.

What the stool samples showed

Researchers collected stool from patients with advanced melanoma before they started treatment, then followed what happened.

Responders had significantly more varied gut bacteria than non-responders. They also carried more of particular groups, notably the Ruminococcaceae family and Faecalibacterium, both major producers of the compounds bacteria make from fibre.

Non-responders had less varied communities, and that imbalance tracked with measurably weaker immune cell activity.

A second group published the same week reached a compatible conclusion in a separate melanoma cohort.

The pattern extended to tumour tissue. Patients with favourable gut bacteria had more immune cells infiltrating their tumours, which is the step the treatment depends on.

Source: Gut microbiome modulates response to anti-PD-1 immunotherapy in melanoma patients

Testing whether the bacteria cause it

Association between bacteria and response could run either way, so researchers moved the communities into animals.

Germ-free mice were colonised with stool from responding patients or from non-responding patients, then given tumours and treated.

Mice receiving bacteria from responders controlled their tumours better. Their immune responses were stronger.

Trials in people have since gone further. Early-phase studies transplanted stool from patients who had responded into patients whose melanoma had stopped responding to immunotherapy. Between 20% and 40% of those previously resistant patients showed an objective response.

That's a small number of patients in early trials, not established treatment. It does mean the relationship has been tested in the direction that matters.

Two practical findings worth knowing

A review gathered 95 clinical studies published between 2015 and 2025, and two results stand out for being immediately actionable.

Broad-spectrum antibiotics given within 30 days of starting immunotherapy consistently correlate with worse outcomes. That's now widely enough accepted that oncologists weigh it when antibiotics are being considered.

The second is more surprising. Over-the-counter mixed probiotic supplements also correlated consistently with poorer outcomes. The likeliest explanation is that adding large quantities of a few species disturbs the varied community the response depends on.

Running the other way, pilot studies of high-fibre, plant-forward diets improved immune cell infiltration into tumours.

So the evidence points at variety built through food rather than at supplements.

Source: Clinical Evidence for Microbiome-Based Strategies in Cancer Immunotherapy: A State-of-the-Art Review

What this means for you

A new mole, or an existing one changing in size, shape or colour, needs checking by a doctor promptly, since melanoma caught early is usually curable by surgery alone. In addition, the research above found gut bacterial variety before treatment separating people who respond to melanoma immunotherapy from those who don't, with transplants from responders producing responses in previously resistant patients. A varied, plant-rich diet supports the same gut bacteria this research ties to stronger immune infiltration of tumours, while over-the-counter probiotic supplements tracked with worse outcomes. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.