Can the bugs in my gut really treat my cancer?

Ok, well maybe not yet, but it might not be so far away. The last few years have seen a surge in interest from consumers, researchers, and clinicians alike, wanting to know what microbes – living organisms that are too small to see with the naked eye – can do to help (or hinder) cancer treatment. And while bugs have colonised every nook and cranny of the human body, as the largest single community of microbes it’s the ones in the gut that have received the most attention. Even in the gut, most research to date has concentrated on bacteria, but it’s important to remember that there are lots of other microbes such as viruses and fungi that might be significant, so we’re only just beginning to understand how all the pieces of this puzzle fit together.

What’s the difference between microbes, microbiota and the microbiome?

When studying microbes (also called “microbiota” as a group or community), researchers often want to know whether specific microbes are present, and how much of one is present relative to another. But studying bacteria directly is difficult; they can be hard if not impossible to grow in the laboratory, and there can be many hundreds of different species present in any one sample. So, most of the time researchers analyse the “microbiome” which is the collective genetic information that comes from the entire microbial community. It turns out that the microbiome is relatively easy to measure using modern gene-reading techniques called next generation sequencing. This lets researchers measure everything that’s present in one go, and they can then work backwards using clever analytical tools to understand the role played by individual bacteria.

What do we already know about the microbiome and human health?

Our Haematology colleagues who treat blood cancers like leukaemia have known for a long time that the diversity of the gut microbiome – a measure of how “multicultural” the bacterial community is in our gut – has a huge influence on side-effects and survival after stem cell transplantation for acute leukaemia. The gut microbial community has been implicated in regulating all sorts of non-cancer disease too, like cardiovascular health, allergic disease, and even certain psychiatric conditions. In other situations, specific microbes are important, for example, Fusobacterium is implicated in both the development and spread of bowel cancers.

Is it better to have champion players, or a champion team?

Over the decades, attention has swung from looking at what individual microbes are doing to what the community of microbes is doing, and back again. Several studies have now shown that community-level characteristics (for example, diversity) matter, whereas others have shown the importance of specific bacterial species (e.g. Akkermansia muciniphila, Faecalibacterium prausnitzii, Bifidobacterium longum, Bacteroides spp.) can influence the effectiveness of modern anti-cancer immunotherapies¹ like anti-PD-1 and anti-CTLA-4 drugs. Similarly, gut microbes may influence the likelihood of developing immunotherapy-related side-effects like inflammatory diarrhoea and colitis. But it’s fair to say that despite some common themes, similar studies from around the world reveal a mixed bag of results when it comes to naming “good” and “bad” bacteria, possibly due to differences in lifestyles, diet, and genetics, of the populations studied. Increasingly, it’s felt that what the whole microbial community is doing is more important than what any individual microbe is contributing, and the field of “metagenomics” is looking at just that. So, as a general rule, it seems that a champion team beats a team of champions after all.

Can I change the microbes in my gut to improve my cancer treatment?

Now; possibly. In the future; probably. The theory is that cancer patients may have the wrong kinds or amounts of microbes in their gut, called “dysbiosis”, which makes their immune systems somehow less able to fight cancer. Already there have been studies showing that faecal microbiota transplantation (“FMT” for short) – basically giving processed faecal material from a healthy patient to a cancer patient – might promote better responses to cancer immunotherapy², and treat immunotherapy-related side effects like colitis. What isn’t clear is exactly what features reliably define a “good” or a “bad” microbiome, therefore it’s also not yet known who would make the ideal faecal transplant donor. A considerable amount of effort is underway worldwide to help answer these questions through research and clinical trials. FMT is promising, but is definitely still an experimental treatment in cancer patients.

Is there anything special I should be eating?

We are what we eat, but more importantly, we are what the bugs in our gut eat, because a lot of the nutrients we need are pre-digested or even created by those microbes from the food we consume. Currently there’s no strong data to indicate that eating anything unusual – including supplements and exotic ingredients – is helpful, but more research in this area is needed. Preliminary data suggests that exposure to antibiotics or probiotic supplements may be detrimental, but individual circumstances may make these necessary³ Evidence is building that dietary fibre and resistant starches might be the most important element of the diet to promote a good anti-cancer immune response. The recommended Australian dietary fibre intake is ≥25grams per day (g/d) for women and ≥30g/d for men yet less than 30% of adults meet this level. In one study of melanoma patients, for every 5g/d increase in dietary fibre intake the risk of melanoma progression decreased by nearly 30%. Until more well-designed studies in cancer patients can answer the question of what to eat, for now, it seems like the same advice that is good for general health applies equally to cancer patients receiving immunotherapy. The acclaimed academic and food writer Michael Pollan put it best: “Eat food. Not too much. Mostly plants”.⁴

At the end of the day, we could do better at acknowledging the vast numbers of microscopic hangers-on that we share our lives with, because our health depends on it!

1. Routy et al., Science 2018;359(6371):91-7, Gopalakrishnan et al., Science 2018;359(6371):97-103, Matson et al., Science 2018;359(6371):104-8 [↩]
2. Routy et al., Nat Med 2023, Baruch et al., Science 2021;371(6529):602-9 [↩]
3. Routy et al., Science 2018;359(6371):91-7, Spencer et al., Science 2021;374(6575):1632-40 [↩]
4. Michael Pollan. “In Defense of Food: An Eater’s Manifesto”. Penguin Press. 2008 [↩]