Have you ever eaten something because it has “probiotics”? Or perhaps something that contains “prebiotics”? If you’ve done either of those things, there’s a very good chance that you’ve heard of a concept called the microbiome before, or at the very least, your gut microbes. The microbiome is the entire population of bacteria, fungi, and viruses that live on or even in our bodies at all times. We eat food with pro- and prebiotics because they either contain microbes that are good for our health or feed the microbes already in our gut. And it turns out that feeding our microbiome is rather important, because there are actually more microbes on or in our bodies than human cells…

That means that, in terms of number of cells, we’re only about half human, maybe even a bit less… But, in terms of actual mass and volume, we’re still almost entirely human. Because microbes are so small, they don’t actually take up much space. For a 70kg (154lbs) person with 30 to 37 trillion human cells, those 38 trillion bacteria weigh only about 0.2kg (0.66lbs). Even so, with how many microbes live on our bodies, it’s unsurprising that they have such a big impact on our health. And so, today, we’re talking about the role they play in protecting us from cancer and promoting it.

One of the best examples of where microbes play a role in cancer progression is in the gut. That might be because the gut bacteria get so much attention for their role in other aspects of our health. But our gut microbiome also has a clear relationship with cancer. Several studies have shown that decreased diversity and balance of our gut microbiome are associated with gastric and colorectal cancer. This might not be a surprise, because microbes have been shown to protect against cancer in several different ways. The first way is by activating our immune system. Certain strains of bacteria have been associated with increased infiltration of immune cells into the tumour. This is critical because the access of immune cells to a tumour site, or rather the inaccessibility, is a major barrier that prevents effective tumour killing by our immune system. So, if bacteria can increase the presence of immune cells in a tumour, it can lead to more effective tumour killing. Certain strains of bacteria have even been shown to improve the efficacy of certain immune therapies in aggressive breast cancer. In some cases, bacteria don’t even need to recruit immune cells to kill cancer; they can simply do it themselves. By secreting certain toxins or spores, some bacteria can actually directly kill cancer cells. So, by either recruiting immune cells to kill cancer, or by killing cancer themselves, bacteria can help prevent tumour growth.

Unfortunately, not all bacteria are good bacteria. In fact, microbes can be one of the main risk factors for cancer. Helicobacter pylori is a spiral bacterium commonly found in the gut. It’s also one of the main risk factors for stomach cancer. That’s because it can burrow into the lining of the stomach, causing inflammation and interfering with the immune response. Although inflammation is usually beneficial and helps resolve infections, chronic inflammation actually increases one’s risk of cancer. So, with Helicobacter infections, chronic inflammation can promote stomach cancer. Viruses, on the other hand, have an extensive history in cancer. Several different viruses are the primary risk factors for cancer.

But HPV isn’t the only cancer-causing virus; there are plenty of cancer-causing viruses, called tumour viruses. Viruses can be particularly potent in terms of causing cancer because they can, on top of causing chronic inflammation, quite literally enter our healthy cells and can undermine cellular processes that keep our cells safe. This causes cells to divide too rapidly and makes them incredibly challenging to detect as they hide within our own cells. This is really only the tip of the iceberg when it comes to tumour viruses, but maybe we can go deeper into that another time…

The microbiome is a rich and deeply complicated part of being a person. With just as many bacterial cells as human cells, it’s crucial for countless aspects of our health, including cancer prevention. Having a diverse and balanced gut microbiome can help stave off cancer, and some of these microbes can even help kill cancer, either by recruiting immune cells or by killing cancer directly. On the other side of the coin, some microbes, viruses in particular, are the leading risk factors for some types of cancer. Today, scientists are delving deep into both sides of this coin: finding new ways to use cancer-killing microbes to treat cancer, but also expanding our knowledge of tumour viruses’ function, and how being proactive might be the best option.

I hope you learned a little something this week, but next week we’re talking about what can happen after cancer, discussing some of the long-term side effects of cancer and cancer treatment.

See you next week!

If you missed last week’s letter, you can read “Setting The Stage” to learn a little more about how cancer is staged and what it means.