The Bidirectional Relationship Between Cancer and the Microbiome

Throughout the human body, there are trillions of microbes that have made themselves at home. Our microbiome consists of bacteria, fungi, and viruses that interact with our cells and help drive our bodies at a cellular level.

In recent years, research has begun to focus on how the microbiome might impact different diseases and conditions in human health. Largely, studies have shown the potential to improve clinical outcomes by manipulating the microbiome in various ways. In particular, more research is investigating the relationship between the microbiome and cancer. While there’s still much to learn about this complex relationship, a better understanding of how the microbiome impacts cancer development and treatment could have huge clinical benefits.

What is the microbiome?

A microbiome consists of microorganisms in a specific environment. In this case, that environment would be different parts of your body like your skin, mouth, and especially your gut. Your gut microbiome is home to billions of bacteria, with 100 billion resident bacteria in each gram of stool. Together, these bacteria impact a variety of different processes in your body such as  hormone metabolism, inflammation, and much, much more.

Every person is born with a microbiome that continues to change and develop as they grow. Many different factors can impact your individual microbiome. Some of these are environmental factors such as where you live, whether you spend time around animals, or what you eat. Age, genetics, and even some diseases also affect the community of microorganisms in your personal microbiome.

 How does the microbiome impact cancer?

Recently, more studies have begun to investigate how the microbiome impacts cancer. 

 Some of the earliest studies looking at the microbiome and cancer focused on colon cancer. It’s been shown that having an imbalanced microbiome, or what scientists call “dysbiosis”, can lead to some types of cancer, including colon cancer. This imbalance could be due to lack of certain types of bacteria or to changes in the amounts of different bacteria. These variables factor into how researchers measure diversity in the microbiome. Alternatively, this can also work the other way around. In some cases, cancer is what changes the composition of the microbiome. For example, bone tumors with high levels of hydroxyproline (an amino acid found in collagen) change the bone microbiome by increasing the number of bacteria capable of degrading hydroxyproline.

While having more bacterial diversity is good in the gut, greater microbiome diversity in the female reproductive tract has been associated with human papillomavirus (HPV). One way in which HPV infection, the major cause of cervical cancer, can be prevented is through vaccination. Although vaccines and cervical cancer screening have been largely effective in reducing cervical cancer rates in the US, prevention is dependent on access to these medical services.

Your gut microbiome can also impact other types of cancer that don’t develop in your gut, such as breast cancer. Breast cancer can be heavily impacted by levels of hormones in your body, especially certain hormone-positive types of breast cancer. Studies have shown that your gut microbiome and the bacteria that inhabit it can impact hormone metabolism and, ultimately, the levels of hormones circulating through your body. High levels of circulating hormones, such as estrogen, are associated with an increased risk of developing cancer.

Even further, the gut microbiome can affect a person’s response to medical treatments, such as immunotherapy for skin cancer. Some research has indicated that having a diverse microbiome could improve the response to immunotherapy.

 How do diet, medications, or supplements affect your microbiome?

For a long time, scientists have understood that different diets lead to different communities of bacteria in the gut. It’s been shown that your microbiome can change as quickly as three or four days after drastically changing your diet. For example, the microbiome composition of meat-eaters is significantly different compared to vegetarians and vegans. Some diets designed to treat certain diseases, such as inflammatory bowel disease (IBD), also change the microbiome. This can be important to cancer prevention, as IBD increases the risk of developing colon cancer.

 Some medications can also drastically alter your microbiome. One example of this is antibiotics, which can deplete the good bacteria in your gut at the same time as fighting your infection. However, supplements such as prebiotics and probiotics have been investigated to replenish the good bacteria and rebalance your microbiome. Prebiotics can be consumed as pills or in the form of high-fiber foods like fruits and vegetables. These help to feed the good bacteria in your gut and grow their populations. On the other hand, probiotics directly repopulate the good bacteria in your microbiome. Probiotics can also be consumed as pills or in the form of fermented foods like yogurt, kombucha, or kimchi. Prebiotics and probiotics are used primarily to alleviate side effects from other cancer therapies, such as chemotherapy and radiation. However, it should be noted that recent studies have shown that some probiotic treatments may make cancer immunotherapy treatment less effective.

How can studying the microbiome help improve the lives of cancer patients?

Better understanding how the microbiome impacts cancer, and vice versa, can lead to improved treatment methods and positively impact the quality of life of patients.

 In recent years, fecal transplants have grown in popularity as a way to treat an imbalanced microbiome in various gastrointestinal diseases, such as Crohn’s, ulcerative colitis, and irritable bowel syndrome. A fecal transplant is a stool sample taken from a healthy donor and then given to the patient. Sometimes this is as easy as swallowing a pill, while other times fecal transplants can be done by infusing stool during a colonoscopy or through your nose. Many cancer treatments, such as radiation and chemotherapy, can negatively impact the composition of your microbiome and diminish the good bacteria you need in your gut. These bacteria can be replenished through fecal transplants. However, research is still being done to understand how fecal transplants can positively affect the microbiome, as well as how to prevent negative impacts from more dangerous bacteria.

 A study published in October 2021 showed that changing the microbiome can positively impact prostate cancer therapy. Prostate cancer is frequently treated with androgen-deprivation therapy (ADT), which depletes the androgens needed for prostate tumors to grow. Unfortunately, patients often become resistant to ADT and alternative treatment options are limited. However, this study found that patients resistant to ADT had a microbiome composition capable of making androgens. Resistance to ADT could then be corrected by treating this imbalance in the microbiome, either through treatment with antibiotics, fecal transplant, or Prevotella species (a bacteria that is depleted in resistant patients).

 Ultimately, research has only begun to touch on how the microbiome can be manipulated to improve cancer treatment and the lives of patients.