New research in mice has discovered that disrupting the gut microbiome may result in more aggressive breast cancer.

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One factor that may drive the spread of breast cancer is the population of gut bacteria.

Although the outlook of people with breast cancer has improved dramatically in recent years, predicting and preventing the spread of cancer to other parts of the body (metastasis) continues to be a major challenge in the medical community.

Recent estimates place the number of women living with metastatic breast cancer in the United States at 154,794.

Around 5–9% of new breast cancer cases are already in metastasis at the time of diagnosis, according to some estimates.

There are several factors that influence the likelihood of breast cancer spreading. One of them is the hormone receptor status.

For instance, hormone receptor-positive breast cancer, which accounts for the majority of breast cancer cases, is driven by the hormones estrogen or progesterone. It usually responds well to treatment. Hormone receptor-negative cancers, on the other hand, tend to spread faster.

Another factor that may influence the spread of breast cancer "is having a high level of [immune] cells called macrophages present within the tissue," explains Melanie Rutkowski, Ph.D., of the Department of Microbiology, Immunology and Cancer Biology at the University of Virginia in Charlottesville.

"There have also been studies that have demonstrated that increased amounts of the structural protein collagen in the tissue and tumor also lead to increased breast cancer metastasis," continues Rutkowski.

She and her colleagues have recently conducted a study in mice that revealed another factor with a crucial role in breast cancer metastasis: gut bacteria.

Rutkowski and team disrupted the natural gut bacteria of mice with breast cancer using powerful antibiotics. The researchers have now published their findings in the journal Cancer Research.

Disrupting gut bacteria causes inflammation

The researchers used a mouse model of hormone receptor-positive mammary cancer. They altered the rodents' natural gut bacteria balance by giving them powerful antibiotics and performing a fecal microbiota transplant of dysbiotic, or macrobiotically unbalanced, fecal contents.

"When we disrupted the microbiome's equilibrium in mice by chronically treating them [with] antibiotics, it resulted in inflammation systemically and within the mammary tissue," Rutkowski reports.

"In this inflamed environment, tumor cells were much more able to disseminate from the tissue into the blood and to the lungs, which is a major site for hormone receptor-positive breast cancer to metastasize," she explains.

"These findings suggest that having an unhealthy microbiome, and the changes that occur within the tissue that are related to an unhealthy microbiome, may be early predictors of invasive or metastatic breast cancer," continues Rutkowski.

"Ultimately, based upon these findings, we would speculate that an unhealthy microbiome contributes to increased invasion and a higher incidence of metastatic disease."

Melanie Rutkowski, Ph.D.

She cautions, however, that the findings do not mean that antibiotics are dangerous for people with breast cancer by any means. She also warns of the dangers of generalizing these results from mice to humans.

To recreate the gut bacteria imbalance the scientists observed in the study, humans would have to take far more antibiotics than the amount doctors usually prescribe, Rutkowski says.

However, the researchers advise maintaining a healthy microbiome by adopting a healthful "diet, high in fiber, along with exercise, sleep — all of those things that contribute to positive overall health."

"If you do all of those things," Rutkowski adds, "in theory, you should have a healthy microbiome. And that, we think, is very much associated with a favorable outcome in the long term for breast cancer."