Bacteria differ between healthy breasts and those with cancer, and beneficial bacteria may actually be protecting women against cancer. These preliminary findings may ultimately lead to the use of probiotics to protect women against breast cancer.1

Breast tissue is not sterile; it contains a diverse population of bacteria. The authors, led by Gregor Reid, PhD, of Western University, London, Ontario, Canada, investigated the hypothesis that a host’s local microbiome could be modulating the risk of breast cancer development.

Breast tissues were collected from 58 women who were undergoing lumpectomies or mastectomies for either benign (13 women) or cancerous (45 women) tumors, as well as from 23 healthy women who had undergone breast reductions or enhancements. DNA sequencing identified bacteria from the tissues, and then the bacteria were cultured to confirm that they were alive.

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Women with breast cancer had elevated levels of Escherichia coli and Staphylococcus epidermidis, which are known to induce double-stranded breaks in DNA in HeLa cells (a type of cultured human cell).

“Double-strand breaks are the most detrimental type of DNA damage and are caused by genotoxins, reactive oxygen species, and ionizing radiation,” the investigators explained. The repair mechanism for double-stranded breaks is highly error prone, and such errors can lead to cancer development.

In contrast, the healthy breast had a higher prevalence of Lactobacillus and Streptococcus, which are considered to be health-promoting bacteria. Both of these bacterial groups have anticarcinogenic properties.

Reid explained that the research was motivated by the knowledge that breast cancer decreases with breast feeding.

“Since human milk contains beneficial bacteria, we wondered if they might be playing a role in lowering the risk of cancer. Or, could other bacterial types influence cancer formation in the mammary gland in women who had never lactated? To even explore the question, we needed first to show that bacteria are indeed present in breast tissue,” said Reid. (They had shown the presence of bacteria in breasts in earlier research.)

However, lactation might not even be necessary to improve the bacterial flora of breasts.

“Colleagues in Spain have shown that probiotic lactobacilli ingested by women can reach the mammary gland,” said Reid. “Combined with our work, this raises the question, should women, especially those at risk for breast cancer, take probiotic lactobacilli to increase the proportion of beneficial bacteria in the breast? To date, researchers have not even considered such questions, and indeed some have balked at there being any link between bacteria and breast cancer or health.”

Reid explained that probiotics might allow an increase in the abundance of beneficial bacteria at the expense of harmful ones. Antibiotics targeted bacteria that aid cancer also might be an option to improve breast cancer management.

In any case, something keeps bacteria in check on and in the breasts, as it does throughout the rest of the body, said Reid. “What if that something was other bacteria — in conjunction with the host immune system? We haven’t answered this question, but it behooves experts in the field to now consider the potential.”


1. Urbaniak C, Gloor GB, Brackstone M, et al. The microbiota of breast tissue and its association with tumours. Appl Environ Microbiol. 2016 Jun 24. doi:10.1128/AEM.01235-16. [Epub ahead of print]