Breast tissue microbiota

This paper is part of the 2018 Microbiome Special Issue of NMJ. Download the full issue here.

Relation

Hieken TJ, Chen J, Hoskin TL, et al. The microbiome of aseptically collected human breast tissue in benign and malignant diseases.Scientific reports. 2016;6:30751.

Objective

To determine differences between resident microbiomes in breast tissue vs. skin and in malignant vs. non-malignant breast tissue samples.

Draft

Observational cohort study

Participant

Thirty-three women scheduled to undergo breast surgery at the Mayo Clinic had their postoperative samples analyzed. Approximately half of the women were diagnosed with breast cancer (n=17), and half were diagnosed with benign breast disease (BBB; n=16). All breast cancer patients were estrogen and progesterone receptor positive and 29% were HER2/neu receptor positive (n=4). One participant with cancer dropped out of the analysis. Of the 15 participants with breast cancer, 10 had stage I disease and 5 had stage II disease, and 13% of all breast cancer patients had lymph node involvement.

Notably, there were some differences in the characteristics of the two groups (women with cancer and women with BBB). First, the mean age of each group and, accordingly, menopausal status were significantly different. The overall median age of the cohort was 60 (range 33–84); the median age was 75 (range 44-84) for women with invasive cancer versus 49 (range 33-70) for women with BBD (p = 0.001). Of women with cancer, 86.7% were peri-/postmenopausal and 13.3% premenopausal, while 53.9% of women with BBD were peri-/postmenopausal and 46.2% premenopausal (P=0.02). The time from incision to specimen collection was also statistically different between the two groups (median 82 min vs. 52 min in those with cancer and those without, respectively;P=0.0001).

The presence of a change in the flora before the disease occurs means that we may one day be able to stratify the risk of developing breast cancer based on the microbiome present in the tissue.

Study parameters assessed: Intraoperative tissue samples of the breast and overlying skin were analyzed for microbial DNA signatures using 16S rDNA tag sequencing. Buccal swabs and breast skin swabs were also obtained and analyzed in the same manner.

Key insights

Different microbial communities existed in breast tissue compared to samples of overlying skin tissue, breast skin swabs, or buccal swabs. When comparing women with cancer to women with BBD, significant differences in the microbial community were found. In particular, several taxa that are less abundant overall are enriched in cancer tissue relative to BBD tissue, includingFusobacterium,Atopobium,Gluconacetobacter,Hydrogenophaga,andLactobacilli. Finally, the nearby disease-free tissue in cancer patients compared to the nearby normal tissue in patients with BBD differed significantly across taxa (P=0.009).

Practice implications

The authors' first claim is that this study "confirms the existence of a distinct breast microbiome and differences between the breast tissue microbiome in benign and malignant diseases." The first part of this may be a bit of news to naturopaths who have compromised infant health by altering the mother's flora, or who have recommended putting a little infant probiotic on the nipple before feeding. We have long assumed that organisms originated in the breast. Perhaps we based this knowledge on the 2 studies from the 1980s^1.2this suggested the existence of a distinct breast flora, or perhaps we simply believed in the lack of evidence. According to the authors of the current study, studies from the 1980s that found various bacteria in the breast were widely dismissed, with critics pointing out that the bacteria were likely contaminants from the skin.

Interestingly, while the existence of endogenous bacteria in the breast appears to be news in medicine, it also appears to have been an "open secret" in plastic surgery circles. These bacteria are suspected to be the cause of a subclinical infection responsible for capsular contracture after implantation.³Regardless, the study reviewed here confirms our long-held belief that the breast has its own unique microbiome. That much is crystal clear.

The more intriguing aspect of the study discussed here is the presence of different microbes in cancerous breast tissue compared to BBB. The dominant taxonomy was not different, Bacteroidetes and Firmicutes dominated both samples. The differences lay in the higher levels of the normally very sparse flora:Fusobacterium,Atopobium,Hydrogenophaga,GluconacetobacterandLactobacilli(P<0.05). The last one can get our attention, providedLactobacillispp are considered beneficial insects.Lactobacilli, like all these bacteria, are only associated with the cancer, not the cause. The function of these bacteria and exactly how they interact with the various components of the stroma is not yet known.

There have been two previous studies that used molecular (rather than culture) techniques to analyze breast cancer tissue. Xuan and colleagues examined breast cancer tissue compared to normal tissue from the same donor and found that this was the caseMethylobacterium radiotoleranswas enriched in cancer tissueSpingomonas yanoikuyaewas enriched in the normal controls.⁴Additionally, they found that flora diversity was inversely associated with disease extent, with patients with advanced disease having lower diversity in the breast biome. However, it was a very small study with only 20 participants, which the authors of the study discussed here criticized as having a high potential for contamination for methodological reasons.

The second study, published by Urbaniak and colleagues, examined the breast microbiome in 81 women from Canada and Ireland with and without breast cancer.⁵The study was intended to definitively determine whether live bacteria (not just their DNA) were present in the presumed sterile breast tissue. The group found bacteria through both molecular and culture techniques, with Proteobacteria being the dominant phylum. Apart from this, this is also the dominant phylum found in human breast milk.⁶The study was not designed to evaluate differences between normal and cancerous tissue or between Canadian and Irish women.

Since the publication of the study reviewed here, Wang and colleagues have confirmed that the breast microbiome in women with breast cancer is different from the microbiome in normal breast tissue. The microbiome of the mouth, urinary tract and breast tissue was determined in 57 women with cancer and 21 women without cancer.⁷The authors found that the breast microbiome was significantly different between the two groups (P=0.03), mainly driven by the presence of Methylobacterium in cancer tissue. In addition, several gram-positive organisms includingCorynebacterium(P<0.01),Staphylococci(P=0.02),Actinomyces(P< 0.01) andPropionibacteriaceae(P< 0.01) were more common. In contrast to the current studyLactobacillispp were not enriched in breast cancer tissue. However, the presence ofLactobacilliin the urine of postmenopausal women was lower than that of premenopausal women. Oral microbiomes did not differ.

Note that while the molecular studies above show some similarities, much of the data to date is inconsistent. This is due to several factors, including the immense complexity of the microbiome, inherent differences in techniques, expected ethnic differences in biomes, and the small number of participants in each study. In summary, we can confidently say that there is a unique microbial niche within the breast itself, and breast cancer is significantly different in its microbiome signature from normal breast tissue. The details of these 2 results will also be flushed out in the future.

A unique aspect of the study discussed here is that the non-diseased tissue close to the malignancy also harbored different flora compared to the nearby tissue in patients with BBB. That's fascinating. The presence of a change in the flora before the disease occurs means that we may one day be able to stratify the risk of developing breast cancer based on the microbiome present in the tissue. This would be a means to better determine the risk of sporadic breast cancer.

Consistent with the popular metaphor of the body's microbial niches as ecosystems, integrative practitioners are uniquely trained to improve breast flora in the context of overall health. In the modern reductionist medicine model, individual strains are touted as specific to breast health. In fact, there are numerous patented therapeutic probiotics that follow this line of thought.⁸This would be equivalent to spreading a single plant seed or a handful of plants and expecting a complex and healthy ecosystem to emerge. While specific strains may eventually emerge in research, it will always be the body's entire environment that must be nurtured for the proper establishment of the breast microenvironment and its microbiota.

This is not to say that the use of certain bacterial strains is never indicated. Several Lactobacillus strains have been associated with increased immune recognition, reduced tumor growth, and increased survival in rodent models of breast cancer.⁹These include certain strains ofL casei,L PlantarumandL reuteri. That's interesting given thatLactobacillispp were enriched only in breast cancer tissue in the current study. The role of bacteria also remains to be determined.

There are currently no human outcome data to suggest that there are specific probiotics that help prevent breast cancer or its recurrence. In the absence of evidence, we often fall back on our philosophically based understanding of health and illness. In short, this can be understood as optimizing the overall health of the organism by providing all the necessary components of elements that are woven into the larger landscape of life on the planet. In the context of the breast microbiome, this is certainly our best choice.