The microbiome of the human mammary gland stroma: composition and perturbations

The breast stroma contains a microbiome

Female breast stroma contains a distinct microbiome which differs from that found in breast skin, buccal swabs, and human milk. The human breast tissue microbiome has been predominantly studied in the context of breast cancer and benign breast disease.

• A meta-analysis by Rad et al 2025 identified "Proteobacteria, Firmicutes, Actinobacteriota, and Bacteroidota as the dominant phyla in breast tissue, with Staphylococcus and Corynebacterium frequently detected across studies. ... Variability in bacterial genera was observed across primer sets and studies."

• Breast tissue microbiome varies with stromal composition, including with the percent of fatty tissue and fibrosis. (Stromal fat is lower and fibrosis higher in benign disease comopared to the stroma from women with breast cancer.)

Studies of the breast stroma microbiome again emphasise the complex interplay found within the stromal microenvironment.

The stroma microbiome changes if breast cancer develops

Research shows clear links between the breast tissue stromal alterations associated with breast cancer and the stromal microbiome.

• The Rad et al 2025 meta-analysis stated: "While microbial diversity was similar between cancer and cancer-adjacent tissues, they both exhibited a lower diversity compared to normal and mastitis-affected tissues."

• There are distinct differences between stromal microbial communities in breast tissue from women with breast cancer compared to women with benign breast disease. Certain microbial communities including Lactobacillus are enriched in maligant breast tissue.

• The breast tumour microenvironment is profoundly impacted by the resident microbiome, with bacterial signaling molecules modulating breast cancer cell viability and proliferation in vitro.

• High Staphylococcus abundance in the breast stroma of women with breast cancer was associated with a 4.1 fold increased mortality risk. I propose this can be explained by an elevated immune response in the context of more aggressive disease, discussed in Possums Breastfeeding & Lactation in the context of the elevated Staphylococcus counts in milk during an episode of breast inflammation.

Selected references

Hieken TJ, Chen J, Hoskin TL et al. The microbiome of aseptically collected human breast tissue in benign and malignant disease. Sci Rep. 2016 Aug 3;6:30751. doi: 10.1038/srep30751. PMID: 27485780; PMCID: PMC4971513.

Hieken TJ, Chen J, Chen B et al. The breast tissue microbiome, stroma, immune cells and breast cancer. Neoplasia. 2022 May;27:100786. doi: 10.1016/j.neo.2022.100786. Epub 2022 Mar 30. PMID: 35366464; PMCID: PMC8971327.

Rad SK, Yeo KKL, Wu F et al. A systematic review and meta-Analysis of 16S rRNA and cancer microbiome atlas datasets to characterize microbiota signatures in normal breast, mastitis, and breast cancer. Microorganisms. 2025 Feb 19;13(2):467. doi: 10.3390/microorganisms13020467. PMID: 40005832; PMCID: PMC11858161.