In recent years, greater appreciation for microbes inhabiting human body sites has emerged. In the female mammary gland, milk has been shown to contain bacterial species, reaching the ducts from the skin. Researchers have also discovered a diverse population of bacteria within tissue collected from sites all around the breast in women ages 18 to 90, not all of whom had a history of lactation.
However, scientists have uncovered a potential role of bacteria found within breast tissue in the prevention and progression of breast cancer. Studies show that healthy breast tissue has a higher abundance of beneficial bacteria, while women with breast cancer have a higher relative abundance of Enterobacteriaceae, Staphylococcus, and Bacillus. This suggests that the microbiome may shift as a result of disease.