The microorganisms that live symbiotically in human beings are increasingly recognized
as important players in health and disease. The largest collection of these microorganisms is found
in the gastrointestinal tract. Microbial composition reflects both genetic and lifestyle variables
of the host. This microbiota is in a dynamic balance with the host, exerting local and distant
effects. Microbial perturbation (dysbiosis) could contribute to the risk of developing health problems.
Various bacterial genes capable of producing estrogen-metabolizing enzymes have been identified.
Accordingly, gut microbiota is capable of modulating estrogen serum levels. Conversely, estrogen-like
compounds may promote the proliferation of certain species of bacteria. Therefore, a crosstalk
between microbiota and both endogenous hormones and estrogen-like compounds might synergize
to provide protection from disease but also to increase the risk of developing hormone-related
diseases. Recent research suggests that the microbiota of women with breast cancer differs from
that of healthy women, indicating that certain bacteria may be associated with cancer development
and with different responses to therapy. In this review, we discuss recent knowledge about the
microbiome and breast cancer, identifying specific characteristics of the human microbiome that may
serve to develop novel approaches for risk assessment, prevention and treatment for this disease