TABLE 1.

Association of gut and breast tissue microbiome with breast cancer metastasis.

Microbiome type	Study type	Microorganisms involved	Mechanism	Impact on metastasis	References
Gut microbiome	In vitro and in vivo	Fusobacterium nucleatum	Bacterial colonization suppresses accumulation of tumor‐infiltrating T cells and NK cells	Promotes metastasis	[83]
	Clinical	Clostridiaceae, Faecalibacterium, Ruminococcaceae	β‐Glucuronidase producing microorganisms convert conjugated inactive form of estrogen to deconjugated biologically active form	Promotes metastasis	[35]
	Clinical	Bifidobacterium, Blautia, Faecalibacterium, Prausnitzii	Alteration of the enterohepatic circulation of estrogens and/or the metabolism of phytoestrogens	Not specified	[45]
	In vitro and in vivo	Staphylococcus xylosus, Lactobacillus animalis, and Streptococcus cuniculi	Enhances resistance to FSS by reorganizing the actin cytoskeleton	Promotes metastasis	[15]
	In vitro and in vivo	Escherichia coli	Indole‐propionic acid reduces expression of vimentin, FGFBP1, Snail, and β‐catenin; and upregulates expression of E‐cadherin to suppress epithelial–mesenchymal transition	Inhibition of cancer metastasis	[87]
	In vitro and in vivo	Escherichia coli	Cadaverine (produced by the intestinal microbiome) reduces motility and metastatic nature of cancer stem cells by restoring epithelial–mesenchymal transition	Inhibition of cancer metastasis	[88]
	Clinical	Lactobacillus, Bifidobacterium, Escherichia coli, and so forth	Regulation of inflammation and immune response	Inhibition of cancer metastasis	[89]
	In vivo	Streptococcus, Campylobacter and Moraxellaceae	Steroid hormone biosynthesis by these bacterial species influences bone metastasis	Promotes metastasis	[17]
	In vivo	Antibiotic‐induced commensal dysbiosis	Dysbiosis enhanced levels of circulating tumor cells and metastasis in the lungs	Promotes metastasis	[86]
Breast tissue microbiome	In vitro and in vivo	Staphylococcus aureus	S. aureus induces autophagy‐dependent neutrophil extracellular traps that increase breast cancer cell metastasis	Promotes metastasis	[90]
	In vitro and in vivo	Staphylococcus and Lactobacillus	Invasion of tumor cells with bacteria trigger certain changes in tumor cell behavior which include the metastatic property	Promotes metastasis	[15]
	Clinical	Proteobacteria spp. and Listeria spp.	Influences expression profiles of genes involved in epithelial–mesenchymal transition	Promotes metastasis	[91]
	Clinical	Bacteroides fragilis	Breast tumor progression and metastasis through the secretion of the B. fragilis toxin (BFT)	Promotes metastasis	[92]
	Clinical, in vitro, and in vivo	Fusobacterium nucleatum	F. nucleatum‐derived extracellular vesicles enhanced breast cancer cell metastasis via toll‐like receptor 4	Promotes metastasis	[93]