Alterations in anti-tumor immune responses and chronic inflammation are important factors influencing the occurrence of various tumors. Microbial dysbiosis (gut microbiota that express the enzyme β-glucuronidase), characterized by chronic inflammation and immune evasion, can promote tumorigenesis in breast cancer (Buchta Rosean et al., 2019; Arnone and Cook, 2022). Gut microorganisms (e.g. Proteobacteria and Firmicutes) can modulate the immune system through regulating lymphocyte proliferation via bacterial metabolites and influencing chronic inflammation and estrogen metabolism (Van der Merwe et al., 2021; Zhang et al., 2021). Furthermore, commensal bacteria (e.g. Helicobacter hepaticus) also promotes the occurrence and distant metastasis of breast cancer by influencing the anti-tumor immune functions of IL-6 and neutrophils in the tumor microenvironment (Lakritz et al., 2015; Rutkowski et al., 2015). These studies indicate that immune system disorders induced by intestinal microecological dysbiosis play an important role in the development of breast cancer.

Toll-like receptors (TLRs) are essential components of innate immune responses. They serve as pattern recognition receptors (PRRs) that detect various pathogens, including commensal microbiota. TLR4/MyD88 stimulation by Fusobacterium nucleatum (F. nucleatum) promotes tumor development via NF-κB activation in CRC (Yang et al., 2017; Yu et al., 2017; Zhang et al., 2019). Similar TLR4 and NF-κB activation has been reported in breast cancer cells stimulated by bacterial LPS, leading to the expression of inflammatory factors and apoptotic proteins (Rajput et al., 2013). Moreover, microorganisms can drive malignant progression at extra-mucosal sites via TLR5-dependent signals to increase the expression of systemic IL-6 and immunosuppressive γδ T cells to regulate tumor-promoting inflammation (Rutkowski et al., 2015). These studies indicate that intestinal microorganisms and their metabolites promote the growth and metastasis of breast cancer cells by regulating TLR-mediated innate immune responses.

Estrogen exposure is an important factor that affects the occurrence and development of breast cancer. The “estrobolome” refers to the collection of intestinal bacterial genes whose metabolites can metabolize estrogen (Plottel and Blaser, 2011). Estrogen is mainly produced by the ovaries, adrenal glands, and adipose tissues, circulates in the blood in the form of free or conjugated estrogen, and combines with its metabolites in the liver to form conjugated estrogen. Conjugated estrogen is metabolized into water-soluble molecules and excreted in urine or bile. The conjugated estrogen in bile can be decomposed by bacteria with β-glucuronidase activity in the intestine, and after reabsorption and re-entry into the circulation, it increases the bioavailability of estrogen. Circulating estrogen stimulates growth and proliferation of breast cells (Kwa et al., 2016). Therefore, the enterohepatic circulation of estrogen can affect the levels of estrogen and its metabolites in the circulatory system and may ultimately contribute to the risk of hormone-driven breast cancer.

The process by which the intestinal “estrobolome” regulates the enterohepatic circulation and reabsorption of estrogen is also influenced by host factors, such as age, diet, and antibiotics. The effects of antibiotics and diet are discussed below.

The gut microbiome plays a significant regulatory role in modulating responses to both traditional and immune therapies (Battaglia et al., 2024). They can regulate local inflammation and gut barrier function by targeting drug metabolism, modulating immune responses, and secreting different metabolites, ultimately affecting chemotherapy outcomes (Alexander et al., 2017; Sampsell et al., 2020). Chemotherapy drugs may also affect chemotherapy-induced weight gain and neurological side effects by regulating microbial diversity (Terrisse et al., 2021).

HER2 inhibitors such as trastuzumab have a good therapeutic effect on HER2-positive breast cancer. However, antibiotic treatment changes the composition of the intestinal flora, reduces dendritic cell activation and IL12p70 secretion, leads to changes in anti-tumor immunity in the tumor microenvironment, and ultimately reduces the therapeutic activity of trastuzumab (Di Modica et al., 2021). This study indicates that intestinal microbial dysregulation can regulate the host immune system and ultimately affect the therapeutic effects of chemotherapeutic drugs.

The immune checkpoint blockade (ICB) is a new-generation immunotherapeutic strategy for various cancers. Commensal gut bacteria can suppress inflammation, reshape primary and acquired immune responses, and reprogram the TME in murine models and patients, thereby influencing ICB efficacy (Xue et al., 2024). Jia et al. reported that the gut microbial metabolite indolepropionic acid (IPA) enhances immunotherapy efficacy by modulating T cell stemness in cancers (Jia et al., 2024). Additionally, using a murine model of gut microbiota dysbiosis, Shi et al. found that Lactobacillus and its metabolite lactic acid promote breast cancer progression, particularly triple-negative breast cancer (TNBC), by affecting the anti-tumor activities of immune cells in the TME (Shi et al., 2023). These results indicate that intestinal microbiota is expected to become a potential target for breast cancer treatment.

A large part of the physiological regulatory function of microorganisms is exerted through their metabolites such as short-chain fatty acids (SCFAs), bile acids, and inosine. These metabolites enter blood circulation, serve as significant modulators of the TME, and influence immune cell differentiation signals and the release of substances from both immune cells and tumors (Jaye et al., 2022a; Yang et al., 2023b).

The origin and colonization of intratumoral microbiota remain uncertain. Microbiota may colonize tumor tissues through three primary mechanisms: mucosal destruction, hematogenic invasion, and adjacent tissue migration (Cao et al., 2024; Guo et al., 2024). Additionally, the immunosuppressive, hypoxic, and nutrient-rich environments in tumors may facilitate microbial colonization and reproduction ( Figure 1 ).

F. nucleatum primarily presented in the oral cavity and gastrointestinal tract, has been implicated in various cancers, including breast cancer. This bacterium may promote tumorigenesis by affecting the infiltration and function of immune cells. Transient bacteremia during periodontal disease can facilitate oral F. nucleatum invasion into the bloodstream, leading to its translocation to the mammary glands. F. nucleatum can dock to tumor tissues via its surface-exposed lectin, Fap2, which recognizes the host Gal-GalNAc and enhances breast tumor growth and metastatic progression (Abed et al., 2016; Parhi et al., 2020). In addition, Bacteroides have been detected in canine breast tumors, as well as in the oral and gut microbiomes, suggesting a potential route of dissemination from the mouth to the gastrointestinal tract and ultimately to distant mammary tissue (Zheng et al., 2022). These studies indicate that oral microorganisms are a source of colonizing microorganisms in the breasts.

Adjacent normal tissues may also serve as potential sources of the intratumoral microbiota. The skin-related bacteria Staphylococcus epidermidis and Micrococcus luteus have been identified in mammary tumors, suggesting that these microbes may access the mammary duct through the nipple and disseminate to the mammary gland via the lobules and ducts (Urbaniak et al., 2014; Bernardo et al., 2023a). The similarities between the microbiome communities in tumors and adjacent normal tissues support the possibility of adjacent tissue invasion.

Methylobacterium radiotolerans, Bacillus, Enterobacteriaceae, Staphylococcus, Ralstonia, Bacillaceae, and Burkholderiaceae are more abundant in breast cancer and their adjacent tissues, whereas Sphingomonas yanoikuyae, Acetobacter aceti, Lactobacillus vini, and Lactobacillus paracasei are more abundant in healthy breast tissues (Xuan et al., 2014; Urbaniak et al., 2016; Hoskinson et al., 2022; German et al., 2023). Analysis of microbiome-immune networks in the breast has revealed that Anaerococcus, Caulobacter, and Streptococcus are crucial hubs in benign tissues but are absent in tumor tissues (Tzeng et al., 2021). Taken together, these studies indicate that both healthy breast tissues and breast cancer tissues have unique microbial environments.

The intratumoral microbiota of patients with breast cancer also varies according to race and sex. Metagenomic analyses have identified race-associated microbial biomarkers, such as Pseudomonas and Methylobacter in tumors from Asian women, and Amycolatopsis in tumors from black women (Siddharth et al., 2021; Parida et al., 2023b). Notably, Ralstonia was most enriched in non-Hispanic Black patients, whereas Xanthomonadales were more prevalent in non-Hispanic white patients (Smith et al., 2019). Differences in the breast microbiomes of men and women have also been reported. Tenericutes, particularly Mesoplasma and Mycobacterium, are implicated in breast carcinogenesis in both sexes (Niccolai et al., 2023). Dysbiosis extends throughout the breast tissue in females and is more localized to the tumor site in males.

Microbial signatures differ among breast cancer subtypes. Actinomyces signatures have been detected across all breast cancer types, with higher signal intensities observed in patients with HER2+ breast cancer (Banerjee et al., 2018). A larger cohort analysis revealed that each subtype had unique microbial signatures, with ER+ breast cancer showing the most diverse tumor microbiome and TNBC exhibiting the least diversity. Notably, higher abundances of Bacillus, Mucor, Nodaviridae, Toxocara, and Trichophyton in TNBC samples were significantly correlated with a better prognosis (Banerjee et al., 2021).

Although the direct effects of the intratumoral microbiota on breast cancer have been less frequently reported, evidence suggests that the intratumoral microbiota may regulate the development of breast cancer through inducing genomic instability and DNA mutations, activating carcinogenic pathways, promoting inflammatory responses, and modulating the local immune microenvironment to facilitate invasion (Xie et al., 2022; Yang et al., 2023a; Cao et al., 2024) ( Figure 2 ).

A study by Gao et al. demonstrated that F. nucleatum can induce PD-L1 expression through activating STING signals, leading to accumulation of IFN-γ+ CD8+ lymphocytes (Gao et al., 2021). This could potentially improve the efficacy of ICBs in the treatment of CRC. Similarly, the pathogenic bacterium Salmonella typhimurium exerts potent anti-tumor effects by activating the immune system (Zheng et al., 2017; Guo et al., 2020b). These studies indicate that symbiotic microorganisms play important roles in regulating host immune responses and therapeutic effects.

Tumor-resident intracellular microbiome (TRIM) can enhance tumor cell proliferation and metastatic colonization while decreasing chemotherapy efficacy (Sears et al., 2014; Geller et al., 2017; Fu et al., 2022). ETBF-secreted toxins enhance cancer cell stemness and chemoresistance by activating NUMB phosphorylation, which leads to lysosomal degradation and Notch1 activation (Ma et al., 2024). In CRC, F. nucleatum induces resistance to oxaliplatin and 5-fluorouracil (5-FU) by upregulating autophagy through TLR4/MYD88-dependent signals and preventing apoptosis via the upregulation of ANO1/BIRC3 (Yu et al., 2017; Zhang et al., 2019). In addition, F. nucleatum is involved in promoting chemotherapy resistance in esophageal squamous cell carcinoma (Liang et al., 2022) and oral squamous cell carcinoma (Liu et al., 2021a). In an animal model of breast cancer, F. nucleatum colonization was linked to reduced chemotherapy efficacy by activating autophagy-related pathways in cancer cells (Su et al., 2024).

Combining targeted antibacterial treatments with chemotherapy can suppress both TRIM and tumor cells, promoting M2-to-M1-like tumor-associated macrophage repolarization and achieving long-term survival in animal models with no recurrence (Wang et al., 2024). F. nucleatum-mimicking nanovehicles, which fuse cytoplasmic membranes with antibiotic-loaded liposomes, have been shown to selectively target and eradicate tumor-resident bacteria, thereby significantly restoring the effectiveness of chemotherapy (Chen et al., 2024c). The biomimetic nanocarriers improve the immunosuppressive TME induced by intratumoral F. nucleatum and enhance the therapeutic effect of PD-L1 (Geng et al., 2024). These studies indicate that targeting pathogenic microorganisms in tumors may help improve the efficacy of chemotherapy and prevent recurrence.

Additionally, nanoparticles coated with bacteria-derived outer membrane vesicles (OMVs) convert intratumor F. nucleatum into immunopotentiators, releasing pathogen-associated molecular patterns (PAMPs) and enhancing immunochemodynamic therapy efficacy in TNBC (Liu et al., 2023). Encapsulating bacteria-derived extracellular vesicles (BEVs) in nanocloaks can increase immunogenicity and facilitate DC maturation by activating cGAS-STING signaling. This approach, in combination with anti-PD-L1 antibodies, elicits a potent immune response and synergistically inhibits tumor progression and lung metastasis (Zhang et al., 2024). Therefore, bacteria-derived vesicles can effectively improve the immunosuppressive state of the TME, and represent a potential treatment method.

Intratumoral microbiome profiles may serve as valuable tools for predicting patient prognosis and assessing the clinical efficacy of specific drugs. A recent study by Li et al. identified four immune-related intratumor microbiomes (IRIM; e.g., Acidibacillus and Succinimonas) that have potential prognostic value (Li J. et al., 2024). These microbiomes were correlated with immune gene levels and sensitivity to chemotherapeutic agents, particularly tamoxifen and docetaxel.

Microbiota dysbiosis can trigger various immune-mediated diseases by regulating the derived metabolites and host environmental factors. miRNAs have emerged as critical mediators of host-microbiome interactions, with bidirectional effects observed between the microbiome and miRNAs in carcinogenesis (Yang et al., 2017; Yuan et al., 2019; Wang et al., 2021).Yang et al. Found that patients with both high amount of tissue Fusobacterium nucleatum DNA and miR21 had a higher risk of poor prognosis (Yang et al., 2017). Laborda-Illanes et al. revealed an increase in the expression of miR-149-5p, miR-20b-5p, and miR-342-5p in metastatic breast cancer (Met-BC) patients, compare with non-metastatic breast cancer (nonMet-BC) patients (Laborda-Illanes et al., 2024). The Met-BC group exhibited an increase in several pathogenic and pro-inflammatory species, including Streptococcus epidermidis, Haemophilus influenzae, Corynebacterium aurimucosum, and Corynebacterium kroppenstedtii, while the NonMet-BC group displayed higher levels of probiotic bacteria, such as Parabacteroides distasonis, Lactobacillus iners, Blautia obeum, and Faecalibacterium prausnitzii (Laborda-Illanes et al., 2024). These studies suggest that consideration of both intratumoral miRNA expression and microbiota changes could aid in precision treatment for breast cancer.