Firmicutes in gut microbiota: hormonal influence on breast cancer in obese individuals

Abstract

Breast cancer (BC) is among the leading causes of death in women. The risk factors of BC are various, among which obesity has been proved to be closely related to the incidence and progression of BC, especially in postmenopausal women. Obesity leads to elevated estrogen levels, increased inflammatory factors, and insulin resistance, which are considered to increase the risk of BC. Microbial dysbiosis has emerged as an important element in the development and progression of various cancers, including BC. Meanwhile, Firmicutes, a phylum enriched in the gut microbiota, can affect the metabolism of estrogen and progesterone, which explains part of its role in promoting the development of BC. This review highlights the “obesity–microbiota–hormone–breast cancer” axis, focusing on Firmicutes influence on BC in hormone levels. We also explore potential methods to prevent obesity-related BC and improve patient survival outcomes by regulating gut microbiota. To identify relevant studies, we conducted a comprehensive literature search in PubMed, Web of Science, and Scopus up to April 2025 using a combination of MeSH terms and free-text keywords related to obesity, microbiota, hormones, and BC.

Graphical Abstract

Introduction

In 2001, Joshua Lederberg used the word “microbiome”, defining it as “the ecological community of commensal, symbiotic, and pathogenic microorganisms that literally share our body space” [1]. And the advent of metagenomics and next generation sequencing has helped scientists learn more about how gut microbiota affect human health. in the past decades. The World Health Organization lists over 100 human carcinogens, including microbiota. Nowadays, dysbiosis of the gut microbiota is recognized as a hallmark of cancer [2]. Digestive system cancers, due to their tumor microenvironment being in direct contact with the gut microbiota, have been extensively studied [3–10]. Research on other types of cancer, especially BC, has been little investigated in the field of microbiology.

BC is one of the most common tumors in women worldwide, and despite significant progress in its diagnosis and treatment, there were 2.3 million new cases and 666,000 deaths in 2022 [11]. The pathogenesis of BC remains unclear, while obesity [12] and estrogen levels [13] can contribute to an increased likelihood of it.

Obesity is highly correlated with the incidence of postmenopausal BC [14]. Obese individuals have distinct characteristics in their gut microbiota, with differences in microbial composition compared to normal individuals [15, 16]. Besides, one of the most prominent roles of the human microbiota is the regulation of steroid hormone metabolism, which is in relation to BC. The Firmicutes phylum is known to affect hormone metabolism in the host’s body. Estrogen is the main regulator of the gut microbiota, and the gene pool of gut microbiota that can metabolize estrogen is called the “estrobolome” [17]. Firmicutes phylum genera such as Lactobacillus and Enterococcus can regulate estrogen metabolism through the β – glucuronidase [18]. Endogenous estrogens are the most important risk factor in BC development, especially in postmenopausal women. Firmicutes may significantly influence the occurrence and progression of BC in obese individuals through hormonal regulation. Understanding the mechanisms behind this effect could offer a novel perspective on the pathogenesis of BC.

Women with BC have distinct microbiota profiles compared to those without cancer [19]. Given the interrelationship between gut microbiota dysbiosis and obesity, it is crucial to understand the connection between obesity and BC through the lens of gut microbiota.

In this review, we summarize key evidence supporting the pivotal role of Firmicutes in regulating hormone levels and its involvement in the development and metastasis of BC in obese individuals. Furthermore, we propose the “obesity-microbiota-hormone-breast cancer axis” as a mechanism influencing BC development, integrating microbiology, endocrinology, and oncology. We also explore potential future interventions targeting the gut microbiota for BC. These approaches could contribute to the development of microbiota-based therapies. Additionally, they may enhance the clinical management of BC.

Obesity and BC: the microbiome’s role

Body mass index (BMI) has been inversely associated with premenopausal and positively associated with postmenopausal BC [20]. Obesity, defined as BMI ≥ 30 kg/ m², is a major risk factor for adverse outcomes in BC [21–23]. And BC is the main cause of death among obesity related female cancers [24–27]. Obese women with a BMI greater than 40 have an increased risk of disease recurrence, regardless of menopausal or hormone receptor status [28], and their risk of death is 2.12 times of women with normal weight [29]. However, the underlying molecular mechanisms by which obesity causes BC have not been elucidated.

Notably, obesity creates a cancer-friendly environment by inducing chronic inflammation [30, 31], increasing hormone levels [32], and activating proteins and signaling pathways [33, 34] that promote cancer development. Research shows that breast tumor cells and fat cells are the main components of breast tumor matrix and can interact directly [35, 36]. Leptin, a hormone secreted by adipocytes, plays a critical role in regulating obesity and metabolic functions [37]. Excessive adiposity often leads to metabolic disorders, resulting in abnormal secretion of insulin, insulin-like growth factor 1 (IGF-1), leptin, and adiponectin [38]. With the increase of circulating leptin, the activation of JAK/STAT pathway in tumor cells is also enhanced [39]. Proteins related to JAK and STAT signaling transduction play a crucial role in the growth, division, specialization, and lifespan of cancer cells [40]. Moreover, obesity causes fat cells to upregulate chemokines, promoting the production of proinflammatory proteins, proangiogenic factors, and macrophages, which trigger systemic inflammation and immune dysregulation. Together, they support to the generation and progression of malignant tumors [41].

In addition to the systemic metabolic changes mentioned above, the gut microbiota plays a significant role in the incidence and treatment outcomes of BC. Obesity has been shown to cause dysbiosis of the gut microbiota, leading to significant changes in microbial diversity [42, 43].Recently, 16 S rRNA sequencing and metagenomics analyses have shown a marked reduction in gut microbial alpha diversity in obese people [44]. Similarly, Barone et al. observed that obese women with uncontrolled eating behavior were mostly characterized by low-diversity microbial communities with limited species richness and weak interspecies connections [45]. In a recent work, Chen et al. proposed that high fat diet (HFD) is associated with the rapid progress and poor prognosis of BC [46]. The gut microbiota mediated by HFD is involved in the development of obesity related tumors in these cancers. They further proposed a novel mechanism involving a “gut–bone marrow–tumor” axis in HFD-related cancer development [46].

Goedert et al. reported that postmenopausal women with BC exhibit altered fecal microbiota composition, with a lower alpha diversity independent of estrogen [47]. The opposite results were indicated by Zhu et al., who observed that in premenopausal BC patients, the gut microbiota composition was not significantly different from that of healthy premenopausal women [48]. However, postmenopausal patients and postmenopausal controls showed different abundance: 38 species were enriched in postmenopausal patients [48]. And it is important to note that the two above clinical studies have not considered the different types of BC. A separate study analyzed microbiota diversity across the four main subtypes of BC: ER-positive, triple-positive, Human Epidermal Growth Factor Receptor 2(HER2)-positive, and triple-negative (TNBC). The results revealed unique and strong microbial signatures for each subtype, though some components were shared [49]. All four subtypes showed a predominance of Proteobacteria, followed by Firmicutes [49].

Overview of gut microbiota and firmicutes

The gut microbiota, a collective term for the microbial community residing in the human gut, has emerged as one of the most compelling areas of research in microbiology [50], medicine [51], genetics [52], and other related fields in recent years. Disruption of gut microbiota is increasingly recognized as a characteristic of cancer [53]. However, only a limited number of microbiota have been identified as directly initiating tumorigenesis or distorting the immune system to produce tumor-tolerant environments.

The human gut microbiota contains dozens of bacterial phyla, including Bacteroidetes and Firmicutes [54, 55]. In a typical healthy human colon, about 95% of the bacteria belong to four major phyla. Specifically, 59% are Firmicutes, 24% are Bacteroidetes, 9% are Actinobacteria, and 3% are Proteobacteria [49]. Preclinical studies have suggested that certain Firmicutes can enhance the anticancer response after migrating through damaged gut mucosa. These findings indicate a mechanistic connection between harmful and therapeutic effects [56]. Firmicutes, which are also called the low mol% G + C Gram positive prokaryotes (Fig. 1). Major taxa to be included are Alicyclobacillus, Bacillus, Clostridium, Enterococcus, Erysipelothrix, Eubacterium, Haloanaerobium, Heliobacterium, Lachnospira, Lactobacillus, Leuconostoc, Listeria, Paenibacillus, Peptococcus, Ruminococcus, Staphylococcus, Streptococcus, Syntrophomonas, Thermoactinomyces, Thermoanaerobacter, Veillonella and 229 additional genera [57]. Faecalibacterium prausnitzii, a key butyric acid producer, is among the beneficial bacteria in the Firmicutes phylum. Butyric acid has anti-inflammatory effects, supports bacterial enzyme activity, and protects the gut from intestinal pathogens [58]. Lactic acid bacteria, commonly found in yogurt and other fermented dairy products, are also part of this phylum. These bacteria produce short-chain fatty acids, including acetate, as well as lactic acid and antimicrobial compounds, which help prevent pathogen colonization [59, 60]. However, the Firmicutes phylum also contains some harmful bacteria, such as Clostridium perfringens, which causes gastrointestinal infections [61], and Staphylococcus aureus, a common cause of severe infections [62].

Fig. 1.

General characteristics of Firmicutes in the human gut, including their key features, specific effects, and current research focus on the Firmicutes/Bacteroidetes (F/B) ratio. For details, consult text. (Created with Adobe Illustrator 2020)

The Firmicutes to Bacteroidetes ratio (F/B ratio) plays a key role in preserving gut microbial balance. Changes in this ratio are associated with various pathologies [63]. Recent studies consistently support a relationship between an increased F/B ratio and obesity. The changes in Firmicutes and Bacteroidetes abundance were first observed in obese animals and humans, where Firmicutes levels increased and Bacteroidetes levels dropped [64–66].

A study comparing stool samples from Japanese individuals with different body weights found that non-obese participants had an average Firmicutes level of 37.0 ± 9.1%, while obese participants had 40.8 ± 15.0%. For Bacteroidetes, non-obese individuals showed 44.0 ± 9.8%, whereas obese individuals had 37.0 ± 14.0% [67]. Similar findings were reported in a study of 61 Ukrainian adults, where the F/B ratio showed a significant correlation with BMI. Individuals with an F/B ratio ≥ 1 were 23% more likely to be overweight than those with an F/B ratio < 1 [16]. In a study conducted in Qatar, comparing 37 obese and 36 lean individuals, the F/B ratios of the obese and lean subjects were 2.25 ± 1.83 and 1.76 ± 0.58, respectively [68]. Among Kazakh [69] and Belgian [70] students, the F/B ratio in the obese group was significantly higher than that in the control group.

Firmicutes and hormonal modulation

The regulation of estrogen is crucial for women’s health, as it influences a wide range of physiological processes, including reproductive health [71, 72], bone density [73], and cardiovascular function [74]. Gut microbiota play an essential role in estrogen metabolism, as evidenced by studies showing that using antibiotics, which disrupt gut microbes, can lower the amount of estrogen in the blood [75]. Changes in the balance between Firmicutes and Bacteroidetes have also been linked to increased levels of circulating estrogen. This shift may raise the risk of developing BC [76]. Dysbiosis of the microbiota can also lead to obesity [77], abnormal production of sex hormone [78, 79], and also increased reabsorption of hormones in the gut by breaking their bonds with glucuronide [80].

Estrogen, the main sex hormone in women, is released into the bloodstream. It is then inactivated in the liver through binding with glucuronic acid and other compounds, which enhances its solubility in water. This modification allows estrogen to be excreted via the kidneys in urine, with some also excreted through bile into the intestine. Microbially secreted β-glucuronidase can metabolize estrogens from its conjugate forms to deconjugated forms [17]. Finally, estrogen can return to the enterohepatic circulation in a biologically active form. Plottel and Blaser coined the term “estrobolome” to refer to the collective set of bacterial genes in the gastrointestinal tract that are involved in estrogen metabolism [17]. In human gastrointestinal tract, the most important β-glucuronidase encoding genes are β-Glucuronidase (GUS) genes [81]. Most of the bacteria in Firmicutes also possess genes that are part of the “estrobolome”, which can regulate endogenous estrogen metabolism through the hepatic intestinal circulation by regulating the activity of β-glucuronidase, thus affecting the circulation and excretion of estrogen levels. A recent study mapped the β-glucuronidase enzymes in gut microbes and found 112 new GUS types [82]. These fell into six groups and were found in four major bacterial phyla: Bacteroidetes, Firmicutes, Verrucomicrobia, and Proteobacteria [82]. Among them Bacteroidetes presents highest abundance and diversity of GUS enzymes [82]. Ervin et al. conducted the first in vitro analysis of 35 human gut microbial GUS enzymes to assess their ability to reactivate two estrogen glucuronides: estrone-3-glucuronide and estradiol-17-glucuronide. Their results revealed that gut microbial GUS enzymes play a key role in the estrobolome [83].

Many researchers have focused on the role of Firmicutes in hormone metabolism. The following summarizes selected studies on members of Firmicutes. Studies have shown that Bacteroides melaninogenicus exhibits a high affinity for both estrogen and progesterone, capable of metabolizing these hormones [84]. Further research revealed that Escherichia coli Nissle 1917 can promote progesterone levels in serum and follicular fluid supernatant [85]. Beyond metabolizing estrogens, Firmicutes are also involved in synthesizing estrogen-like compounds or mimics from dietary sources [86–90].Eubacterium limosum contributes to the activation of estrogenic compounds by demethylating the hydroxyl group [91]. McIntosh et al. reported a strong link between levels of systemic estrogens and certain gut bacteria, especially those in the Clostridia class of Firmicutes, such as the Ruminococcaceae family [80]. Overall, current research indicates a direct and strong link between the Firmicutes community and total systemic estrogen. This association appears to be independent of any specific phylogenetic class or cluster. Further investigation is needed to explore its functional and metabolic significance. However, the interaction between the gut microbiota and hormonal may not be unidirectional.

Hormones significantly regulate gut microbiota composition and function, impacting various aspects of host physiology. Further research in microbial endocrinology has revealed that catecholamines [92], estrone and progesterone [84] can promote bacterial growth. Catecholamines influence the expression of bacterial virulence genes [93], increasing bacterial virulence [94]. In contrast, estriol and estradiol decrease bacterial virulence [95]]. Progesterone has been shown to induce the growth of lactobacilli in ovariectomized mice and help prevent anxiety like behavior and depression [96].

Previous studies on the members and functions of Firmicutes related to estrogen levels through enterohepatic recirculation have highlighted the interaction between Firmicutes and hormones like estrogen and progesterone. This interaction plays a role in several pathways that influence women’s health. These interactions may influence various physiological aspects, including fertility, obesity, diabetes, and cancer. The Firmicutes-hormone axis represents a critical link between gut microbiota and hormonal regulation, highlighting its potential as a therapeutic target for improving women’s health outcomes.

Mechanisms linking firmicutes and BC in obese populations

As we mentioned above, a key characteristic of the gut microbiota in obese individuals is its dysbiosis, which is marked by an overrepresentation of Firmicutes [57]. Firmicutes are part of the estrogenic microbiota, possessing complex and powerful mechanisms that significantly influence estrogen metabolism [17]. Estrogen, particularly in its role as a driver of estrogen receptor-positive BC, is an important pathogenic factor in BC development [13]. Therefore, we are striving to establish an “obesity-microbiota-hormone-breast cancer” axis, combining microbiology, endocrinology and oncology in multiple disciplines, in order to deepen our understanding of the pathogenesis of BC and provide insights for clinical strategies to better prevent and treat the BC.

Estrogen receptor-positive BC is the most common subtype [97] and is closely linked to estrogen levels. Several studies have demonstrated an association between gut microbiota composition and systemic estrogen. For instance, research on healthy postmenopausal women found that greater microbial diversity correlated with higher urinary levels of estrogen and its metabolites [98]. Similar findings were observed in African postmenopausal women, where both gut and oral microbiota were linked to circulating estrogen levels [99]. In BC patients, higher levels of Clostridium leptum and Clostridium coccoides were found in more advanced stages [100]. These species belong to the Firmicutes phylum [100]. They are known to produce β-glucuronidase, an enzyme that may promote the reactivation of estrogen in the gut.

Firmicutes have been shown to significantly affect estrogen and progesterone levels, thereby regulating the development and progression of cancer. It is clear that the dysbiosis observed in the gut microbiota of obese individuals increases the risk of BC by influencing hormone regulation. The “obesity-microbiota-hormone-breast cancer” axis provides a valuable framework for understanding this physiological process and offers potential avenues for future research and therapeutic interventions.

Influence of gut microbiota on the BC treatments

New evidence suggests that the gut microbiota may play a crucial role in BC development and treatment by influencing drug efficacy and toxicity, including responses to chemotherapy, immunotherapy, and other targeted therapies (Table 1). The gut microbiota is capable of metabolizing over 40 different drugs and may influence the efficacy of many more [101]. Drug exposure activates metabolic and stress response pathways across various microbial phyla, with Firmicutes playing a key role [102]. Moreover, specific gut microbiota have emerged as important determinants of drug response, influencing clinical outcomes in BC patients [103].

Table 1.

Summary the effect of gut microbiota on BC therapeutics

Therapeutics	Study	Sample source	Main Methodology	Results
Chemotherapy	Ye, Z., 2024[111]	Female BALB/c mice injected with 4T1and MCF-7 breast cancer cells	V3 and V4 16 S rRNA Sequencing	The combination of probiotics and anti-tumor drug doxorubicin has a higher tumor inhibition rate Probiotics used alone could not directly induce anti-tumor effects
	Terrisse, S. et al., 2021[132]	121 specimens from 76 early BC patients	Ion-proton technology (ThermoFisher) Sequencing	Fecal microbiota composition at diagnosis is associated with prognosis in early breast cancer (BC) Adjuvant chemotherapy modifies gut microbial β-diversity Certain commensal bacteria are more abundant in BC patients than in healthy individuals These bacteria may negatively influence prognosis They may also contribute to weight gain and neurological side effects during treatment
Targeted Therapy	Di Modica, M. et al., 2021 [115]	Female FVB/Ncrl mice injected with human HER2-positive MI6 murine mammary carcinoma cells;24 consecutive patients who received neoadjuvant trastuzumab-based chemotherapy	V3 and V4 16 S rRNA sequencing	Antibiotic administration has been shown to reduce the therapeutic efficacy of Trastuzumab, suggesting a direct role of the gut microbiota in modulating its activity. In particular, patients who respond well to Trastuzumab are often enriched with bacterial taxa from the Clostridiales, Bifidobacteriaceae, Turicibacteraceae, and Bacteroidales orders
	Schettini, F. et al., 2023 [116]	14 metastatic breast cancer patients	V3 and V4 16 S rRNA sequencing	Microbiota-targeted interventions have been considered as strategies to improve therapeutic response to CDK4/6 inhibitors
Immunotherapy	Kim, H. et al., 2021 [127]	Female BALB/c mice injected with 4T1 breast cancer cells	16 S rRNA sequencing	Synergistic treatment with B. longum RAPO and anti-PD-1 may enhance anti-tumor responses by reshaping the gut microbial environment

The study by Terrisse, S. et al., 2021 is a registered clinical trial (Clinical trial no. NCT01993498). For other listed studies, clinical trial registration numbers were not applicable or not provided in the original publications

Chemotherapy remains a cornerstone of BC treatment that uses drugs to target and destroy cancer cells [104]. It is often used either as a primary treatment for early-stage cancer or as adjuvant therapy following surgery to eliminate remaining cancer cells and reduce the risk of recurrence [105]. Common chemotherapy agents for BC include anthracyclines (such as doxorubicin), taxanes (such as paclitaxel), and cyclophosphamide [106]. The gut microbiota plays an important role in regulating the efficacy of chemotherapy. Studies have found that the efficacy of chemotherapy drugs such as cyclophosphamide and platinum salts is significantly poorer in germ-free mice and animals treated with antibiotics [107]. Studies have shown that cyclophosphamide promotes the migration of bacteria such as Lactobacillus and Enterococcus to secondary lymphoid organs, including the spleen [56]. This process triggers a specific immune response and promotes the accumulation of cytotoxic T cells in tumors, thereby enhancing the therapeutic effect of cyclophosphamide [56]. Certain bacteria, like Raoultella planticola, can metabolize doxorubicin (DOX), reducing its efficacy and toxicity through deglycosylation [108]. In addition, In murine models of TNBC, depleting gut microbiota with antibiotics improved the therapeutic response to DOX and suppressed metastasis [109]. Pseudomonas aeruginosa has also been found to influence DOX-induced cancer cell death in vitro [110].Therefore, these findings suggest that maintaining a microbiota rich in beneficial bacteria could improve chemotherapy outcomes. In support, combining DOX with probiotics was shown to inhibit breast tumor growth more effectively [111]. The chemotherapy drug irinotecan is converted in the liver and gut into its active form SN-38, which is later detoxified to SN-38-G. In the gut, microbial β-glucuronidase can reconvert SN-38-G into its active form, leading to gastrointestinal toxicity [112]. Use of antibiotics or β-glucuronidase inhibitors can mitigate this side effect [112]. However, the mechanisms linking gut microbiota to chemotherapy response in BC remain poorly understood. At present, the potential of the microbiota to influence the development and progression of BC can be measured from studies of other cancers. Colorectal cancer is the earliest cancer associated with changes in the microbial community. For example, Yu, T. et al. injected F.nucleatum into colorectal tumors and found that F.nucleatum developed resistance to oxaliplatin chemotherapy through an autophagy-dependent pathway [113], indicating the protumorigenic effect of bacteria. Iida, N. et al. also found that gut microbiota can affect the efficacy of the platinum chemotherapy drug oxaliplatin [114].

There is compelling evidence to suggest that the composition of the gut microbiota can influence the efficacy of anti-HER2 therapies in BC, particularly in patients with HER2-positive tumors. Di Modica, M. et al. explored the link between gut bacterial composition and trastuzumab response in HER2-positive patients [115]. Their findings revealed that antibiotic use reduced the effectiveness of anti-HER2 therapy, suggesting a direct role of gut microbiota in trastuzumab activity [115].At the same time, they observed higher abundances of Clostridiales and lower abundances of Bacteroides in patients who showed better responses to trastuzumab [115]. In a study involving 14 metastatic BC patients who received treatment with palbociclib, ribociclib, or abemaciclib combined with fluvoxetine, it was found that the responders had enriched specific microbial communities in their intestines [116]. These findings suggest that manipulation of the gut microbiota, such as through fecal microbiota transplantation, could enhance the effectiveness of targeting therapies by restoring beneficial microbial communities. At present, there is still limited systematic research on the interaction between gut microbiota and the efficacy of endocrine targeted therapy. Further research is needed to understand how gut microbiota metabolizes or affects targeted therapy, which may affect its efficacy and cancer recurrence.

At the same time, in systemic treatments of cancer, the gut microbiota can affect the metabolism of medical drugs, modulating the immune response to treatment [117] and influencing the side effects of therapies [118]. With immune checkpoint inhibitors like Cytotoxic T-Lymphocyte–Associated Protein 4(CTLA-4), Programmed Cell Death Protein 1(PD-1), and Programmed Death-Ligand 1(PD-L1) approved for cancer therapy, the gut microbiota’s role in immunotherapy has gained recognition [119–125]. Preclinical studies indicate that certain microbial populations influence treatment response [120, 126]. B. longum enhanced anti-PD-1 therapy in a murine model of TNBC [127]. However, evidence for the impact of the gut microbiota on these therapies exists largely in the context of metastatic melanoma. CTLA-4 blockade is influenced by Bacteroides species like B. thetaiotaomicron and B. fragilis [121], while Bifidobacterium supports anti-PD-L1 efficacy by promoting dendritic cell function and CD8 + T cell activity [128]. B. longum, Collinsella aerofaciens, and Enterococcus faecium are also linked to better anti-PD-1 outcomes [122]. Clinical evidence highlights the role of the microbiota in ICB efficacy, with responders showing an increased abundance of key bacterial species, including Akkermansia and Alistipes [123]. Other microbes, including Ruminococcaceae, Clostridales, and Faecalibacterium, enhance CD4+/CD8 + ratios while reducing Tregs and suppressor cells, creating a favorable immune environment [124]. Probiotics may further support immune-based therapies by restoring microbial diversity, boosting Natural Killer cell activity, and shifting toward a Th1-biased immune profile [125, 129].

Besides, endocrine therapies, such as selective estrogen receptor modulators, are also affected by gut microbiota. Chen et al. found that tamoxifen and its metabolite 4-hydroxytamoxifen are substrates of bacterial β-glucuronidase, which may limit treatment efficacy by reversing glucuronidation [130, 131].

Additionally, therapeutics can disrupt the gut microbiota composition [132]. Chemotherapy is associated with a reduction in Veillonella species, which is associated with worse prognosis in cancer patients [132]. In a study of 57 BC patients with a history of chemotherapy, the Firmicutes was found to be inversely related to chemotherapy treatment [133]. Chemotherapy has been shown to reduce populations of beneficial gut microbes, including Bifidobacterium and Lactobacillus, while creating favorable conditions for the overgrowth of opportunistic species like Clostridium difficile [134]. In tumor-bearing mice, treatment with cyclophosphamide was associated with a shift in microbial composition, notably reducing the abundance of Bacteroidetes and elevating the relative levels of Firmicutes [135]. In another study, mice colonized with microbiota from BC patients showed altered body weight and nervous system function after chemotherapy, indicating a link between microbial composition and prognosis [132]. Tamoxifen and raloxifene also modify gut microbial communities [136]. These agents negatively affect bacteria like Bacillus stearothermophilus, Klebsiella pneumoniae, and Pseudomonas aeruginosa, among others [136].Interestingly, preoperative gut microbiota profiles in BC survivors have been associated with chronic postoperative pain [137]. This suggests that gut microbiota may serve as a clinical predictor for pain susceptibility and a potential target for alleviation strategies [137].

Our quest for answers: can microbiota be used for BC treatment in obese people

Impact of weight reduction surgery on gut microbiota and potential effects on BC risk

Given that an imbalance in the gut microbiota of obese individuals may contribute to the development of BC, it is worth exploring whether weight reduction surgery can reverse these microbial alterations and mitigate cancer risk. Evidence from 12 animal experiments and 9 clinical studies suggests that weight reduction surgery induces notable compositional shifts within the phylum Firmicutes, including increases in Lactobacillales and Enterococcus, and decreases in Clostridiales, Clostridiaceae, Blautia, and Dorea [138]. These microbial alterations may influence systemic estrogen metabolism. Therefore, weight reduction surgery may substantially reshape the gut microbiota, potentially modulating estrogen metabolism and thereby affecting BC risk.

Influence of diet on gut microbiota and estrogen metabolism in BC risk

Given that there are various ways to lose weight, can diet adjust the gut microbiota and affect estrogen metabolism? Diet is a major factor influencing gut microbial diversity, and the quality and content of the diet can shape the microbiota [139]. In particular, the Mediterranean diet(MD) has been widely studied for its association with gut health and BC outcomes [140]. This dietary pattern emphasizes the intake of vegetables, fruits, legumes, extra virgin olive oil, nuts, fish, and whole grains, while limiting sugar, red and processed meats, and dairy products [141]. Clinical studies have shown that better adherence to the MD is significantly and inversely associated with chemotherapy-induced digestive toxicity [142]. Several studies have supported the protective effect of the MD against breast cancer, particularly among postmenopausal women [143]. Geographically, this inverse association appears to be most pronounced in Asian populations [143]. Ongoing research is investigating how adherence to the MD may differentially influence the risk of developing specific histopathological subtypes of breast cancer [144]. In addition to the Mediterranean diet, the ketogenic diet (KD) has attracted growing interest in the context of breast cancer management. This dietary approach is characterized by high fat and low carbohydrate intake [145]. Some evidence suggests that KD may improve health outcomes and quality of life in women with BC [146]. However, more robust and well-designed clinical trials are needed to conclusively determine the efficacy of KD as a therapeutic strategy in breast cancer. Additionally, the activity of β-glucuronidase is regulated according to diet and bacterial environment. Diets high in fat or protein have been associated with increased β-glucuronidase activity, whereas higher fiber intake appears to reduce it [147]. Thus, dietary modifications may provide a promising avenue for modulating estrogen metabolism and potentially reducing BC risk.

Potential of GUS inhibitors in preventing BC

Given the role of β-glucuronidase in reactivating estrogen metabolites and increasing estrogen bioavailability, could the use of GUS inhibitors help prevent estrogen-driven BC? To date, numerous selective inhibitors targeting human gut microbial β-glucuronidase (GUS) enzymes have been developed [148]. Ervin, S. M. et al. conducted the first comprehensive in vitro study of 35 human gut microbial GUS enzymes and confirmed their ability to convert estrone-3-glucuronide and estradiol-17-glucuronide back into active estrone and estradiol [83]. However, the inhibitor UNC10201652 alone is insufficient to inhibit all GUS enzymes involved in estrogen reactivation [83]. A Loop 1 GUS-specific inhibitor failed to reduce tumor development in the PyMT mouse model of BC [83], suggesting that such inhibition does not comprehensively disrupt estrogen reactivation in the gastrointestinal tract. Despite these limitations, the therapeutic potential of GUS inhibition remains an area of interest, particularly given its theoretical relevance to estrogen regulation. Importantly, the impact of GUS inhibitors on the efficacy of endocrine therapies for BC remains unknown and warrants further investigation.

Potential of fecal microbial transplantation (FMT) in BC treatment

The most radical yet efficient means to modify the gut microbiota is FMT. FMT refers to the process of transferring minimally processed stool from a healthy donor into the gastrointestinal tract of a recipient. This procedure is intended to treat conditions linked to imbalances in the gut microbiota [149]. Recent studies in melanoma patients have demonstrated that transferring gut microbiota from treatment-responsive individuals to non-responders or drug-resistant patients can enhance therapeutic efficacy and overcome resistance [150, 151].While FMT has shown promise in alleviating chemotherapy-induced toxicity in colorectal cancer models [152], its application in BC remains largely unexplored. Such studies could offer novel insights into the role of microbiota modulation in BC treatment. These findings suggest that FMT may have potential as an adjunctive strategy in cancer therapy, and warrant further exploration in the context of BC.

Role of probiotics and prebiotics in BC prevention and treatment

In recent years, probiotics and prebiotics have increasingly entered people’s lives. Does preventive use of probiotics or prebiotics help prevent BC? Probiotics are live beneficial bacteria that help maintain or restore a favorable microbial balance [153–155], while prebiotics are non-digestible fibers that promote the growth of such microorganisms [156]. These microorganisms and their metabolites exhibit anti-estrogenic, anti-inflammatory, and antiproliferative properties, all of which may reduce BC risk [157]. For instance, a Japanese case-control study reported an inverse association between BC incidence and regular intake of Lactobacillus casei containing probiotics [158]. Similarly, milk fermented with Lactobacillus helveticus R389 modulated cytokine profiles in tumor-bearing mice, reducing pro-inflammatory IL-6 and enhancing IL-10, leading to tumor inhibition [159]. In another study, Lactobacillus reuteri intake prevented mammary tumor development in two distinct murine models [160]. Moreover, heat-killed bacterial cells and cytoplasmic extracts of Enterococcus faecalis and Staphylococcus hominis demonstrated anti-proliferative and pro-apoptotic effects in MCF-7 cells [161]. A clinical trial further identified Lactobacillus brevis KABP052 as possessing the highest β-glucuronidase activity among several lactic acid bacteria [162]. Women who consumed a probiotic containing this strain maintained stable serum estrogen levels during menopause, unlike those in the placebo group( (September 17, 2021; approval no. MRS2021091702) [162]. These results suggest a potential role for probiotics in estrogen regulation and BC prevention. Additionally, Bifidobacterium longum RAPO, when administered alongside ICI, enhanced anti-PD-1 efficacy in TNBC models [127]. Beyond naturally occurring bacteria, engineered strains capable of producing tumor-suppressive peptides [163–165], bacteriocin [166], enzymes [167], or bacterial toxins [168–170]have also shown activity against human BC cells. Prebiotics may further influence estrogen metabolism in postmenopausal women through modulation of β-glucuronidase activity [171]. Preclinical studies suggest that probiotics could be potential moderators to prevent and control BC progression. However, further clinical trials or prospective studies are needed to confirm the efficacy of probiotics in clinical management.

Various strategies can help modulate the gut microbiota. These include weight loss surgery, dietary interventions, fecal microbiota transplantation, and the use of probiotics or prebiotics. Each of these approaches has shown potential in reshaping microbial communities. Modifying the microbiota in this way may offer new options for managing estrogen-driven diseases. BC is one key example. These approaches could help address the metabolic aspects of BC, providing new avenues for its prevention and treatment.

Discussion

Gut microbiota is often referred to as “the second genome of the human body,” which play a crucial role in various diseases and has attracted widespread attention, especially in cancer research. Studies increasingly highlight its impact on metabolism, immunity, and disease progression. Research on the gut microbiota in BC development and progression has gained increasing attention in recent years. However, the precise mechanisms regarding how gut microbiota influence BC remain largely unclear. Among the proposed mechanisms, estrogen metabolism has emerged as a key pathway, with dysregulation of estrogen homeostasis recognized as a significant risk factor for hormone-dependent BC [172, 173]. Importantly, the Firmicutes play a distinct role in modulating systemic estrogen levels through β-glucuronidase activity [18], making them a promising target for further investigation. Due to its capacity to influence estrogen metabolism, Firmicutes may serve not only as a biomarker for BC risk but also as a window into underlying molecular mechanisms and opportunities for prevention. While microbial interventions are increasingly common in the treatment of digestive diseases, their application in cancer [174, 175], particularly BC, remains limited. Expanding research on the gut microbiota’s role in cancer could hasten the clinical use of microbiota-targeted therapies, such as diet modulation, selective antibiotics, or probiotics, as adjuncts to conventional BC treatments. The potential for microbiota-based interventions in BC holds promise for improving patient outcomes, reducing side effects, and potentially preventing the disease in high-risk populations.

Obesity is a known risk factor for both gut microbiota imbalance and BC [176]. It often leads to changes in gut microbial composition, usually with an increase in Firmicutes. Obesity is also linked to hormonal changes, such as higher levels of estrogen. In this context, we propose the “obesity–microbiota–hormone–breast cancer” axis as a conceptual framework to better understand BC pathogenesis in obese individuals. This model integrates the interplay between metabolic dysregulation, microbial imbalance, and hormone-driven oncogenic signaling. Investigating this axis may help bridge current knowledge gaps by elucidating how obesity-induced shifts in microbial composition, particularly within Firmicutes, contribute to hormonal imbalances that foster BC development. Establishing this axis provides a comprehensive approach to exploring novel therapeutic targets, enhancing early detection strategies, and improving prevention efforts. This integrated model represents an exciting step toward personalized, microbiota-based interventions, potentially transforming how BC is prevented and treated in the future.

Currently, much of the research on gut microbiota is focused on identifying characteristic microbial profiles associated with healthy individuals versus those with disease. However, there is a significant gap in dynamic studies that track changes in microbiota composition over time or in response to treatment. The variation in experimental protocols, including differences in tissue sources, DNA extraction methods, and microbial analysis techniques, poses challenges for large-scale collaborative research. Standardizing these methodologies would be a significant step forward in advancing our understanding of the relationship between gut microbiota and BC.

Additionally, much of the current research predominantly addresses gut microbiota, neglecting the role of the breast microbiota. This gap in focus is significant. Contrary to earlier assumptions, the human breast is not a sterile organ [177]. Microorganisms may colonize breast tissue via multiple routes, including translocation from the gut, migration through the nipple-areolar orifices, skin contact, lactation-associated nipple-oral contact, and sexual activity [177]. Integrating breast microbiota into BC research is essential for elucidating its potential role in disease initiation and progression [178]. Notably, the microbial composition of breast tissue is distinct from that of the overlying skin, and marked differences have been observed between individuals with benign and malignant breast conditions [179]. Recent studies comparing breast tissue microbiota from invasive and benign BC patients, as well as healthy individuals, have revealed significant differences in microbial communities [180–182]. Researchers identified Lactobacillaceae (Firmicutes), along with Acetobacteraceae and Xanthomonadaceae (Proteobacteria), as the most abundant bacterial in healthy breast tissue. Ralstonia (Proteobacteria) was found to be more prevalent in breast tumor tissues and adjacent non-malignant areas, suggesting a distinct microbial signature associated with malignancy [183, 184]. Furthermore, microbiota composition varies among different molecular subtypes of BC, including luminal A, luminal B, HER2+, and TNBC [49, 185], suggesting that the breast microbiota may contribute to the progression and classification of the disease. The identification of distinct bacterial profiles associated with each BC subtype presents an exciting opportunity for developing microbiota-based biomarkers and therapeutic strategies tailored to specific BC types. Incorporating both the gut and breast microbiota would help evaluate the prognostic potential of specific microbial communities, enabling earlier detection and targeted interventions for those at risk.

Conclusion

This review describes that the Firmicutes in gut microbiota affects the occurrence and treatment of BC by regulating the metabolic level and bioavailability of hormones. Establishing an “obesity-microbiota-hormone-breast cancer” axis model holds significant potential for advancing our understanding of BC pathogenesis. This axis provides a comprehensive framework for exploring how obesity-induced dysbiosis influences the development and progression of BC. The “obesity-microbiota-hormone-breast cancer” axis highlights the critical role of gut microbiota in preventing BC in obese individuals. It opens new pathways for developing prevention and treatment strategies, focusing on microbiota as a key target in BC management.

Abbreviations

BC

Breast cancer

BMI

Body mass index

HFD

High fat diet

HER2

Human Epidermal Growth Factor Receptor 2

GUS

β-Glucuronidase

CTLA-4

Cytotoxic T-Lymphocyte–Associated Protein 4

PD-1

Programmed Cell Death Protein 1

PD-L1

Programmed Death-Ligand 1

TNBC

Triple negative breast cancer

F/B ratio

Firmicutes to Bacteroidetes ratio

DOX

Doxorubicin

MD

Mediterranean diet

KD

Ketogenic diet

FMT

Fecal microbial transplantation

Author contributions

J.C. and Z.Z. conceived the review topic and wrote the main manuscript text. M.M, Q.H., and Z.L. performed literature search and data collection. Y.L., Z.G., and S.W. contributed to drafting specific sections of the manuscript. Y.W. and M.L. prepared the figures and tables. M.L. and L.Z. critically revised the manuscript for important intellectual content. All authors reviewed and approved the final version of the manuscript.

Funding

The authors express their sincere gratitude to all participants for their outstanding cooperation and meaningful contributions. This work was supported by the National Natural Science Foundation of China (Project No. 8227143221).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable. This article is a review and does not involve any studies with human participants or animals performed by any of the authors.

Consent for publication

Each author approved the manuscript before submission for publication.

Competing interests

The authors declare no competing interests.