1. Antibiotics save lives 

Since the discovery of penicillin in 1928, the widespread use of antibiotics has saved millions of lives. The main weapon in the fight against bacterial infections, antibiotics, alongside vaccinations, have added nearly twenty years to life expectancy.¹  

2. Antibiotics destroy species responsible for infections, but also eliminate good bacteria

Gut, vagina, lungs, skin... many parts of the body play host to microorganisms (bacteria, fungi, viruses). Such microbial communities are known as microbiota.² Antibiotics eradicate pathogenic germs responsible for infection but can also destroy certain beneficial bacteria in our microbiota, leading to imbalances (dysbiosis (sidenote: Dysbiosis Generally defined as an alteration in the composition and function of the microbiota caused by a combination of environmental and individual-specific factors. Levy M, Kolodziejczyk AA, Thaiss CA, et al. Dysbiosis and the immune system. Nat Rev Immunol. 2017;17(4):219-232.   ) ³) of varying degrees within these ecosystems. This concerns all of the body’s microbiota, including: the gut, skin,⁴ lung,⁵ ENT,⁶ urinary,⁷ and vaginal microbiota.⁸ 

3. Antibiotics can have side effects 

By inducing a dysbiosis, antibiotics can have harmful effects on health. The main short-term complication is the alteration of bowel movements experienced by some patients. This most often results in diarrhea, with the gut microbiota less able to perform its protective functions. Antibiotic-associated diarrhea is usually mild to moderate⁹ in intensity, but its incidence varies according to age, type of antibiotic, context, etc. It may affect up to 35%^9,10,11 of patients and 80% of children.⁹ In 10%-20% of cases, the diarrhea results from an infection by Clostridioides difficile (C. difficile),¹¹ a bacterium that colonizes the gut microbiota and becomes pathogenic due to certain factors (e.g. antibiotic use). The clinical consequences vary, ranging from moderate diarrhea to much more serious symptoms, or even death.¹¹ 

4. Antibiotics are thought to be responsible for longer-term effects

Diarrhea is not the only symptom of antibiotic-associated dysbiosis. When it occurs early in life, the condition is thought to be responsible for longer-term effects. The perinatal period, characterized by the development of the gut microbiota and the maturation of the immune system, is a particularly sensitive period.¹² Antibiotic-associated dysbiosis during this phase seems to be a risk factor in the development of certain chronic diseases (obesity, diabetes mellitus, asthma, inflammatory bowel disease).¹³ 

 5. Inappropriate use of antibiotics is responsible for antibiotic resistance 

Antibiotic resistance happens when an antibiotic treatment is no longer effective against a bacterial infection.¹ The cause? Antibiotics are only effective against bacteria and have no effect on viruses (e.g. the flu).¹⁴ The inappropriate (e.g. with viral infections) or excessive use of antibiotics – in humans or animals – accelerates antibiotic resistance. Antibiotic resistance leads to longer hospitalizations, higher health care costs and more deaths. For this reason, the issue has become a major public health concern worldwide.¹

6. World Antimicrobial Awareness Week 

Each year, from November 18 to 24, the WHO organizes World AMR Awareness Week, which aims to increase awareness of global antimicrobial (sidenote: Antimicrobial Class of drugs that includes antibiotics (active against bacteria), antiviral agents (active against virus), antiparasitic agents (active against parasites), and antifungal agents (active against fungi). WHO Antimicrobial Resistance; Oct 2020 ) resistance and to encourage best practices among the general public, health workers and policymakers to avoid the further emergence and spread of drug-resistant infections. As an expert on microbiota, the Biocodex Microbiota Institute takes part in this initiative.

Infographics to share with your patients

1. Antibiotics save lives 

Since the discovery of penicillin in 1928, the widespread use of antibiotics has saved millions of lives. The main weapon in the fight against bacterial infections, antibiotics, alongside vaccinations, have added nearly twenty years to life expectancy.¹  

2. Antibiotics destroy species responsible for infections, but also eliminate good bacteria

Gut, vagina, lungs, skin... many parts of the body play host to microorganisms (sidenote: Microorganisms Living organisms that are too small to be seen with the naked eye. They include bacteria, viruses, fungi, archaea and protozoa, and are commonly referred to as “microbes”. What is microbiology? Microbiology Society. ) (bacteria, fungi, viruses). Such microbial communities are known as microbiota.² Antibiotics eradicate pathogenic germs responsible for infection but can also destroy certain beneficial bacteria in our microbiota, leading to imbalances (dysbiosis (sidenote: Dysbiosis Generally defined as an alteration in the composition and function of the microbiota caused by a combination of environmental and individual-specific factors. Levy M, Kolodziejczyk AA, Thaiss CA, et al. Dysbiosis and the immune system. Nat Rev Immunol. 2017;17(4):219-232.   ) )³ of varying degrees within these ecosystems.

This concerns all of the body’s microbiota, including:

• the gut microbiota,
• the skin microbiota,⁴ 
• the lung microbiota,⁵ 
• the ENT microbiota,⁶ 
• the urinary microbiota,⁷ 
• and the vaginal microbiota.⁸ 

3. Antibiotics can have side effects 

By inducing a dysbiosis, antibiotics can have harmful effects on health. The main short-term complication is the alteration of bowel movements experienced by some patients. This most often results in diarrhea, with the gut microbiota less able to perform its protective functions. Antibiotic-associated diarrhea is usually mild to moderate⁹ in intensity, but its incidence varies according to age, type of antibiotic, context, etc. It may affect up to 35%^9,10,11 of patients and 80% of children.⁹ In 10%-20% of cases, the diarrhea results from an infection by Clostridioides difficile (C. difficile),¹¹ a bacterium that colonizes the gut microbiota and becomes pathogenic due to certain factors (e.g. antibiotic use). The clinical consequences vary, ranging from moderate diarrhea to much more serious symptoms, or even death.¹¹ 

 4. Antibiotics are thought to be responsible for longer-term effects

Diarrhea is not the only symptom of antibiotic-associated dysbiosis. When it occurs early in life, the condition is thought to be responsible for longer-term effects. The perinatal period, characterized by the development of the gut microbiota and the maturation of the immune system, is a particularly sensitive period.¹² Antibiotic-associated dysbiosis during this phase seems to be a risk factor in the development of certain chronic diseases (obesity, diabetes mellitus, asthma, inflammatory bowel disease).¹³ 

 5. Inappropriate use of antibiotics is responsible for antibiotic resistance 

Antibiotic resistance happens when an antibiotic treatment is no longer effective against a bacterial infection.¹ The cause? Antibiotics are only effective against bacteria and have no effect on viruses (e.g. the flu).¹⁴ The inappropriate (e.g. with viral infections) or excessive use of antibiotics – in humans or animals – accelerates antibiotic resistance. Antibiotic resistance leads to longer hospitalizations, higher health care costs and more deaths. For this reason, the issue has become a major public health concern worldwide.¹

6. World AMR Awareness Week

Each year, from November 18 to 24, the WHO organizes World AMR Awareness Week, which aims to increase awareness of global antimicrobial (sidenote: Antimicrobial Class of drugs that includes antibiotics (active against bacteria), antiviral agents (active against virus), antiparasitic agents (active against parasites), and antifungal agents (active against fungi). WHO Antimicrobial Resistance; Oct 2020 ) resistance and to encourage best practices among the general public, health workers and policymakers to avoid the further emergence and spread of drug-resistant infections. As an expert on microbiota, the Biocodex Microbiota Institute takes part in this initiative.

On the one hand, they are an extraordinary scientific discovery that saves millions of lives. On the other, their excessive and sometimes inappropriate use can lead to the emergence of multiple forms of resistance in microorganisms (including bacteria, viruses, parasites and fungi). As a result, although they were designed to heal, antimicrobials are becoming less and less effective and, ultimately, if no action is taken, there is a risk that they will no longer be able to protect us from infections.

Antimicrobial resistance would become responsible for almost 700,000 annual deaths worldwide². If nothing changes, infectious diseases could become one of the leading causes of mortality worldwide by 2050, causing up to 10 million deaths².

As a major centre of expertise on the microbiota, the Microbiota Institute has been an active partner for the event since 2020. Throughout November, the Institute will be sharing articles and news, as well as expert videos and downloads on key topics, to enhance your knowledge and help you understand the mid- and long-term effects of antibiotics on the human microbiota. To cite just one example, despite their well-known efficacy against bacteria (and lack of efficacy against viral infection³), they often lead to dysbiosis. This is associated with several well-known problems, such as antibiotic-associated diarrhea.

But there's more! Taking antibiotics is also suspected to increase the risk of multiple chronic diseases⁴ (allergies, asthma, obesity, chronic inflammatory bowel disease, etc.), particularly if they are prescribed in early childhood. So can we do anything about it? Yes! By encouraging good prescription practices to ensure that antibiotics are used properly! But also by educating patients on the risks of dysbiosis associated with excessive and inappropriate use of antibiotics. We are all responsible and we all have a role to play in reducing antimicrobial resistance!

On the one hand, they are an extraordinary scientific discovery that saves millions of lives. On the other, their excessive and sometimes inappropriate use can lead to the emergence of multiple forms of resistance in microorganisms (including bacteria, viruses, parasites and fungi). As a result, although they were designed to heal, antimicrobials are becoming less and less effective and, ultimately, if no action is taken, there is a risk that they will no longer be able to protect us from infections.

Antimicrobial resistance would become responsible for almost 700,000 annual deaths worldwide². If nothing changes, infectious diseases could become one of the leading causes of mortality worldwide by 2050, causing up to 10 million deaths².

As a major centre of expertise on the microbiota, the Microbiota Institute has been an active partner for the event since 2020. Throughout November, the Institute will be sharing articles and news, as well as expert videos, to help you understand the mid- and long-term effects of antibiotics on the human microbiota. To cite just one example, despite their well-known efficacy against bacteria (and lack of efficacy against viral infection³), antibiotics disrupt the balance of our gut microbiota. This imbalance, more commonly known as dysbiosis (sidenote: Dysbiosis Generally defined as an alteration in the composition and function of the microbiota caused by a combination of environmental and individual-specific factors. Levy M, Kolodziejczyk AA, Thaiss CA, et al. Dysbiosis and the immune system. Nat Rev Immunol. 2017;17(4):219-232.   ) , is associated with several well-known problems, such as antibiotic-associated diarrhea. But there's more! Taking antibiotics is also suspected to increase the risk of multiple chronic diseases (allergies, asthma, obesity, chronic inflammatory bowel disease, etc.), particularly if they are prescribed in early childhood.

So can we do anything about it? Yes! Firstly, by ensuring that antibiotics are used sensibly and appropriately. Do not take these medications without a prescription from a healthcare professional. Stick to the specified dose, administration frequency and duration of treatment and do not share your antibiotics with anyone else⁴. And remember, as the French health slogan states:

Antibiotics: handle with care© !

Recommended by our community

A single purpose: explore the connection between prostate cancer and gut microbiota in a group of Japanese men. A single methodology: the profiles of the gut microbiota in men with and without high grade prostate cancer were compared. A single objective: determine whether the composition of the gut microbiota can be used as a new, non-invasive marker of high grade prostate cancer.

In search of a new marker...

The cancer is diagnosed via rectal examination and a number of clinical examinations that assess the severity of the cancer and the risk of progression, and help to determine the treatment options (sidenote: Mohler JL, Antonarakis ES, Armstrong AJ, et al. Prostate Cancer, Version 2.2019, NCCN Clinical Practice Guidelines in Oncology. J Natl Compr Canc Netw. 2019;17(5):479-505. ) . The vital screening and follow-up examination for prostate cancer is the prostate specific antigen (PSA) assay, although it does not accurately identify the grade of the cancer. The grade is assessed on a scale from 1 to 5, following examination of tissue samples from the prostate. At grade 1, patients’ prognosis is generally favourable, so it is recommended that no treatment be given, in order to avoid overtreatment which can often have an adverse impact.  By contrast, patients with grade 2 prostate cancer and above require prompt, appropriate treatment. A new method for grading prostate cancers needs to be developed as a matter of priority to supplement the serum PSA test and avoid invasive examination. 

...In the intestinal microbiota

In a previous study, the authors showed that in mice, obesity, a fatty diet and even certain molecules produced by the gut microbiota tend to promote the proliferation of prostate cancer cells.  These results suggest that the gut microbiota could be used as a biomarker for determining how the cancer will progress. When they analysed the intestinal microbiota of patients who had undergone a prostate biopsy, the researchers found that three groups of bacteria were more abundant in patients with a high grade prostate cancer. To improve the accuracy of the diagnosis, the authors used a mathematical model and identified a further 18 types of bacteria to create the Fecal Microbiome Prostate Index, or FMPI. This FMPI index can be used to identify patients with a high grade prostate cancer with greater accuracy than conventional PSA assay. 
While very encouraging, this study remains localised and the scope of the research is limited, with only men who were Japanese citizens being included. It now needs to be conducted in more patients, with different profiles, thereby confirming what are promising results. 
 

^Source:

^{Matsushita M, Fujita K, Motooka D, et al. The gut microbiota associated with high-Gleason prostate cancer. Cancer Sci. 2021.}

Osteoarthritis, a condition that makes joints seize up and become misshapen, blights the life of more than 3% of the world’s population, especially seniors. 10% of men and 18% of women over 60 are suffering from this painful and debilitating joint disease. It has multiple causes: genetics, gender, age, obesity, sedentary lifestyle… and maybe even the gut microbiota. We already know that our gut bacteria are involved in several inflammatory diseases, and they might also play a role in osteoarthritis-related inflammation.

How an unbalanced gut microbiota correlates with osteoarthritis

To learn more about the relationship between gut microbiota and osteoarthritis in elderly women, researchers compared the stool bacterial composition of 57 women aged 65 on average suffering from osteoarthritis and that of 57 healthy volunteers of the same age (control). The gut microbiota of patients with osteoarthritis was generally less abundant and less diversified. Some beneficial bacteria are less prevalent, such as Bifidobacterium longum which regulates the immune system, or Faecalibacterium prausnitzii, an anti-inflammatory bacterium known for its health benefits in humans. On the contrary, the content of some pathogenic bacteria such as Clostridium ramosum is increased. In addition, some functions of the gut microbiota seem to be disrupted in case of osteoarthritis, thus suggesting a reduced ability to benefit from food.

When could the gut microbiota help with diagnosis, and even treatment?

Since the gut microbiota of patients with osteoarthritis is different from that of controls–thus suggesting that the gut microbiota could be a risk factor–the research team tried to develop a predictive tool for the disease based on the presence of 9 bacteria in the patients’ stools. Their predictive model was proven reliable in the elderly women from the study but has not yet been verified in other patient groups. The presence of these bacteria could well prove to be useful in establishing a diagnosis. These microorganisms could also open up new treatment options for osteoarthritis based on prebiotics or probiotics. Ultimately this could lead to better pain relief and improved quality of life for osteoarthritis patients.

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Coconut oil for the scalp microbiota!

^{Saxena R, Mittal P, Clavaud C. et al. Longitudinal study of the scalp microbiome suggests coconut oil to enrich healthy scalp commensals. Sci Rep 2021; 11: 7220.}

A recent study has shown that coconut oil can help maintain a healthy scalp by improving the scalp microbiota. The researchers compared the impact of coconut oil and a neutral shampoo on the scalp bacterial and fungal microbiota of 140 women with and without dandruff. The scalps of the women with dandruff had a much higher abundance of uncharacterized Malassezia species, fungus known to accelerate the development of dandruff and inflammation. Conversely, the fungus species M. globosa, was found in abundance on the scalps of the women with no dandruff or itching. Treatment with coconut oil brought the ratio of M. globosa to other groups of Malassezia in line with that of healthy scalps. Although no significant differences were observed between the bacterial microbiota of the healthy group and that of the dandruff group, both groups saw an increase in bacteria involved in the metabolism of biotin following coconut oil treatment. Biotin, a B vitamin that is essential for the maintenance of healthy skin and a healthy scalp, is also known to reduce inflammation. Further studies are required to understand the underlying mechanisms, but for the researchers, the positive effect of coconut oil on the composition and function of the scalp microbiota is the first step towards the longer-term restoration of scalp health.

Using recombinant endolysins to treat bacterial vaginosis

^{Landlinger C, Tisakova L, Oberbauer V. Engineered phage endolysin eliminates gardnerella biofilm without damaging beneficial bacteria in bacterial vaginosis ex vivo. Pathogens 2021; 10: 54.}

A study has shown that by using recombinant endolysins of the type 1,4-beta- N-acetylmuramidase encoded on Gardnerella prophages it is possible to eliminate the bacterial biofilm responsible for bacterial vaginosis without damaging the beneficial bacteria of the vaginal microbiota. To this end, the authors generated several engineered endolysins, bacteriophage enzymes that lyse the bacterial wall, via domain shuffling. They compared their bactericidal activity on Gardnerella strains to that of wild-type endolysins. The bactericidal activity of the recombinant endolysins was 10 times that of any wild-type enzyme. When tested against a panel of 20 Gardnerella strains from 4 species (G. vaginalis, G. leopoldii, G. piotii and G. swidsinski), the most active endolysin, called PM-477, showed superior efficacy compared to the antibiotics tested (metronidazole, tinidazole, clindamycin). Furthermore, PM-477 had no effect on beneficial lactobacilli or other species of vaginal bacteria. According to the authors, PM-477 is highly selective for Gardnerella and kills strains of each of the four main species without affecting beneficial lactobacilli or other species typical of the vaginal microbiota. The effect of PM-477 was confirmed by microscopy in mixed cultures of Gardnerella and lactobacilli. To go further and analyze the efficacy of PM-477 in a physiological environment closely resembling the in vivo situation, the researchers treated vaginal swabs from 15 bacterial vaginosis patients and analyzed them by fluorescence in situ hybridization (FISH). They showed that in 13 of the 15 cases, PM-477 eradicated the Gardnerella bacterium and physically dissolved the biofilms without affecting the vaginal microbiota. According to the authors, endolysins are a promising therapeutic alternative to antibiotics for the treatment of bacterial vaginosis.

Gut microbiota, mediterranean diet and cardiovascular disease

^{Wang D, Nguyen LH, Li Y, et al. The gut microbiome modulates the protective association between a Mediterranean diet and cardiometabolic disease risk. Nature Medicine 2021; 27: 333-43.}

Top contributors of disease burden worldwide, cardiometabolic disease such as cardiovascular disease (CVD) and type 2 diabetes (T2D) have been linked to the individualized nature of the gut microbiome (metabolism and immune interactions). While preclinical studies suggest that gut microbiome and diet engage in a two-way relationship, strong clinical evidence is still lacking especially towards cardiometabolic disease risk. The aim of the study was to examine the interplay of a Mediterranean diet (MedDiet), the gut microbiome and cardiometabolic disease risk in a subpopulation of over 300 men from the long-running Health Professionals Follow-up Study (HPFS). A significant interaction between a healthy dietary pattern and the gut microbiome in relation to cardiometabolic disease risk was identified. This study shows that long-term adherence to a healthy MedDiet was associated with the taxonomic and enzymatic variation of the gut microbiome. Dietary patterns explained 0.7% of the variation which is higher than that caused by antibiotic use. Adherence to MedDiet was associated with the enrichment of microbial degradation of dietary fibers and short chain fatty acid fermentation run by anaerobic fiber metabolizers such as F. prausnitzii and E. rectale. Low adherence to MedDiet with use of red or processed meat was associated with increased microbial synthesis of hepatotoxic secondary bile acids carried mainly by C. aerofaciens. The study underlines the ability of the MedDiet to mitigate cardiometabolic disease risk in the absence of Prevotella copri; while increase in the MedDiet index was associated with decreased myocardial infarct risk in P. copri non-carriers, the P. copri carriers had increased risk. Consequently, individual’s gut microbial profile could be used to tailor dietary interventions to prevent CV disease. For P. copri non-carriers, a MedDiet would be the first-line preventive measure, while P. copri carriers might benefit more from exercise or statins to control CV risk.

Antibiotic prophylaxis and resistance in leukaemia patients

^{Margolis EB, Hakim H, Dallas RH, et al. Antibiotic prophylaxis and the gastrointestinal resistome in paediatric patients with acute lymphoblastic leukaemia: a cohort study with metagenomics sequencing analysis. Lancet Microbe 2021 [Epub ahead of print].}

Although antibiotic prophylaxis (AP) can reduce the risk of serious infection in immunocompromised patients the major drawback is antibiotic resistance. Prophylaxis with a broad-spectrum fluoroquinolone may select for antibiotic-resistant microbes and lead to cross-resistance to other antibiotics. In this study, the authors have examined gastrointestinal resistome of children with acute lymphoblastic leukaemia (ALL) to determine the impact of AP on the antibiotic resistance genes (ARGs). Amongst the 49 children with ALL, 31 (63%) received levofloxacin prophylaxis during induction therapy and 18 received no prophylaxis. Trimethoprim-sulfamethoxazole was given for Pneumocystis jirovecii prophylaxis. An increase in the relative abundance of trimethoprim-sulfamethoxazole resistance genes of gut microbiota was detected, which was not modified by levofloxacin prophylaxis. Topoisomerase point mutations of stool bacteria increased during therapy in levofloxacin recipients, but not in the rest of the study population. Levofloxacin target bacterial topoisomerase enzymes that catalyze DNA double-strand break. The increase in the prevalence of fluoroquinolone resistance genes was low and the number of patients with topoisomerase mutations remained small. Although the selective effect of levofloxacin appeared small, an increase in the frequency of fluoroquinolone resistance persisted for at least 2 months after exposure. By contrast, no changes were detected in the aminoglycoside, β-lactam, vancomycin, or multidrug resistance genes after induction therapy suggesting no cross-class resistance to any other antibiotics. In conclusion, fluoroquinolone prophylaxis gives short-term protection against infections but does not increase the risk of cross-resistance to other antibiotics.

The role of Fecal Microbiota Transplantation (FMT) in melanoma treatment

^{Barruch EN, Youngster I, Ben-aBetzalel G, et al. Fecal microbiota transplant promotes response in immunotherapy-refractory melanoma patients. Science 2021; 371: 602-9.}

Immunotherapy to inhibit the programmed cell death-1 (PD-1) checkpoint protein is used for melanoma patients, but only 10-20% obtain complete remission. To increase therapy success, modulation of the gut microbiota has become one of the most promising leads as it shows positive results in preclinical models. However, it has not been investigated in clinical trials. The authors wanted to evaluate immune cell impact of FMT followed by anti-PD-1 immunotherapy in patients with refractory metastatic melanoma. FMT was performed both via colonoscopy and with oral administration of stool capsules followed by reinduction of anti-PD-1 therapy. Stool was obtained from two donors (donor 1 and 2) whose metastatic melanoma had been treated and complete remission reached. No moderate or severe adverse events from FMT were observed. Objective melanoma treatment responses were detected in three patients, they all had received FMT from the same donor (1). One patient achieved complete and two partial remission. After FMT gut microbiota differed from the baseline in all patients and was different depending on the donor (1 or 2). Responders had higher relative abundance of Enterococcaceae, Enterococcus, and Streptocccus australis and lower abundance of Veillonella atypica, but no association between microbial taxa and treatment response was detected. After FMT up-regulation of genes related to the presentation of peptides by antigen-presenting cells (APC) was detected. Responders up-regulated also genes related to APC-activity, innate immunity and interleukin-12. Tumor analysis of all available recipients revealed post-treatment up-regulation of many immune-related gene sets. The study shows that FMT combined with anti-PD-1 therapy is a safe and potentially effective treatment for refractory metastatic melanoma. Modulation of the gut microbiota may overcome resistance to immunotherapy.

Microbiota and breast cancer

^{Costa DA, Nobre JG, Batista MV, et al. Human microbiota and breast cancer – is there any relevant link? – A literature review and new horizons toward personalized medicine. Frontiers Microbiol 2021; 12: 584332.}

In this review, the authors focus on human microbiota throughout life, the links between gut/breast microbiota and breast cancer (BC), and the impact of metabolomics and pharmacomicrobiomics on BC risk and prognosis and treatment choices. While estrogens, high breast density, western- style diet, obesity, alcohol and genetic factors are risk factors for BC, gut microbiota dysbiosis has emerged as a key player in development, treatment, and prognosis of BC through diverse biological processes. β-glucuronidase (BGUS) bacteria modify the enterohepatic circulation of estrogens and may increase the risk of hormone dependent BC. As in the gut, local microbial signatures of breast microbiota in BC patients differ from that of healthy controls. While it is not known whether these findings are a cause or a consequence, a link between breast dysbiosis and BC might exist and is influenced by bacteria and/or their components in local immune microenvironment. Human estrobolome (sidenote: Human estrobolome The aggregate of enteric bacterial genes whose products are capable of metabolising oestrogens (Plottel and Blaser, 2011). ) denotes enteric bacterial genes whose products metabolize estrogens. BGUS of intestinal bacteria deconjugate xenobiotics and estrogens leading to reuptake via enterohepatic circulation. Estrogen produced by BGUS may increase the risk of hormone-dependent BC. Other intestinal bacteria metabolize phytoestrogens that may protect against BC. Some gut bacteria produce equol and enterolignans that may diminish hormone-dependent BC risk. Twenty to 30% of western population have microbes (Coriobacteriaceae family) converting isoflavone to equol that has affinity to estrogen receptors, and antiandrogenic and antioxidant activity. Gut free fatty acid receptors are activated by short chain fatty acids and may participate in tumor suppression. Breast and gut microbiota may modulate BC microenvironment: activate aberrant epithelial proliferation, secretion of growth factors, genome mutations, local metabolic microenvironment and angiogenesis. Gut bacteria may inactivate, e.g., doxorubicin and gemcitabine. Gut microbiota has even a double role in radiotherapy efficacy either beneficial and protective or detrimental and resistant. To conclude, in BC patients, microbiota could serve as prognostic and predictive factors of treatment response. In the future, microbiota modulation may improve the outcomes of BC patients.