Your Microbiome

People with colorectal cancer have different relative proportions of gut bacteria or more diverse gut bacteria than people without colorectal cancer.¹Gao Z, Guo B, Gao R, Zhu Q, Qin H. Microbiota disbiosis is associated with colorectal cancer. Frontiers in Microbiology. 2015 Feb 2;6:20; Hibberd AA, Lyra A et al. Intestinal microbiota is altered in patients with colon cancer and modified by probiotic intervention. BMJ Open Gastroenterology. 2017 Jul 3;4(1):e000145; Geng J, Fan H, Tang X, Zhai H, Zhang Z. Diversified pattern of the human colorectal cancer microbiome. Gut Pathogens. 2013 Mar 7;5(1):2; Sears CL, Garrett WS. Microbes, microbiota, and colon cancer. Cell Host & Microbe. 2014 Mar 12;15(3):317-28. Harmful species also tend to be present in people with colorectal cancer.²Dejea C, Wick E, Sears CL. Bacterial oncogenesis in the colon. Future Microbiology. 2013;8(4):445–460. Examples:

• Enterotoxigenic Bacteroides fragilis [ETBF] is associated with a greater number of early stage carcinogenic lesions and increased risk of colorectal cancer.³Purcell RV, Pearson J et al. Colonization with enterotoxigenic Bacteroides fragilis is associated with early-stage colorectal neoplasia. PLoS One. 2017;12(2):e0171602.
• Higher abundance of Fusobacteria in cancerous tissue compared to normal tissue,⁴Kelly D, Yang L, Pei Z. Gut microbiota, fusobacteria, and colorectal cancer. Diseases. 2018 Dec 11;6(4):109; Marchesi JR, Dutilh BE et al. Towards the human colorectal cancer microbiome. PLoS One. 2011;6(5):e20447; Castellarin M, Warren RL et al. Fusobacterium nucleatum infection is prevalent in human colorectal carcinoma. Genome Research. 2012 Feb;22(2):299-306; Gao Z, Guo B, Gao R, Zhu Q, Qin H. Microbiota disbiosis is associated with colorectal cancer. Frontiers in Microbiology. 2015 Feb 2;6:20; Flanagan L, Schmid J. Fusobacterium nucleatum associates with stages of colorectal neoplasia development, colorectal cancer and disease outcome. European Journal of Clinical Microbiology & Infectious Diseases. 2014 Aug;33(8):1381-90. and its abundance is linked to higher mortality from colorectal cancer⁵Flanagan L, Schmid J et al. Fusobacterium nucleatum associates with stages of colorectal neoplasia development, colorectal cancer and disease outcome. European Journal of Clinical Microbiology & Infectious Diseases. 2014 Aug;33(8):1381-90.
• Fewer butyrate-producing bacteria among people with colorectal cancer compared to controls in observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods studies⁶Bultman SJ. Emerging roles of the microbiome in cancer. Carcinogenesis. 2014;35(2):249–255.
• Difference in the composition of microbes, and especially higher incidence of Actinomyces, Atopobium, Fusobacterium, and Haemophilus, among people with colon adenoma or colorectal cancer compared to healthy controls in a small observational study⁷Kasai C, Sugimoto K et al. Comparison of human gut microbiota in control subjects and patients with colorectal carcinoma in adenoma: terminal restriction fragment length polymorphism and next-generation sequencing analyses. Oncology Reports. 2016 Jan;35(1):325-33.

The connection between the microbiome and colorectal cancer is strong enough that the microbiome has been proposed as a screening tool for early stages of colorectal cancer.⁸Zackular JP, Rogers MA, Ruffin MT 4th, Schloss PD. The human gut microbiome as a screening tool for colorectal cancer. Cancer Prevention Research (Philadelphia). 2014 Nov;7(11):1112-21; Zeller G, Tap J et al. Potential of fecal microbiota for early-stage detection of colorectal cancer. Molecular Systems Biology. 2014 Nov 28;10(11):766.

Antibiotic use and colorectal cancer

Modest evidence of higher risk of colon cancer but lower risk of rectal cancer, mostly among women, among people with the highest use of antibiotics

• 17% higher risk of proximal colon cancer among people with the highest use of antibiotics more than 2 years before diagnosis compared to no use in a very large observational study⁹Lu SSM, Mohammed Z et al. Antibiotics use and subsequent risk of colorectal cancer: a Swedish nationwide population-based study. JNCI: Journal of the National Cancer Institute. 2021 Sep 1:djab125.
• Increased risk of colon cancer among people using antibiotics, even with minimal use or with use 10 or more years prior to diagnosis, compared to no use, and risk was strongest with antibiotics with anti-anaerobic effects, in a very large observational study¹⁰Zhang J, Haines C et al. Oral antibiotic use and risk of colorectal cancer in the United Kingdom, 1989-2012: a matched case-control study. Gut. 2019;68(11):1971–1978.
• A lower risk of rectal cancer, mostly among women, among people with the highest use of antibiotics more than 2 years before diagnosis compared to no use in a very large observational study¹¹Lu SSM, Mohammed Z et al. Antibiotics use and subsequent risk of colorectal cancer: a Swedish nationwide population-based study. JNCI: Journal of the National Cancer Institute. 2021 Sep 1:djab125.

“When asked about the difference between the apparent impact of antibiotic use on the risk of cancer in the colon when compared to the rectum, [senior author Cynthia] Sears commented, ‘We think these differences highlight the differences in biology and likely the microbiome between these two cancer sites. Hence we hypothesize that antibiotics impact disease at these sites differently.’”¹²Harrison P. Antibiotics use and increased risk of colon cancer? Medscape. August 23, 2019. Viewed February 25, 2020.

Esophageal cancer

Preliminary evidence of a link between esophageal cancer and an altered gut microbiome; we don’t know if changes in the microbiome contribute to cancer, or if cancer contributes to changes in the microbiome, or if the link works in both directions

• Differences in salivary microflora among people with esophageal cancer compared to healthy controls in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study¹³Farrell JJ, Zhang L et al. Variations of oral microbiota are associated with pancreatic diseases including pancreatic cancer. Gut. 2012 Apr;61(4):582-8.

Gallbladder cancer

Preliminary evidence of a link between chronic bacterial infection of bile and gallbladder cancer; we don’t know if the infection contributes to cancer, or if cancer contributes to infection, or if the link works in both directions

• Higher incidence of chronic bacterial infection of bile among people with gallbladder cancer compared to controls in a mid-sized observational study¹⁴Sharma V, Chauhan VS, Nath G, Kumar A, Shukla VK. Role of bile bacteria in gallbladder carcinoma. Hepatogastroenterology. 2007 Sep;54(78):1622-5.

Stomach cancer

Preliminary evidence of a link between stomach cancer and an altered gut microbiome; we don’t know if changes in the microbiome contribute to cancer, or if cancer contributes to changes in the microbiome, or if the link works in both directions

• An altered gut microbiome among people with intestinal-type gastric cancer compared to non-atrophic gastritis in a small observational study¹⁵Aviles-Jimenez F, Vazquez-Jimenez F, Medrano-Guzman R, Mantilla A, Torres J. Stomach microbiota composition varies between patients with non-atrophic gastritis and patients with intestinal type of gastric cancer. Scientific Reports. 2014 Feb 26;4:4202.
• A link between Helicobacter pylori infection and risk of gastric cancer, although some evidence that microbes other than H. pylori must also be present to facilitate mucosal movement toward cancer¹⁶Sheflin AM, Whitney AK, Weir TL. Cancer-promoting effects of microbial dysbiosis. Current Oncology Reports. 2014 Oct;16(10):406; Wroblewski LE, Peek RM Jr, Wilson KT. Helicobacter pylori and gastric cancer: factors that modulate disease risk. Clinical Microbiology Reviews. 2010 Oct;23(4):713-39.
• No evidence of a difference in the stomach (gastric) microbiota of people with stomach cancer compared to dyspeptic controls in a small observational study¹⁷Dicksved J, Lindberg M et al. Molecular characterization of the stomach microbiota in patients with gastric cancer and in controls. Journal of Medical Microbiology. 2009 Apr;58(Pt 4):509-516.

• Virtually all invasive cervical cancers worldwide contain human papillomavirus (HPV).¹⁸Walboomers JM, Jacobs MV et al. Human papillomavirus is a necessary cause of invasive cervical cancer worldwide. Journal of Pathology. 1999 Sep;189(1):12-9.

• Substantial differences in the larynx and throat microbiome of people with laryngeal carcinoma compared to people with vocal cord polyps in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study¹⁹Gong HL, Shi Y et al. The composition of microbiome in larynx and the throat biodiversity between laryngeal squamous cell carcinoma patients and control population. PLoS One. 2013 Jun 18;8(6):e66476.

Higher risk of worse cancer progression, spread, and prognosis if certain microorganisms that are normally found in the mouth are present within the lungs. The microbes are thought to trigger an inflammatory response, including immune suppression.²⁰The ASCO Post Staff. The lung microbiome may affect lung cancer pathogenesis and prognosis. The ASCO Post. December 10, 2020. Viewed March 25, 2021.

• Higher presence of lower airway microbiota with oral commensals among people with more advanced stages of lung cancer (stage 3B—4) compared to earlier stages (stage 1—3A) in a mid-sized observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study²¹Tsay JJ, Wu BG et al. Lower airway dysbiosis affects lung cancer progression. Cancer Discovery. 2021 Feb;11(2):293-307.
• Better survival among people with lung cancer with more microbe compositional diversity in the lower airway to compositional similarity in a mid-sized observational study²²Tsay JJ, Wu BG et al. Lower airway dysbiosis affects lung cancer progression. Cancer Discovery. 2021 Feb;11(2):293-307.

• Higher incidence of ovarian cancer or BRCA1 mutation status among women with lower levels of the protective bacteria Lactobacillus in the microbiota in the cervix, especially in younger women, compared to higher levels in a mid-sized observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study²³Nené NR, Reisel D et al. Association between the cervicovaginal microbiome, BRCA1 mutation status, and risk of ovarian cancer: a case-control study. Lancet Oncology. 2019 Aug;20(8):1171-1182.

• Altered oral microbes among people with pancreatic cancer compared to healthy controls in 3 small observational studies²⁴Farrell JJ, Zhang L et al. Variations of oral microbiota are associated with pancreatic diseases including pancreatic cancer. Gut. 2012 Apr;61(4):582-8; Matsukawa H, Iida N et al. Dysbiotic gut microbiota in pancreatic cancer patients form correlation networks with the oral microbiota and prognostic factors. American Journal of Cancer Research. 2021 Jun 15;11(6):3163-3175; Zhou W, Zhang D et al. The fecal microbiota of patients with pancreatic ductal adenocarcinoma and autoimmune pancreatitis characterized by metagenomic sequencing. Journal of Translational Medicine. 2021 May 18;19(1):215.
• A substantially more abundant microbiome in the pancreas among people with pancreatic cancer compared to healthy controls in a small observational study²⁵Castellino AM. Pancreatic microbiome influences cancer and its treatment. Medscape. March 23, 2018. Viewed April 15, 2018; Pushalkar S, Hundeyin M et al. The pancreatic cancer microbiome promotes oncogenesis by induction of innate and adaptive immune suppression. Cancer Discovery. 2018 Apr;8(4):403-416.

• Differences in microbial diversity and relative abundance among people with obesity compared to people of normal weight in reviews of observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods studies³⁵Magne F, Gotteland M et al. The Firmicutes/Bacteroidetes ratio: a relevant marker of gut dysbiosis in obese patients? Nutrients. 2020 May 19;12(5):1474; Hartstra AV, Bouter KE, Bäckhed F, Nieuwdorp M. Insights into the role of the microbiome in obesity and type 2 diabetes. Diabetes Care. 2015 Jan;38(1):159-65.
• Lower ratio of Bacteroidetes to Firmicutes species, which increases with weight loss, among obese people compared to people of normal weight in a review³⁶Ley RE, Turnbaugh PJ, Klein S, Gordon JI. Microbial ecology: human gut microbes associated with obesity. Nature. 2006 Dec 21;444(7122):1022-3.
• Higher incidence of obesity among children with lower ratio of Bacteroidetes to Firmicutes species in a review of observational studies³⁷Indiani CMDSP, Rizzardi KF et al. Childhood obesity and Firmicutes/Bacteroidetes ratio in the gut microbiota: a systematic review. Childhood Obesity. 2018 Nov/Dec;14(8):501-509.
• Lower fecal microbial diversity, showing higher amounts of Bacteroidetes and decreased amounts of Clostridium cluster XIVa among obese males with metabolic syndrome compared to lean healthy people in an observational analysis in a small RCTrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects³⁸Vrieze A, Van Nood E et al. Transfer of intestinal microbiota from lean donors increases insulin sensitivity in individuals with metabolic syndrome. Gastroenterology. 2012 Oct;143(4):913-6.e7.

Some research shows that categorizing individuals according to 2 microbial enterotypes (dominance of either Prevotella or Bacteroides) may be useful in predicting responses to diets and drugs. Knowing your enterotype could help in personalizing obesity management.³⁹Christensen L, Roager HM, Astrup A, Hjorth MF. Microbial enterotypes in personalized nutrition and obesity management. American Journal of Clinical Nutrition. 2018 Oct 1;108(4):645-651.

• Increased insulin sensitivity and gut microbial diversity among males with metabolic syndrome treated with small intestinal infusions of microbiota from lean donors (allogenic) compared to baseline in a small RCTrandomized controlled trial, a study design in which people are randomly assigned to either an experimental group or a control group to compare the outcomes from different treatments; an RCT is considered a strong design for determining a therapy’s effects⁴⁰Vrieze A, Van Nood E et al. Transfer of intestinal microbiota from lean donors increases insulin sensitivity in individuals with metabolic syndrome. Gastroenterology. 2012 Oct;143(4):913-6.e7.
• Lower levels of beneficial butyrate-producing bacteria, Roseburia and Faecalibacterium prauznitzii, in the gut microbiota of people with type 2 diabetes compared with healthy people in mid-sized observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods studies⁴¹Karlsson FH, Tremaroli V et al. Gut metagenome in European women with normal, impaired and diabetic glucose control. Nature. 2013 Jun 6;498(7452):99-103; Qin J, Li Y et al. A metagenome-wide association study of gut microbiota in type 2 diabetes. Nature. 2012 Oct 4;490(7418):55-60.
• Higher risk of insulin resistance among postmenopausal obese Caucasian females with higher fecal concentrations of Lactobacillus gasseri, Streptococcus mutans, or Escherichia coli in an observational study⁴²Hartstra AV, Bouter KE, Bäckhed F, Nieuwdorp M. Insights into the role of the microbiome in obesity and type 2 diabetes. Diabetes Care. 2015 Jan;38(1):159-65.
• More marked insulin resistance among people with low bacterial richness (number of gut microbial genes) compared with people with high bacterial richness in a mid-sized observational study⁴³Le Chatelier E, Nielsen T et al. Richness of human gut microbiome correlates with metabolic markers. Nature. 2013 Aug 29;500(7464):541-6.

Gut microbiota play an important role in estrogen metabolism.⁴⁴Bultman SJ. Emerging roles of the microbiome in cancer. Carcinogenesis. 2014;35(2):249–255. Some colonic microflora metabolites display estrogenic and/or antiestrogenic activity.⁴⁵Larrosa M, González-Sarrías A, García-Conesa MT, Tomás-Barberán FA, Espín JC. Urolithins, ellagic acid-derived metabolites produced by human colonic microflora, exhibit estrogenic and antiestrogenic activities. Journal of Agricultural and Food Chemistry. 2006 Mar 8;54(5):1611-20.

Neutropenic fever is linked to a loss of microbiota-derived protective metabolites among people with acute myeloid leukemia.⁴⁶Rashidi A, Ebadi M et al. Loss of microbiota-derived protective metabolites after neutropenic fever. Scientific Reports. 2022 Apr 15;12(1):6244.

The gut microbiota dampens the growth of potential pathogenic bacteria. Several gut bacteria can induce a specific immune response. Examples:⁴⁷Scheithauer TP, Dallinga-Thie GM, de Vos WM, Nieuwdorp M, van Raalte DH. Causality of small and large intestinal microbiota in weight regulation and insulin resistance. Molecular Metabolism. 2016 Jun 10;5(9):759-70.

• The exopolysaccharides of adherent Bifidobacterium brevis reduce the production of inflammatory cytokines.
• Short-chain fatty acids (SCFAs) produced by many Clostridium spp. stimulate the production of the anti-inflammatory interleukin (IL)-10 by regulatory T cells. Read more about SCFAs in the section titled The role of fiber and short-chain fatty acids in What is your microbiome? ›
• Segmented filamentous bacteria (SFB) stimulate the development of T helper cells.

Disturbances in the balance between the host immune system and gut microbiota can create or contribute to inflammation and can contribute to inflammatory bowel disorder⁴⁸Koh A, De Vadder F, Kovatcheva-Datchary P, Bäckhed F. From dietary fiber to host physiology: short-chain fatty acids as key bacterial metabolites. Cell. 2016 Jun 2; 165(6):1332-1345. and other diseases.⁴⁹Belkaid Y, Hand TW. Role of the microbiota in immunity and inflammation. Cell. 2014 Mar 27;157(1):121-41. Loss of microbial diversity due to chemotherapy is also linked to higher risk of infections from microbes.

• Higher risk of microbiologically documented infections within the 90 days after induction chemotherapy neutrophil recovery among people with acute myelogenous leukemia who lost microbial diversity during chemotherapy in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵⁰Galloway-Peña JR, Smith DP et al. The role of the gastrointestinal microbiome in infectious complications during induction chemotherapy for acute myeloid leukemia. Cancer. 2016 Jul 15;122(14):2186-96.

• A link between microbiota disruption and intestinal inflammation in people with inflammatory bowel diseases in a mid-sized observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵¹Morgan XC, Tickle TL et al. Dysfunction of the intestinal microbiome in inflammatory bowel disease and treatment. Genome Biology. 2012 Apr 16;13(9):R79.
• More pronounced inflammatory symptoms (phenotype) among people with low bacterial richness compared with people with high bacterial richness in a mid-sized observational study⁵²Le Chatelier E, Nielsen T et al. Richness of human gut microbiome correlates with metabolic markers. Nature. 2013 Aug 29;500(7464):541-6.
• More pronounced low-grade inflammation among obese or overweight people with lower microbial gene richness compared to greater richness in a small observational study⁵³Cotillard A, Kennedy SP et al. Dietary intervention impact on gut microbial gene richness. Nature. 2013 Aug 29;500(7464):585-8.

• More fear of cancer recurrence among women with invasive breast cancer with a higher relative abundance of the Bacteroides genus, and higher fear among women with a history of chemotherapy and lower microbial diversity and lower relative abundance of Firmicutes and higher relative abundance of Bacteroidetes at the phylum level, and higher relative abundance of Bacteroides and lower relative abundance of Lachnospiraceae and Ruminococcus at the genus level in a mid-sized observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵⁴Okubo R, Kinoshita T et al. Impact of chemotherapy on the association between fear of cancer recurrence and the gut microbiota in breast cancer survivors. Brain, Behavior, and Immunity. 2020 Mar;85:186-191.
• More anxiety among people with breast cancer with changes in frequency of certain gut bacteria genera (Faecalibacterium, Prevotella, Bacteroides) compared to baseline in a small observational study⁵⁵Paulsen JA, Ptacek TS et al. Gut microbiota composition associated with alterations in cardiorespiratory fitness and psychosocial outcomes among breast cancer survivors. Supportive Care in Cancer. 2017 May;25(5):1563-1570.

• Changes in cardiorespiratory fitness among people with breast cancer with changes in frequency of certain gut bacteria genera (Faecalibacterium, Prevotella, Bacteroides) compared to baseline in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵⁶Paulsen JA, Ptacek TS et al. Gut microbiota composition associated with alterations in cardiorespiratory fitness and psychosocial outcomes among breast cancer survivors. Supportive Care in Cancer. 2017 May;25(5):1563-1570.

• More fatigue among people with breast cancer with changes in frequency of certain gut bacteria genera (Faecalibacterium, Prevotella, Bacteroides) compared to baseline in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵⁷Paulsen JA, Ptacek TS et al. Gut microbiota composition associated with alterations in cardiorespiratory fitness and psychosocial outcomes among breast cancer survivors. Supportive Care in Cancer. 2017 May;25(5):1563-1570.

• Higher incidence of diarrhea among people receiving radiotherapy for abdominal tumors who experienced a progressive modification in their microbial diversity compared to those with stable diversity in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁵⁸Manichanh C, Varela E et al. The gut microbiota predispose to the pathophysiology of acute postradiotherapy diarrhea. American Journal of Gastroenterology. 2008 Jul;103(7):1754-61.
• Higher incidence of diarrhea as a result of pelvic radiotherapy among people with alterations in microbial diversity, richness, and the Firmicutes/Bacteroidetes ratio before radiotherapy compared to people without alterations in a small observational study⁵⁹Wang A, Ling Z et al. Gut microbial dysbiosis may predict diarrhea and fatigue in patients undergoing pelvic cancer radiotherapy: a pilot study. PLoS One. 2015 May 8;10(5):e0126312.
• Microbiota influence the occurrence of diarrhea related to treatment with camptothecin-11 (CPT-11). Treatments to inhibit the enzymes in bacteria that reactivate the drug in the gut have reduced diarrhea.⁶⁰Bultman SJ. Emerging roles of the microbiome in cancer. Carcinogenesis. 2014;35(2):249–255.

• Higher risk of psychoneurological symptoms (PNS) among people with head and neck cancer with a greater change in microbes (decrease in microbial evenness) after radiation therapy compared to before therapy; people with higher PNS had higher abundances in phylum Bacteroidetes, order Bacteroidales, class Bacteroidia, and four genera (Ruminiclostridium, Tyzzerella, Eubacterium fissicatena, and DTU089), while the low PNS cluster had higher abundances in family Acidaminococcaceae and three genera (Lactococcus, Phascolarctobacterium, and Desulfovibrio) in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁶¹Bai J, Bruner DW et al. Gut microbiome associated with the psychoneurological symptom cluster in patients with head and neck cancers. Cancers (Basel). 2020 Sep 6;12(9):2531.

• Worse quality of life scores (physical functioning, vitality, and mental health subscales) among people with breast cancer with higher levels of Ruminococcus or Dorea microbes compared to lower levels in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁶²Smith KS, Tissier A, Bail JR, Novak JR, Morrow CD, Demark-Wahnefried W, Frugé AD. Health-related quality of life is associated with fecal microbial composition in breast cancer survivors. Support Care Cancer. 2022 Dec 13;31(1):10.

• More sleep dysfunction among people with breast cancer with changes in frequency of certain gut bacteria genera (Faecalibacterium, Prevotella, Bacteroides) compared to baseline in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁶³Paulsen JA, Ptacek TS et al. Gut microbiota composition associated with alterations in cardiorespiratory fitness and psychosocial outcomes among breast cancer survivors. Supportive Care in Cancer. 2017 May;25(5):1563-1570.

Changes in appetite and obesity: modest evidence of a link between the large intestine microbiome and changes in several factors related to appetite and obesity; we don’t know which direction the link operates, or if it operates in both directions

• The large intestine microbiome affects obesity through changes in energy harvest, appetite, satiety signals, and disturbed immune and metabolic signaling in a review of the literature⁶⁴Scheithauer TP, Dallinga-Thie GM, de Vos WM, Nieuwdorp M, van Raalte DH. Causality of small and large intestinal microbiota in weight regulation and insulin resistance. Molecular Metabolism. 2016 Jun 10;5(9):759-70.

Depression: preliminary evidence of a link between microbiome composition and depression; we don’t know which direction the link operates, or if it operates in both directions

• Higher proportion of Bacteroidales, lower proportion of Lachnospiraceae, and other changes in the microbiome of people with depression compared to other people in a small observationala type of study in which individuals are observed or certain outcomes are measured, but no attempt is made to affect the outcome (for example, no treatment is given); an example is a study that records people’s diets, but doesn’t try to alter their diets, and looks for patterns of disease or other outcomes related to different foods study⁶⁵Naseribafrouei A, Hestad K et al. Correlation between the human fecal microbiota and depression. Neurogastroenterology and Motility. 2014 Aug;26(8):1155-62.