What Do Gut Microbes Do? — Guests Who Eat the Leftovers, Guests Who Hold the Room

Facing the Phrase "Good Lactic Acid Bacteria for the Gut"

Pharmacy and supermarket shelves are lined with probiotic products promising "gut health." Advertisements claim that bacteria in the gut change digestion and immunity, and even mood and weight. Meanwhile, quite a few people have experienced diarrhea after taking antibiotics for a bacterial infection.

Rarely do we get a chance to learn separately about what lives in the gut, what it has actually been shown to do, and where the hopes still outrun the facts.

This article starts at the very end of the tube through which food passes: the large intestine. It positions the small creatures living at the end of the tube as guests, and follows what these guests do and what happens when they are away. At the end, it sorts out where expectations of these guests outpace the evidence. The big picture of how food travels from mouth to anus is in another installment of this series, the one covering the big picture of digestion.

The End of the Tube, The Large Intestine

Food passes through the stomach and small intestine, where most of it is broken down and absorbed. The small intestine is a long, thin tube between the stomach and large intestine, and most of the nutrients in food are absorbed here. The small intestine gathers amino acids, sugars, fats, vitamins, and minerals from digested food (PMID 42330921).

The large intestine is a thick tube that follows the small intestine, gathering water and salt from what remains and creating stool to expel. The large intestine is shorter in length and wider in diameter than the small intestine. If the small intestine is where nutrients are gathered, the large intestine is where water is reclaimed from the leftovers and waste is compacted and expelled (PMID 42330921).

At the end of this tube, guests live. Gut microbes are small living things, including bacteria, that gather and live in the human gut, especially the large intestine. Bacteria live here in the trillions, along with fungi and viruses (PMID 35163038).

The microbiome refers together to the microorganisms living in a single spot in the body and the genes those microorganisms possess. In this article, it refers to those in the gut. It is a term that adds the list of what the guests know how to do to the roster of guests.

How many guests are there? A commonly cited figure was that bacteria in the body outnumber human cells by at least ten to one (PMID 26824647). Two studies that recalculated this number in 2016 reported that the ratio is much closer to 1 to 1 (PMID 26824647, PMID 27541692). One of those studies estimated that a reference man weighing 70 kg has roughly 38 trillion bacteria and about 30 trillion human cells, and the combined weight of all the bacteria was about 0.2 kg (PMID 27541692).

The large intestine is the end of the tube where water and salt are gathered from what the small intestine left behind, and the bacteria living there as guests are in the same digit count as human cells when looking at the whole body.

A Guest List That Differs for Everyone

Even among healthy people, the microbes living in the gut differ noticeably. Diet, environment, genetics, and the microbes first met in childhood appear to be involved in these differences, but much of the variance remains unexplained. Yet even when the guest lists differ, the metabolic pathways those microbes possess are stable across individuals (PMID 22699609). Although the rosters are different, the list of jobs they know how to do is similar.

The roster keeps changing from birth. An infant's gut microbiota begins to resemble the adult form around the age of three, with the mode of delivery, infant feeding, adult diet, and antibiotics cited as factors that shape this composition (PMID 26269668).

Diet is considered a key factor determining the gut microbiota. However, in trials that try changing the diet, changes in the overall composition have been very small, usually involving shifts in the numbers of only a few species (PMID 32238220).

Because differences among individuals are large, there is not even a consensus on what composition constitutes a "normal gut microbiota." The methods for measuring microbes are also not standardized and continue to change (PMID 39663462). When what is normal has not been determined, the standard for calling a particular composition a bad state becomes shaky to that same extent.

The composition of the gut microbiota varies greatly from person to person, changes depending on age, diet, and drugs, and what constitutes a normal composition has not yet been agreed upon.

The Guest Who Eats What Is Left

The starches in rice and bread are broken down in the small intestine, but plants also contain many carbohydrates that humans cannot break down. Digestive enzymes are proteins that chop large molecules in food into small pieces the body can absorb. Human digestive enzymes cannot break down many of the complex carbohydrates and polysaccharides found in plants (PMID 28165863). The pathway by which starch is broken down in the small intestine is covered in another part of this series, the one dealing with the big picture of digestion. Dietary fiber is the portion of plant-derived carbohydrates that cannot be broken down by human digestive enzymes.

The guest eats what the host leaves behind. The carbohydrates that humans fail to break down are instead degraded by gut microbes, yielding short-chain fatty acids in the process (PMID 28165863). This breakdown is a fermentation performed by gut microbes living in an oxygen-free environment (PMID 23821742).

Short-chain fatty acids are small fatty acids produced when gut microbes ferment dietary fiber, with acetic acid, propionic acid, and butyric acid being the most representative. What the guest leaves behind returns once more to the host. Short-chain fatty acids are an important fuel for the cells lining the inside of the intestine (PMID 37596634). They perform essential tasks for keeping the intestine intact, such as regulating acidity inside the gut, promoting mucus production, and acting on intestinal immunity (PMID 32865024). Pathways involving appetite, energy expenditure, and blood sugar control have also been reported (PMID 32865024).

Here, the layers of evidence diverge. The data supporting these effects come mostly from animal studies, with few well-controlled human studies. This review by a panel of experts from ILSI Europe concluded that mechanistic data and limited human data strongly suggest that increasing short-chain fatty acids could be a valuable strategy for preventing gut dysfunction, obesity, and type 2 diabetes. At the same time, they concluded that long-term, well-controlled human trials are urgently needed (PMID 32865024). There are also many conflicting results, and data measuring how much is actually produced and used in the gut are scarce (PMID 23821742).

Whether eating more fiber increases short-chain fatty acids has not yielded a uniform direction either. Pooling human studies in which healthy adults were fed dietary fiber, among studies measuring total short-chain fatty acids, seven showed a clear increase and five showed no clear change. The results varied depending on the type, structure, and amount of fiber, as well as the measurement method (PMID 35807739).

That gut microbes ferment dietary fiber—which humans cannot break down—to produce short-chain fatty acids is established, but whether this prevents metabolic disease in humans has not yet been confirmed by human trials.

Guests Guarding Their Seats

The intestine must take in nutrients and water while blocking microorganisms and toxins (PMID 42330921). The intestinal barrier is a fence made by the cells lining the inside of the intestine and the mucus on top of them, letting nutrients and water pass while blocking microorganisms and toxins. The cells of the intestinal barrier build chemical and physical fences that keep gut microorganisms separated from the body's immune cells, and the gut microorganisms in turn help keep this fence intact and allow the intestinal immune system to establish itself (PMID 32340570).

The fence can be shaken. Various factors such as a Western diet, antibiotics, alcohol, stress, and age affect the intestinal barrier, and when the barrier is disturbed for a long time, microbial components can cross into the body and lead to mild, persistent inflammation throughout the body. The connection that an intact intestinal barrier and intestinal inflammation go hand-in-hand in intestinal diseases is well established, and that the disruption of the intestinal barrier and the disruption of the gut microbial composition may contribute to metabolic diseases, autoimmune diseases, and aging-related diseases is a stage pointed to by recent studies (PMID 35151560).

Another job the guests do is guarding their seats. Colonization resistance is the ability of the microorganisms originally living in the intestine to prevent pathogens that entered from the outside from settling and multiplying. This is one of the primary jobs of gut microorganisms (PMID 36539611). The resident microorganisms release substances that block pathogens, compete for food, support the intestinal barrier, and even use viruses that attack bacteria. The mechanisms are not yet fully understood (PMID 31167904). Eliciting human immune responses is also one of those methods (PMID 36539611). If guests fill the room and eat the food first, the incoming pathogens have fewer seats to sit on and less food to eat.

This power weakens when the guest list is disrupted. Dysbiosis is a state where the composition of gut microorganisms is disrupted and loses balance. Dysbiosis increases the risk of pathogen infections and the excessive multiplication of potentially harmful strains among the resident bacteria (PMID 36539611). When antibiotics, gastric acid-reducing drugs, diabetes drugs, and certain psychiatric medications change the composition, colonization resistance is disrupted, creating an opportunity for pathogens entering from the outside to take root (PMID 31167904).

Together with the intestinal barrier, gut microorganisms prevent pathogens from taking root, and when medications alter their composition, that power can be disrupted.

The Seat Left Empty — Clostridioides difficile Infection

There is a bacterium that is the leading cause of infectious diarrhea acquired in hospitals (PMID 27158839). Clostridioides difficile infection is an infection in which the bacterium Clostridioides difficile multiplies in the large intestine, releases toxins, and causes diarrhea and inflammation of the colon. Below, it is referred to simply as C. diff infection. Healthy people can also catch it outside of hospitals (PMID 38726585).

This bacterium forms spores. Spores are a form of the bacterium that remains dormant, wrapped in a tough shell, and C. diff can linger in the gut in this state without causing symptoms. A healthy gut microbiome prevents C. diff from taking hold. When dysbiosis caused by antibiotics disrupts that protective force, the spores wake up and release toxins. Symptoms range widely from mild diarrhea to life-threatening, severe inflammation of the colon and death (PMID 41039149).

Taking antibiotics, being of advanced age, and staying in a hospital or nursing facility are the most important risk factors, and the bacteria spread from stool to mouth (PMID 30945014). C. diff infection is treated with specific antibiotics (PMID 27158839). However, it returns in nearly one-third of infected people, and traditional antibiotic treatment can further disrupt the disturbed gut microbiome (PMID 37096495).

Clostridioides difficile infection is an infection in which spores wake up and release toxins in a gut often disrupted by antibiotics, and it returns in roughly one in three cases.

A Treatment That Admits Guests Anymore, Fecal Microbiota Transplantation

For recurrent Clostridioides difficile infection, there is a treatment that admits guests back in wholesale. Fecal microbiota transplantation is a treatment that transfers stool from a healthy donor into a patient's gut to restore disrupted gut microbes.

The evidence for this treatment comes from randomized trials. A randomized controlled trial is a test where participants are divided randomly like drawing lots, giving the drug to only one side and comparing the results with the other side. When pooling 6 randomized trials involving 320 adults with recurrent Clostridioides difficile infection (most people with severely compromised immunity were excluded), the group that received the transplantation had about 1.9 times the rate of resolution of recurrent infection compared to the group that received other treatments such as antibiotics. This magnitude means that if three people are treated with transplantation, one more person gets better (PMID 37096495). The certainty of evidence is a grade—rated as high, moderate, low, or very low—reflecting how much we can trust the pooled results. The certainty of this result was moderate (PMID 37096495).

Safety was not concluded in the same analysis. Serious adverse events and deaths occurred too infrequently to judge, and the researchers wrote that larger national registry data may be needed to see short- and long-term risks. People with compromised immunity could not be concluded because there were few participants (PMID 37096495). One review summarizes that guidelines recommend this treatment to prevent recurrence after two recurrences, and that the cure rate at that time is close to 90% (PMID 36844708).

The 2024 American Gastroenterological Association (AGA) guidelines suggested using fecal microbiota transplantation in adults with normal immunity and recurrent Clostridioides difficile infection to prevent recurrence after completing standard antibiotic therapy, selecting some patients. For adults with severely compromised immunity, they suggested not using any method. When Clostridioides difficile infection severe enough to require hospitalization does not respond to standard antibiotics, they suggested adding conventional transplantation in selected patients (PMID 38395525).

The same guidelines suggested not using conventional transplantation to treat inflammatory bowel disease and one common bowel condition we will look at later, leaving it as an exception only within clinical trials. They concluded that this treatment cannot yet be recommended for other digestive diseases (PMID 38395525).

Prior to transplantation, donors are rigorously selected through questionnaires, interviews, blood tests, and stool tests. One review notes that, based on current evidence, it is generally safe with few side effects, but long-term outcomes arising after a long time following transplantation are not yet fully elucidated (PMID 30181015).

Fecal microbiota transplantation is a treatment whose effectiveness is recognized by guidelines in some patients with recurrent Clostridioides difficile infection, and is still a treatment within clinical trials for other digestive diseases.

The Evidence on Lactic Acid Bacteria Products and Probiotics

Getting diarrhea during or after taking antibiotics is a common side effect (PMID 22570464).

Probiotics are live microorganisms intended to provide a health benefit when consumed. What are commonly called lactic acid bacteria products fall under this category, and other microorganisms like yeasts are also used (PMID 22570464). Prebiotics are substances that microorganisms in the body selectively use to provide a health benefit. This is the definition re-established by an international panel of experts in 2017, and the panel noted that this name can only be given when the effect of benefiting health has been confirmed (PMID 28611480).

In a 2012 analysis pooling 63 randomized trials involving 11,811 people, the group that took probiotics had about 0.58 times the risk of antibiotic-associated diarrhea. This magnitude means that if 13 people take them, one person avoids diarrhea. However, because the results varied greatly across trials and many trials did not properly record the strains used, it was not possible to distinguish how effects differed by person, antibiotic, or product (PMID 22570464).

A placebo is a fake medicine that looks identical but has no active pharmacological ingredient. In a 2017 analysis pooling 31 randomized trials involving 8,672 people, the proportion of people who experienced C. difficile diarrhea was 1.5% in the group that took probiotics and 4.0% in the group that took a placebo or nothing, and the certainty of the evidence was moderate (PMID 29257353).

That difference was tied to conditions. In trials that enrolled people whose original risk of developing C. difficile diarrhea exceeded 5%, the figures were 3.1% versus 11.6%, whereas in trials with a risk of 5% or lower, there was no difference. This distinction came from a post-hoc subgroup analysis that was not predetermined. The rate at which C. difficile was detected in stool did not decrease, standing at 15.5% versus 17.0% (PMID 29257353). The researchers concluded that short-term use alongside antibiotics appeared safe and effective for patients who were not immunodeficient and not severely debilitated, and noted that while further research is needed, hospitalized patients—especially those at high risk—should be informed of the benefits and harms (PMID 29257353).

Guidelines reach a different judgment. The same analysis noted that clinical guidelines at the time did not recommend using probiotics to prevent C. difficile diarrhea, while adding that among the preventive methods covered in guidelines, probiotics had the highest level of evidence (PMID 29257353). The Infectious Diseases Society of America (IDSA) guidelines compiled in 2020 also do not recommend probiotics for preventing C. difficile infection (PMID 32003951). The analyses and the guidelines diverge in their judgments.

Where the effects come from is also an open question. Although some clinical data show that probiotics reduce antibiotic side effects, no data show that this effect occurs because they protect or restore the gut microbiome (PMID 39663462). In most studies, the impact of probiotics on the gut microbiome was not reported. One review suggests that the action of probiotics stems less from an ability to establish themselves among the gut microbiome, and more from sharing genes and substances, supporting a perturbed gut microbiome, and acting directly on intestinal and immune cells (PMID 32010640). They are closer to short-term visitors than guests.

Effects vary by strain and by disease. In one review, many commercial products had no trials confirming their effects, and among 22 conditions with sufficient evidence from 249 trials, 15 showed strong or moderate evidence for at least one condition (PMID 30586435).

Market rules do not focus on verifying efficacy either. Unless they claim to cure a specific disease, probiotics are regulated as dietary supplements, and regulation focuses on whether advertising claims are justified rather than on efficacy, safety, and quality. Because products with vastly different ingredients can be sold under the same brand name, results from one product cannot be generalized to another. This review noted that regulations vary by region and concluded that current regulations fall short of protecting consumers and doctors, especially for products used in the dietary management of serious diseases (PMID 29378309).

The evidence for probiotics varies by strain, product, and condition; has accumulated under certain conditions such as preventing diarrhea when used with antibiotics; and analyses and guidelines diverge in their judgments regarding the prevention of C. difficile infection.

Association and Cause — Where Expectations Get Ahead of Facts

Reports have poured in that gut microbes differ even in obesity, diabetes, liver disease, cancer, and diseases where the brain slowly breaks down. From 2013 to 2017, papers dealing with gut microbes numbered 12,900, which was four-fifths of the papers on this topic published over the past 40 years (PMID 29934437).

One review summarizing this trend attached a warning. When data comparing the gut microbes of people with and without a disease overflow, it is easy to wrongly claim that a certain bacterium prevents or causes a disease. Eating habits, medications taken, the speed at which the gut moves, and the frequency and firmness of bowel movements also change the composition of gut microbes and must therefore be considered together (PMID 29934437).

Observational studies are studies that do not interfere with people's lives, leaving them to live as they normally do and watching what happens afterwards. If studies comparing disease and gut microbes take this shape, even if a difference is seen, which side came first is not established. Dysbiosis has been associated with various inflammatory diseases and infections, and to read these studies, one must sort out individual differences and whether the disturbance is a cause or a result (PMID 28512250).

Depression is one example. When gathering 24 observational studies involving 2,817 people comparing patients with major depressive disorder and healthy people, the overall composition of gut microbes differed between the two groups, and there was no clear difference in diversity within a single person. In 19 trials from the same review involving 1,119 people, probiotics and preparations combining probiotics with prebiotics slightly reduced depressive symptoms in patients with major depressive disorder for 4 to 9 weeks, while prebiotics alone showed no such effect. The research team concluded that it is premature to use them in clinical practice before studies comparing them to antidepressants and following patients for longer periods come out (PMID 35562885).

The gut-brain axis is a pathway through which the gut and brain send and receive signals in both directions via nerves, hormones, and immune substances. This pathway involves immune, hormonal, and neural routes together (PMID 31123355). One review notes that research over the past 20 years has established the ability of gut microbes to alter brain activity and behavior, and conversely, that brain signals also change the composition and work of gut microbes. This abstract does not distinguish whether the evidence came from animals or humans (PMID 39743581). The idea that short-chain fatty acids bridge the gap is still at the stage of speculation, studies examining this directly are rare, and in humans they are even rarer (PMID 31123355).

One review notes that expectations that gut microbe research would provide quick solutions for chronic diseases with complex mechanisms have sometimes been inflated, while also covering realistic possibilities for use in diagnosis and new treatments (PMID 33263886). On the side where evidence is established are colonization resistance, seen earlier, and transplantation treatment for recurrent C. difficile infection.

Many connections between gut microbes and diseases like obesity and depression are at the stage of association, animal studies, or short trials, and whether those differences are the cause or the result of a disease must be sorted out separately.

Irritable Bowel Syndrome

There is a bowel disease that 5 to 10% of otherwise healthy people have at any given time (PMID 33049223). Irritable bowel syndrome is a condition where, despite no noticeable structural abnormalities on tests, abdominal pain repeats along with changes in the shape or frequency of bowel movements. Most cases alternate between getting better and getting worse. The best-known risk factor is acute infection of the gut, and it is more common in young women and those who also experience psychological difficulties (PMID 33049223).

It is well established that even without visible damage, the signals exchanged between the gut and the brain are scrambled. As a result, gut motility becomes disrupted, the gut feels stimuli more sensitively, and the brain processes gut signals differently (PMID 33049223). Reflecting this understanding, the British Society of Gastroenterology (BSG) guidelines reclassified this condition as a disorder of gut-brain interaction (PMID 33903147). Changes in the gut microbiome are also cited as candidate mechanisms, but belong to the side where results are less consistently reproduced (PMID 33049223).

Most cases are diagnosed by listening to the course of symptoms, and tests are performed only as much as strictly necessary. The approach differs if there are warning signs such as weight loss or blood in the stool, or if there is a family history of inflammatory bowel disease or celiac disease (PMID 33049223). Joint guidelines from several Italian societies recommend a colonoscopy if warning signs are present (PMID 36517261). The American College of Gastroenterology (ACG) guidelines suggest that diagnosing directly based on symptoms, rather than ruling out other diseases one by one, allows appropriate treatment to begin sooner. The same guidelines suggest a blood test to screen for celiac disease in patients with diarrhea, and a stool test to screen for inflammatory bowel disease (PMID 33315591).

The foundation of treatment includes explaining the disease, dietary adjustments, water-soluble dietary fiber, and drugs that relieve bowel spasms. If symptoms are severe, various drugs chosen according to the predominant bowel movement pattern and psychological therapy are also used (PMID 33049223).

There is one dietary approach that has been tested the most. The low-FODMAP diet is a dietary method that reduces several types of sugars and sugar alcohols that are poorly absorbed in the gut and easily fermented. In an analysis pooling 13 randomized trials and 944 people, the low-FODMAP diet lowered the risk of overall symptoms failing to improve to about 0.67 times compared to a typical diet, and ranked highest among the compared methods. However, most trials were conducted in specialized clinical centers, and the effects of the reintroduction and personalization phases—where restricted foods are brought back and tailored to the individual—were not examined (PMID 34376515). What this diet does to nutrition and the gut microbiome when continued long-term also needs more study (PMID 33585949). The American College of Gastroenterology guidelines recommend that irritable bowel syndrome patients try a low-FODMAP diet for a set period to reduce overall symptoms (PMID 33315591). This diet is introduced as something to use when general lifestyle and dietary advice does not bring improvement (PMID 34376515).

In an analysis pooling 82 trials and 10,332 people, certain combinations or strains of probiotics could be helpful, but the certainty of the evidence was low or very low in almost all analyses (PMID 37541528). Guidelines from Italian societies conditionally recommended probiotics (PMID 36517261). Fecal microbiota transplantation is suggested by the American College of Gastroenterology guidelines not to be used for irritable bowel syndrome outside of clinical trials (PMID 38395525).

Irritable bowel syndrome is a common disease understood as scrambled signals between the gut and the brain, even when structural abnormalities are not seen on tests.

What Happens to Your Gut Guests After You Take Antibiotics

Let us imagine a person who starts taking antibiotics for a bacterial infection. Antibiotics are medicines that cure bacterial infections. At the same time, it is also well established that antibiotics shake up the composition and work of the gut microbiome (PMID 39663462). In studies of children, the diversity and number of species in the gut were reduced after antibiotics, and one class of antibiotics was associated with a reduced number of species for twice as long as another class (PMID 33651651).

When the guests decrease, empty spaces open up. When antibiotics change the composition, colonization resistance is disrupted, which can create an opportunity for pathogens coming from the outside to take hold (PMID 31167904). If this person is older or staying in a hospital, the risk factors for C. diff infection overlap (PMID 30945014). If spores of C. diff were present in the gut, the spores can wake up in the disrupted space, release toxins, and diarrhea can begin (PMID 41039149).

When and to what extent the guests return after finishing antibiotics is difficult to summarize in one word. There is no consensus on what a normal composition is, and the methods for measuring it are not standardized (PMID 39663462).

C. diff infection is treated with antibiotics, but it returns in nearly one out of three cases (PMID 37096495), and for some patients who have relapsed, the transplant treatment we saw earlier is used within the conditions of the guidelines (PMID 38395525). One of the core strategies for reducing C. diff infection is managing antibiotics so that they are used properly only when truly necessary (PMID 32003951).

Antibiotics cure infections while shaking up the guest list, C. diff infection can arise in those empty spaces, and treatment after a relapse is determined by guidelines with attached conditions.

Where Else to Look

The question that remains unsolved is whether the differences in gut microbes seen in people with a disease are the cause or the result of that disease (PMID 28512250).

Distinguishing between the association seen in observation and the effect confirmed by testing can be followed with a different drug in "What Do GLP-1 Drugs Do Beyond Weight? — Signal, Association, and Confirmation". The distance between a change in gut microbe composition and a person's improvement shares the shape of the problem addressed by "Better Numbers, Better Patients? — An Introduction to Surrogate Endpoints".

The path by which nutrition is absorbed in the small intestine can be seen in another piece in this series, "How Does Digestion Work? — A Tube Through the Body, and a Wall Only Small Pieces Can Cross", and stomach acid and drugs that reduce it in "What Do the Stomach and Esophagus Do? — A Pouch That Holds Acid, and Its Two Doors". Colorectal cancer and its screening were not covered in this article, and there is no published article yet that can follow this one.

Source Papers

• "Gut mechanics." (2026) — PMID 42330921 — The small intestine extracts amino acids, sugars, lipids, vitamins, and minerals; the large intestine reclaims water and electrolytes and compacts the remainder for elimination; and the epithelium acts as a selective barrier that allows nutrients and water to pass while blocking microbes, antigens, and toxins.
• "Gut Microbiota and Short Chain Fatty Acids: Implications in Glucose Homeostasis." (2022) — PMID 35163038 — Gut microbiota consist of trillions of bacteria, fungi, and viruses, with acetic acid, propionic acid, and butyric acid being the most abundant short-chain fatty acids, produced through the fermentation of dietary fiber and resistant starch.
• "Diet and the gut microbiome: from hype to hypothesis." (2020) — PMID 32238220 — Diet is considered a key determinant of microbial composition, but dietary intervention studies show very small effects on overall composition and change only a minority of species, amidst the coexistence of hundreds of species in the large intestine.
• "Are We Really Vastly Outnumbered? Revisiting the Ratio of Bacterial to Host Cells in Humans." (2016) — PMID 26824647 — Re-examining the long-held conventional wisdom of a bacterial-to-human cell ratio of 10:1 or greater reveals it to be much closer to 1:1.
• "Revised Estimates for the Number of Human and Bacteria Cells in the Body." (2016) — PMID 27541692 — For a 70 kg reference male, there are 3.8×10^13 bacteria and 3.0×10^13 human cells—the same order of magnitude—with a total bacterial mass of roughly 0.2 kg.
• "Structure, function and diversity of the healthy human microbiome." (2012) — PMID 22699609 — Even healthy individuals have vastly different microbial compositions that remain largely unexplained (involving diet, environment, genetics, and early-life exposures), yet the metabolic pathways harbored remain stable despite differences in composition.
• "Role of the normal gut microbiota." (2015) — PMID 26269668 — Composition resembles that of an adult by around 3 years of age, shaped by delivery mode, infant feeding, adult diet, and antibiotics.
• "Antibiotic-perturbed microbiota and the role of probiotics." (2025) — PMID 39663462 — Antibiotic-induced microbial perturbation is well established, and while some clinical data suggest probiotics reduce antibiotic side effects (including C. difficile diarrhea), there are no data causally linking them to microbial protection and recovery, hampered by a lack of consensus on normal composition, unstandardized measurements, and individual variation.
• "Dietary fiber and prebiotics and the gastrointestinal microbiota." (2017) — PMID 28165863 — Human digestive enzymes fail to digest most complex carbohydrates and plant polysaccharides, which are instead metabolized by microbes to produce short-chain fatty acids (acetic acid, propionic acid, and butyric acid).
• "The role of short-chain fatty acids in the interplay between diet, gut microbiota, and host energy metabolism." (2013) — PMID 23821742 — Short-chain fatty acids are the end products of dietary fiber fermentation by anaerobic gut microbes, with many conflicting results and a lack of quantitative data on actual fluxes.
• "Short-chain fatty acids in diseases." (2023) — PMID 37596634 — Short-chain fatty acids are the primary metabolites generated by bacterial fermentation of dietary fiber and serve as an important fuel for intestinal epithelial cells.
• "Short chain fatty acids in human gut and metabolic health." (2020) — PMID 32865024 — Short-chain fatty acids are essential for gut integrity via acidity, mucus production, epithelial fueling, and mucosal immunity, and they participate in regulating appetite, energy expenditure, blood glucose, and immunity; data stem mostly from animal studies with human studies being limited, leading an expert panel to conclude that increasing them may be a valuable strategy for preventing gut dysfunction, obesity, and type 2 diabetes, though long-term well-controlled human studies are urgently needed.
• "Effects of Dietary Fibers on Short-Chain Fatty Acids and Gut Microbiota Composition in Healthy Adults: A Systematic Review." (2022) — PMID 35807739 — Across 44 dietary fiber intervention studies in healthy adults, total short-chain fatty acids increased significantly in 7 studies and showed no change in 5, depending on fiber type, structure, dose, and analytical method, yielding no uniform increase in a single direction.
• "Interaction Between the Microbiota, Epithelia, and Immune Cells in the Intestine." (2020) — PMID 32340570 — Epithelial cells act as chemical and physical barriers that separate microbes from immune cells, while microbes maintain the epithelial barrier and shape mucosal immunity; compositional perturbation is frequently observed in autoimmune and chronic inflammatory diseases.
• "Gut barrier disruption and chronic disease." (2022) — PMID 35151560 — Genetics, Western diets, antibiotics, alcohol, circadian disruption, stress, and aging impact the barrier, and chronic disruption leads to the translocation of microbial components and systemic low-grade inflammation, with the relationship in gut diseases being well established and recent research suggesting contributions to metabolic, autoimmune, and aging-related diseases.
• "Microbiota-mediated colonization resistance: mechanisms and regulation." (2023) — PMID 36539611 — Pathogen colonization resistance is a major function of microbes, and dysbiosis increases the risk of infection and the proliferation of harmful commensals via mechanisms including inter-microbial competition and the induction of host immunity.
• "Gut Microbiota and Colonization Resistance against Bacterial Enteric Infection." (2019) — PMID 31167904 — Colonization resistance mechanisms (secretion of antimicrobial substances, nutritional competition, barrier support, and bacteriophages) are not fully elucidated, and antibiotics, proton pump inhibitors, diabetes medications, and antipsychotics alter composition and disrupt colonization resistance, providing opportunities for pathogens to establish.
• "Clostridium difficile infection." (2016) — PMID 27158839 — The leading cause of healthcare-associated infectious diarrhea, posing a particular risk in the elderly and in dysbiosis following antibiotics, with frequent recurrences, treated with specific antibiotics, and fecal microbiota transplantation showing promise for recurrences.
• "Clostridioides Difficile: A Concise Review of Best Practices and Updates." (2024) — PMID 38726585 — Occurs even in healthy community individuals, with the US FDA recently approving two fecal microbiota-based products for recurrence prevention.
• "Clostridioides difficile pathogenesis and control." (2026) — PMID 41039149 — Forms spores and can reside asymptomatically in the gut, healthy microbes resist C. difficile colonization, dysbiosis commonly induced by antimicrobials permits spore germination and toxin production, and symptoms range from mild diarrhea to fulminant colitis and death.
• "Clostridium difficile infection: review." (2019) — PMID 30945014 — Transmitted via the fecal-oral route, with the most important risk factors being antibiotics, advanced age, and hospital or nursing home stay.
• "Fecal microbiota transplantation for the treatment of recurrent Clostridioides difficile (Clostridium difficile)." (2023) — PMID 37096495 — Nearly one-third of infected individuals experience recurrence, traditional antibiotic treatment can worsen dysbiosis, and across 6 RCTs totaling 320 immunocompetent adults, resolution of recurrent infection yielded an RR of 1.92 and an NNTB of 3 (moderate certainty), while conclusions on safety cannot be drawn due to a low number of serious adverse events and deaths, and no conclusions can be drawn for immunocompromised individuals.
• "Fecal Microbiota Transplantation." (2023) — PMID 36844708 — Fecal microbiota transplantation involves transferring stool from a healthy donor into a patient's gut, and guidelines recommend it after two recurrences to prevent further episodes, with cure rates approaching 90%.
• "AGA Clinical Practice Guideline on Fecal Microbiota-Based Therapies for Select Gastrointestinal Diseases." (2024) — PMID 38395525 — Suggests selective use of fecal microbiota-based therapies for recurrent C. difficile in immunocompetent adults after standard antibiotics, conventional transplantation selectively for mild-to-moderate immunocompromise, opposes it for severe immunocompromise, suggests conventional transplantation selectively for severe or fulminant hospitalized patients failing standard treatment, opposes use outside of clinical trials for inflammatory bowel disease and irritable bowel syndrome, and cannot yet recommend it for other digestive diseases.
• "Fecal microbiota transplantation: Review and update." (2019) — PMID 30181015 — Donors are rigorously screened through questionnaires, interviews, blood tests, and stool tests, and while generally safe, long-term outcomes are not fully understood.
• "Probiotics for the prevention and treatment of antibiotic-associated diarrhea: a systematic review and meta-analysis." (2012) — PMID 22570464 — Antibiotic-associated diarrhea is a common side effect; across 63 RCTs totaling 11,811 individuals, the RR was 0.58 and the NNT was 13, with substantial heterogeneity and poor strain documentation preventing conclusions on differences by population, antibiotic, or formulation, and including predominantly Lactobacillus species alongside yeasts and others.
• "Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics." (2017) — PMID 28611480 — Updated definition of prebiotics: a substrate that is selectively utilized by host microorganisms conferring a health benefit, with health benefits needing to be demonstrated.
• "Probiotics for the prevention of Clostridium difficile-associated diarrhea in adults and children." (2017) — PMID 29257353 — Across 31 RCTs totaling 8,672 participants, C. difficile diarrhea occurred in 1.5% versus 4.0% (RR 0.40, moderate certainty), post-hoc subgroup analysis showed efficacy only when baseline risk was >5% (3.1% versus 11.6%) with no difference at ≤5%, stool detection showed no reduction at 15.5% versus 17.0%, guidelines at the time did not recommend prophylactic use, and the conclusion was that short-term co-administration appears safe and effective in patients without severe immunosuppression or debility, and high-risk hospitalized patients should be informed of benefits and harms.
• "Clostridioides difficile Infection: Update on Management." (2020) — PMID 32003951 — Antibiotic stewardship is a core strategy for reducing infections, and the Infectious Diseases Society of America does not recommend probiotics for preventing C. difficile infection.
• "How Probiotics Affect the Microbiota." (2019) — PMID 32010640 — Most studies fail to report an effect on the microbiota, effects stem from sharing genes and metabolites and directly acting on epithelial and immune cells rather than colonization, and strains cannot be expected to have identical efficacy.
• "Choosing an appropriate probiotic product for your patient: An evidence-based practical guide." (2018) — PMID 30586435 — Efficacy is strain- and disease-specific, most commercial products lack evidentiary trials, and out of 249 trials, 15 of the 22 well-supported species (68%) had strong or moderate evidence for at least one disease.
• "The Unregulated Probiotic Market." (2019) — PMID 29378309 — Regulations vary by region, and unless making disease claims, products are regulated as dietary supplements, focusing on the justification of claims rather than efficacy, safety, or quality, with different formulations sold under the same brand name; conclusion: current regulations are inadequate for protecting consumers and physicians.
• "Human gut microbiome: hopes, threats and promises." (2018) — PMID 29934437 — Between 2013 and 2017, 12,900 papers on the gut microbiota were published (four-fifths of the total over 40 years), reporting changes in obesity, diabetes, liver disease, cancer, and neurodegenerative diseases, though patient-healthy comparisons can lead to false claims of bacterial causality, as dietary habits, medications, gut motility, and stool frequency and consistency all affect composition.
• "Introduction to the human gut microbiota." (2017) — PMID 28512250 — Dysbiosis is associated with many inflammatory diseases and infections, and interpretation requires accounting for individual variation and distinguishing cause from effect.
• "The Gut Microbiome in Depression and Potential Benefit of Prebiotics, Probiotics and Synbiotics: A Systematic Review of Clinical Trials and Observational Studies." (2022) — PMID 35562885 — Across 24 observational studies (2,817 individuals) there was no difference in alpha diversity but a difference in beta diversity, and across 19 intervention trials (1,119 individuals) probiotics and synbiotics slightly reduced major depressive disorder symptoms over 4 to 9 weeks while prebiotics did not, making clinical application premature prior to antidepressant comparisons and long-term follow-up.
• "Microbiota-neuroepithelial signalling across the gut-brain axis." (2025) — PMID 39743581 — Research over the past 20 years has established the capacity of gut microbiota to regulate brain activity and behavior, with brain signals also influencing microbial composition and function in bidirectional communication.
• "The role of short-chain fatty acids in microbiota-gut-brain communication." (2019) — PMID 31123355 — The microbiota-gut-brain mediated role of short-chain fatty acids remains speculative with unclarified pathways, and direct studies are rare, especially in humans.
• "Our Microbiome: On the Challenges, Promises, and Hype." (2020) — PMID 33263886 — Explores exaggerated expectations that microbiome research will yield quick fixes for complex chronic diseases, alongside realistic possibilities for diagnosis and treatment.
• "Irritable bowel syndrome." (2020) — PMID 33049223 — Characterized by abdominal pain and changes in stool form and frequency, affecting 5-10% of healthy people with a relapsing-remitting course, with acute enteric infection being the best-known risk factor, psychiatric comorbidities and young women being common, gut-brain communication disorders (motility abnormalities, visceral hypersensitivity, and central processing alterations) being well established, microbial changes being a poorly reproducible mechanism, diagnosis relying on clinical history in the absence of warning signs, and treatment resting on education, dietary modification, soluble fiber, and antispasmodics.
• "British Society of Gastroenterology guidelines on the management of irritable bowel syndrome." (2021) — PMID 33903147 — Reclassified among functional gastrointestinal disorders as a disorder of gut-brain interaction.
• "Italian guidelines for the management of irritable bowel syndrome: Joint Consensus from the Italian Societies of: Gastroenterology and Endoscopy (SIGE), Neurogastroenterology and Motility (SINGEM), Hospital Gastroenterologists and Endoscopists (AIGO), Digestive Endoscopy (SIED), General Medicine (SIMG), Gastroenterology, Hepatology and Pediatric Nutrition (SIGENP) and Pediatrics (SIP)." (2023) — PMID 36517261 — A chronic disorder of gut-brain interaction, recommending colonoscopy if warning signs are present, with probiotics conditionally recommended.
• "ACG Clinical Guideline: Management of Irritable Bowel Syndrome." (2021) — PMID 33315591 — Proposes positive diagnostic strategies rather than diagnoses of exclusion, recommends a limited trial of a low FODMAP diet for global symptom improvement, and suggests gut-directed psychological therapies.
• "Efficacy of a low FODMAP diet in irritable bowel syndrome: systematic review and network meta-analysis." (2022) — PMID 34376515 — Across 13 RCTs totaling 944 participants, a low FODMAP diet ranked first for non-improvement of overall symptoms compared to habitual diet with an RR of 0.67, with most being secondary and tertiary care trials, and reintroduction and personalization effects unstudied.
• "Efficacy of a low-FODMAP diet in adult irritable bowel syndrome: a systematic review and meta-analysis." (2021) — PMID 33585949 — A low-FODMAP diet reduces symptoms and improves quality of life compared to control diets, with nutritional adequacy and long-term effects on gut microbiota requiring further research.
• "Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis." (2023) — PMID 37541528 — Across 82 trials totaling 10,332 participants, certain combinations and strains may be helpful, but GRADE certainty is low to very low across nearly all analyses.
• "Association between antibiotics and gut microbiome dysbiosis in children: systematic review and meta-analysis." (2021) — PMID 33651651 — Antibiotic exposure in children is associated with decreased microbial diversity and abundance, with macrolides leading to a decrease in abundance that persists twice as long as that seen with penicillins.