High Bacteroidetes in Stool: Meaning, Risks and Next Steps
Key Gut Species: Bacterial Strains Commonly Found in the Gut
What Do High Bacteroidetes in Stool Mean?
A high Bacteroidetes reading on a stool test means this large group of bacteria makes up a larger share of your gut community than the test's reference range suggests. Bacteroidetes are not harmful in themselves — they are normal and essential residents of a healthy gut. A high reading is best understood as a signal worth exploring, not a disease.
Most stool tests report relative abundance, meaning they show how large each bacterial group is compared with the others. When Bacteroidetes are high, other groups such as Firmicutes are usually lower by comparison. That is why a single percentage tells you little: what matters more is the whole pattern, including microbial diversity, diet, symptoms, medications and recent antibiotic use.
Bacteroidetes vs. Bacteroides: What's the Difference?
The terms Bacteroidetes and Bacteroides are often used interchangeably, but they are not the same. Bacteroidetes is a phylum, one of the major branches of the bacterial family tree. Bacteroides is a genus within that phylum and one of its most abundant and best-studied members. Most stool tests report the phylum level, while species-level tests may report specific Bacteroides species.
Bacteroidetes is one of the four main bacterial phyla in the human gut, alongside Firmicutes, Actinobacteria and Proteobacteria. Together, Bacteroidetes and Firmicutes usually make up most of the gut community. Bacteroides belongs to the family Bacteroidaceae.
What Are Bacteroides and Why Do They Matter?
Bacteroides species are anaerobic bacteria that live mainly in the colon, the organ that is home to the vast majority of the human microbiome. They are among the most common gut bacteria and perform several important jobs:
- Breaking down complex carbohydrates and plant fibers that human digestive enzymes cannot process.
- Producing short-chain fatty acids such as acetate and propionate, which nourish colon cells and support the intestinal barrier.
- Contributing to the production of vitamins, including vitamin K and some B vitamins.
- Helping to regulate immune responses.
- Competing with potentially harmful microbes for space and nutrients.
Because they are so common in the colon, Bacteroides are often the most abundant bacteria found in stool. Finding them there is completely normal.
Common Bacteroides Species
Several species dominate the human gut:
- Bacteroides thetaiotaomicron — an adaptable species that specializes in digesting plant fibers and supports gut barrier function.
- Bacteroides fragilis — a common resident of the healthy gut and the anaerobic bacterium most frequently linked to infections when it escapes into other parts of the body.
- Bacteroides uniformis and Bacteroides vulgatus — abundant species involved in fiber fermentation and short-chain fatty acid production.
Each species interacts with diet and the immune system a little differently, which is why species-level information can be more useful than a phylum-level percentage.
Are Bacteroidetes Good Bacteria?
Yes. In a well-functioning gut, Bacteroidetes are valuable contributors rather than harmful invaders. They:
- Ferment dietary fiber and resistant starch, releasing short-chain fatty acids that feed the cells lining your colon.
- Help maintain a healthy mucus barrier between gut contents and the intestinal wall.
- Influence immune development and help keep inflammatory responses in check.
- Occupy niches that would otherwise be available to less desirable microbes.
A moderate or even high presence of Bacteroidetes is therefore not a problem in itself. The more useful question is whether a high proportion reflects a loss of diversity elsewhere.
The Firmicutes-to-Bacteroidetes (F/B) Ratio
The F/B ratio is one of the most widely cited microbiome metrics. Firmicutes is the other major phylum in the human gut, and together with Bacteroidetes it typically accounts for most of the community. Early research suggested that a high F/B ratio was linked to obesity, but later studies have produced conflicting results. Most researchers now regard the F/B ratio as an oversimplified indicator rather than a reliable measure of health.
A very low F/B ratio means Bacteroidetes dominate, which can happen with certain dietary patterns. The ratio is most useful when considered alongside symptoms, diet and other diversity measures.
Why Might Bacteroidetes Levels Be High?
Several factors can shift the gut toward higher Bacteroidetes proportions:
- A diet high in animal fat and protein and low in fiber.
- Antibiotic use, which can suppress other groups while Bacteroidetes recover and expand.
- Low overall microbial diversity, which makes dominant groups look larger in relative terms.
- Differences in stool consistency and transit time.
- Chronic stress, poor sleep or excess alcohol.
Identifying which of these may apply to you is more useful than trying to push one number down in isolation.
Foods and Dietary Patterns That Influence Bacteroidetes
Bacteroides are not found in meaningful amounts in food; they colonize the gut. What you eat influences how abundant they become. Dietary patterns higher in animal fat and protein and lower in fiber have been associated with higher Bacteroidetes proportions. Meanwhile, a varied, fiber-rich diet supports a broader community, including many Bacteroides species.
If you want to support Bacteroides specifically, foods rich in fermentable plant fibers may help. These include oats, legumes, apples, onions, garlic, leeks and Jerusalem artichokes, which provide resistant starch and other fibers that gut bacteria can use. The goal is not to maximize one group; it is to feed a wide range of beneficial bacteria.
Key Gut Species: Bacterial Strains Commonly Found in the Gut
Health Risks and Conditions Associated With Elevated Bacteroides
Elevated Bacteroides levels have been observed alongside a range of conditions. Microbiome studies — including those published in peer-reviewed journals — show associations, not proof of cause and effect. A stool test cannot diagnose any of these conditions.
What Diseases Do Bacteroides Cause?
Bacteroides do not cause one specific disease. In the colon, they are usually harmless residents. Problems arise when they reach parts of the body where they do not belong, usually through a breach in the intestinal wall after surgery, injury or severe disease. Bacteroides fragilis is the anaerobe most often isolated from abdominal abscesses, wound infections and bloodstream infections. These infections can be serious and, in some cases, life-threatening, and they require prescription antibiotics that target anaerobic bacteria. A high Bacteroidetes percentage on a stool test is a completely different situation.
Certain Bacteroides strains have also been investigated in relation to colorectal cancer, but this research is not conclusive and does not mean Bacteroides cause cancer.
Inflammatory Bowel Disease (IBD)
People with ulcerative colitis and Crohn's disease often have an altered gut microbiome. Some studies have found increased proportions of certain Bacteroides species alongside reduced diversity. One hypothesis is that some Bacteroides strains interact with a compromised intestinal barrier in ways that fuel inflammation. Whether these shifts are a cause or a consequence of IBD is still unclear.
Obesity and Metabolic Health
Early microbiome research suggested that people with obesity tended to have more Firmicutes and fewer Bacteroidetes, and that weight loss increased Bacteroidetes. More recent studies have not confirmed a consistent pattern. The relationship between the F/B ratio and body weight is now considered much more complex than originally proposed. What is better supported is that overall dietary quality shapes both the microbiome and metabolic health.
Autoimmune and Inflammatory Conditions
Because Bacteroides interact closely with the immune system, researchers have looked for links between their abundance and autoimmune conditions such as rheumatoid arthritis and multiple sclerosis. Some studies report differences in affected individuals, but findings vary. No causal relationship has been established.
The Gut-Brain Axis and Mental Health
The gut and brain communicate through immune signals, metabolites and the vagus nerve. Several studies have reported differences in Bacteroides abundance in people with mood disorders, including bipolar disorder. Early research has also explored links between the gut microbiome and neurological conditions such as Alzheimer's disease. These findings are preliminary, and researchers do not yet know whether microbial differences contribute to mental health conditions or result from them. A high Bacteroidetes reading is not an indicator of any mental health condition.
Signs of an Unbalanced Gut Microbiome
High Bacteroidetes levels rarely cause specific symptoms by themselves. People concerned about gut health are more likely to notice general signs of imbalance:
- Bloating, gas or discomfort after meals.
- Diarrhea, constipation or alternating bowel habits.
- Ongoing fatigue or poor sleep.
- Food intolerances that seem to develop over time.
These symptoms have many possible causes. If they persist, they are worth discussing with a healthcare professional regardless of what a stool test shows.
How Stool Tests Measure Bacteroidetes — and What Results Really Tell You
Understanding how microbiome tests work will help you read your results sensibly.
Relative Abundance Is Not the Full Story
Consumer stool tests almost always report relative abundance — the percentage of each bacterial group within the total community. If Bacteroidetes account for 60% of the bacteria detected, that does not mean you have more Bacteroidetes in absolute terms than someone with a 40% reading. It only means Bacteroidetes occupy a larger share. When one group's percentage rises, other groups' percentages must fall. This makes relative abundance useful but easy to misinterpret.
Different Tests, Different Resolution
Two technologies dominate home microbiome testing:
- 16S rRNA sequencing identifies bacteria by analyzing a conserved gene. It is affordable and widely used, but it usually provides genus-level information and cannot reliably show what the bacteria are doing.
- Shotgun metagenomics sequences all the DNA in a sample. It provides species-level detail and can indicate microbial functions, such as whether genes related to fiber digestion or inflammation are present.
If you want to understand a high Bacteroidetes reading, shotgun metagenomics generally offers more context than 16S sequencing alone.
There Is No Universal Normal Range
A healthy gut microbiome varies widely between individuals. Age, sex, geography, diet, stool consistency and medication use all shape the community. Testing companies also use different reference databases and methods, so results are not directly comparable across brands. Treat your result as a snapshot of a single moment, not a definitive verdict on your health.
What a High Reading Does and Does Not Mean
A high Bacteroidetes percentage is not a diagnosis. Clinicians may use microbiome data as one piece of context alongside symptoms, medical history and other tests, but they do not treat a single elevated group as a disease. If your result shows high Bacteroidetes, the practical next step is to review your diet and lifestyle, and to raise any persistent digestive symptoms with a healthcare professional.
How to Support a Healthier Gut Microbiome
If your test shows high Bacteroidetes, the aim is not to eliminate them. Bacteroidetes belong in your gut. The aim is to support greater diversity so that no single group dominates the community.
Eat a Wide Variety of Plant Foods
Dietary fiber is the fuel that supports a broad range of beneficial bacteria. Aim for a wide variety of vegetables, fruits, legumes, whole grains, nuts and seeds across the week, rather than the same few sources every day. Diversity of plant foods is one of the best-supported ways to encourage microbial diversity.
Balance Animal Foods With Fiber-Rich Choices
A pattern that is heavy in animal foods and light on plant fiber has been linked to higher Bacteroidetes proportions and lower diversity in some studies. You do not need to eliminate animal foods; simply pair them with plenty of plant fiber to support a more even community.
Include Prebiotics and Fermented Foods
Prebiotic-rich foods such as garlic, onions, leeks, asparagus, oats and bananas feed beneficial bacteria. Fermented foods such as yogurt, kefir, sauerkraut and kimchi introduce live microbes and have been associated with greater diversity in some studies. If you take a probiotic supplement, choose one with strains that have been researched for your specific goal, and remember that the evidence for general-purpose probiotics is mixed.
Protect Your Microbiome From Major Disruptors
The most effective way to maintain a healthy gut is to avoid things that damage it:
- Broad-spectrum antibiotics, when they are not genuinely necessary.
- Chronic stress and poor sleep.
- Excess alcohol.
- Long stretches of highly processed, low-fiber eating.
If you do need antibiotics, support your gut afterward with a varied, fiber-rich diet.
What Not to Do
Do not try to treat a high Bacteroidetes reading with over-the-counter antimicrobials, extreme elimination diets, or antibiotics obtained without a prescription. These can reduce diversity further and make an imbalance worse. Do not change or stop any prescribed medication based on a stool test result.
When to Speak With a Healthcare Professional
Consider discussing your results with a doctor or registered dietitian if:
- You have persistent digestive symptoms such as pain, bloating, diarrhea or constipation.
- You have a diagnosed condition such as IBD, IBS or an autoimmune disease.
- You recently completed a course of antibiotics and are concerned about your gut health.
- You take medication that affects your immune system or digestive tract.
A clinician can place your result in the context of your overall health and help you decide whether any further investigation is needed.
Frequently Asked Questions
How Can You Tell if Your Gut Bacteria Are Out of Balance?
There is no reliable way to tell from symptoms alone. A stool test can estimate the relative abundance of bacterial groups, but a high percentage for one group does not mean you have too many bacteria overall. Persistent digestive symptoms are a more meaningful reason to seek advice than a single test reading.
What Antibiotics Are Effective Against Bacteroides Bacteria?
Infections caused by Bacteroides outside the colon are treated with prescription antibiotics that target anaerobic bacteria, such as metronidazole, carbapenems, or beta-lactam/beta-lactamase inhibitor combinations. These are used for confirmed infections and must be prescribed and monitored by a clinician. Antibiotics are not a tool for treating a high Bacteroidetes percentage on a stool test.
Is Bipolar Disorder Linked to Gut Bacteria?
Some studies have reported differences in Bacteroides abundance in people with mood disorders, including bipolar disorder, but the research is early and inconsistent. Researchers do not yet know whether microbial differences contribute to these conditions or result from them. A stool test cannot screen for or diagnose any mental health condition.
What Foods Can Help Increase Bacteroidetes?
Bacteroides are not found in foods, but fermentable plant fibers can support them. Foods such as oats, legumes, apples, onions, garlic, leeks and Jerusalem artichokes provide resistant starch and other fibers that gut bacteria can use. For overall gut health, focus on a wide variety of plant foods rather than targeting one group.
What Is the Firmicutes-to-Bacteroidetes Ratio?
The F/B ratio compares the two dominant bacterial phyla in the gut. Early research linked a high ratio to obesity, but later findings have been conflicting. Most researchers now consider it an oversimplified metric that is best interpreted alongside symptoms, diet and diversity.
Conclusion
A high Bacteroidetes reading on a stool test is information, not a diagnosis. It reflects the current balance of your gut community and can be a prompt to look at your diet, lifestyle and any symptoms you have been ignoring. Bacteroidetes — and the Bacteroides genus within them — are essential residents of a healthy gut. The goal is not to drive their numbers down, but to build a more diverse, resilient microbiome through varied plant foods, sensible lifestyle choices and medical guidance when you need it.
Find more details about Key Gut Species
You can find more information on the related topics below.
Read more: High Bacteroidetes in Stool — Meaning, Risks and Next Steps
Your Gut Has a Story. Read It — Then Fix Potential Problems
Full microbiome sequencing + Gut Health Index. Metabolic pathways, diversity, keystone species. Personalized plans available (diet, supplements, diary, recipes). EU lab + Maastricht University spin-off + GDPR-safe.