Firmicutes vs Bacteroidetes: What Your F/B Ratio Means

Last updated: 18 October 2026

Firmicutes and Bacteroidetes are the two largest groups of bacteria in most people's guts. Both groups break down food your body cannot digest on its own, and research has linked the balance between them to how much energy you get from food, how you store fat and how your body regulates inflammation. That is why gut microbiome tests, including AMILI's, report the ratio as a marker of metabolism and energy use.

In short:

• Your F/B ratio is a marker of metabolism and energy use. It compares two groups of gut bacteria that research has linked to how much energy you get from food and how you store fat.
• A high ratio means relatively more Firmicutes. Studies, mainly in mice, have associated this pattern with extracting more energy from the same food and storing more fat.
• A low ratio means relatively more Bacteroidetes. Research suggests this may reflect less energy extracted from food, and a lower ratio has been reported in people with inflammatory bowel disease.
• Your ratio shifts with what you eat and with age, so it is a gut result that can change over time.
• The AMILI Gut Microbiome Test places your ratio in a low, optimum or high range, so you can see where your balance sits and track it between tests.

What are Firmicutes and Bacteroidetes?

Firmicutes and Bacteroidetes are phyla, one of the broadest levels scientists use to group bacteria by shared ancestry. Think of each phylum as a large extended family: the members are related, but each one has its own job.

In 2021, the two groups were formally renamed Bacillota (Firmicutes) and Bacteroidota (Bacteroidetes). Many studies and test reports still use the older names, and so does this article. Each group contains bacteria you may have heard of:

• Firmicutes include Lactobacillus, the genus found in yoghurt and many fermented foods, along with gut residents such as Clostridium, Ruminococcus, Faecalibacterium and Roseburia.
• Bacteroidetes in the gut are dominated by two genera, Bacteroides and Prevotella.

Both groups break down parts of your food that your own body cannot digest, mainly dietary fibre, and turn them into different short-chain fatty acids. Many of the gut's main butyrate producers are Firmicutes, including Faecalibacterium prausnitzii, one of the most abundant species in a healthy gut, and Roseburia. Butyrate is the main fuel for the cells lining your colon. Bacteroidetes are the main source of the propionate your gut bacteria make from carbohydrates.

What is the Firmicutes to Bacteroidetes ratio?

The Firmicutes to Bacteroidetes ratio is the amount of Firmicutes in a stool sample compared to the amount of Bacteroidetes.

Researchers link the ratio to energy use because of a series of studies from Washington University. In 2005, genetically obese mice were found to have about 50% fewer Bacteroidetes than their lean littermates, and proportionally more Firmicutes. In 2006, germ-free mice (mice raised with no gut bacteria) that received gut bacteria from obese mice gained significantly more body fat than mice that received bacteria from lean mice, and the obese-type bacteria were better at harvesting energy from the diet. In a human study the same year, 12 people with obesity followed a low-calorie diet for a year, and their share of Bacteroidetes rose as they lost weight.

In people, later studies have been less consistent. A 2014 meta-analysis that re-ran five human datasets with the same methods concluded that the ratio did not consistently separate lean from obese gut microbiomes. A 2016 re-analysis of 10 studies found no significant link between obesity and the ratio, or between obesity and either group on its own. A 2020 review concluded that it is currently difficult to link the ratio to a particular health status.
So your ratio is best read as one signal of how your gut bacteria handle energy. It cannot diagnose a condition by itself, and it means most when interpreted alongside the rest of your report and other tests.

Why your F/B ratio can change

Three factors move the ratio, which is why a result is best read against a reference range measured with the same method:

1. Diet. In a study of 10 volunteers, switching to an animal-based diet for five days raised bile-tolerant bacteria, including Bacteroides, and lowered Firmicutes that break down plant fibre, such as Roseburia, Eubacterium rectale and Ruminococcus bromii. The shift showed up about a day after the new food reached the gut.
2. Age. In a study of 62 people, the ratio was about 0.4 in infants, 10.9 in adults aged 25 to 45, and 0.6 in adults aged 70 to 90.
3. Lab method. How a sample is processed changes the result. In an international comparison of laboratory protocols, the way DNA was extracted from stool had the largest effect of any technical step, partly because some bacteria have tougher cell walls that are harder to break open. The 2020 review names differences in sample processing and sequencing as a main reason studies disagree.

This is also why two labs can report different ratios for the same person, and why comparing your number with a range found online is unreliable. The most useful benchmark is your own. Test once to learn your starting ratio, then retest with the same test after a change in diet, or every so often as you age, and compare each result with your last.

What does it mean if Firmicutes is high in your stool?

Research has associated this pattern with extracting more energy from food and with changes in fat storage, metabolic function and inflammation, though in people the link has varied between studies. Firmicutes also include many of the bacteria that turn fibre into butyrate, so the species behind a high count matter. Three things on your report give the number more meaning:

• Which Firmicutes are high. A high count driven by fibre fermenters such as Faecalibacterium and Roseburia reads differently from one driven by a few species crowding out the rest.
• Diversity. A wider range of species is generally associated with a healthier, more stable gut.
• Your diet. If you eat plenty of plant fibre, the Firmicutes that feed on it are likely to be higher.

What does it mean if Bacteroidetes is high in your stool?

Research suggests this may reflect less energy extracted from food, and a 2020 review describes a lower ratio as a pattern seen in people with Crohn's disease and ulcerative colitis, two forms of inflammatory bowel disease (IBD). These findings come from people who were already diagnosed, and a stool test ratio cannot show whether you have IBD.

A high Bacteroidetes reading also makes more sense once you know which of its two main genera is behind it, Bacteroides or Prevotella, because each tends to follow a different long-term diet. In a US study of 98 people, guts dominated by Bacteroides were linked to diets higher in animal protein and fat, and guts dominated by Prevotella to diets higher in carbohydrates. These dominant patterns are called enterotypes. In a 10-day controlled feeding part of the same study, gut bacteria shifted within 24 hours of a diet change, but nobody's enterotype changed.

Why your F/B ratio depends on who you're compared with

A ratio only means something against a reference point because typical gut make-up differs between populations because diets and lifestyles differ. In a study of 303 school-age children across 10 cities in five Asian countries, most children in Khon Kaen in Thailand and in Yogyakarta and Bali in Indonesia had Prevotella-dominated guts, linked to diets with more rice and foods such as tempeh. Most children in China, Japan and Taiwan had guts dominated by Bifidobacterium and Bacteroides. Country explained more of the differences than city did, and children in Bangkok leaned towards the second pattern, which the authors linked to more urban diets.

So the same ratio can be typical in one population and unusual in another. AMILI was founded partly because the major reference databases included few Asian gut microbiomes. The AMILI Gut Microbiome Test places your F/B ratio in a low, optimum or high range, so you can see where your balance sits and track it between tests. Here is how the AMILI Gut Microbiome Test works.

Can you change your Firmicutes to Bacteroidetes ratio?

Your diet can shift the balance within days, but there is no target ratio to aim for. In the five-day diet study above, the Firmicutes that ferment plant fibre fell quickly on an animal-based diet. In the enterotype study, each person's dominant pattern held steady through 10 days of controlled eating, which fits the finding that enterotypes follow long-term eating habits.

Probiotics have been studied for their effect on the ratio, mostly in mice and rats. A 2020 review found the effects differed by strain: some strains lowered the ratio in obese mice, others reduced weight gain without changing it, and in a human trial of Lactobacillus gasseri SBT2055 in adults with a higher BMI, the ratio did not change. The authors concluded that probiotics cannot be used as a universal therapy for obesity and that more strain-level research is needed. If you take a probiotic, judge it by the evidence for its specific strains.

What to do with your F/B ratio result

Use the ratio as a signal of how your gut bacteria handle energy, and read it alongside the diversity and other results on your report. Whatever your range, begin with one small step that feeds a wider variety of gut bacteria: add one plant food you don't usually eat to your meals this week.

This article is for general information and isn't a substitute for personalised medical advice. If you have ongoing gut symptoms, speak with a doctor.

Frequently Asked Questions

1. Are Firmicutes good or bad?
   Neither, as a group. Firmicutes include butyrate producers such as Faecalibacterium prausnitzii and the lactobacilli used in fermented foods, as well as some species that can cause infection. What matters is which species are present and in what balance.
2. What is a normal Firmicutes to Bacteroidetes ratio?
   Ranges differ between labs, because the ratio varies with age, diet, population and test method. Your result is best read against a reference range measured with the same method, such as the low, optimum and high ranges in the AMILI Gut Microbiome Test.
3. Is Lactobacillus a Firmicute?
   Yes. Lactobacillus belongs to the Firmicutes, and in 2020 the genus was split into 25 genera, so many species now have new names such as Lactiplantibacillus plantarum. Bifidobacterium, the other common probiotic genus, belongs to a different phylum, Actinobacteria (now Actinomycetota).
4. Why were Firmicutes and Bacteroidetes renamed?
   In 2021, phylum names were brought under the formal rules for naming bacteria, which base each name on a reference genus and give it the ending "-ota". Firmicutes became Bacillota, after Bacillus, and Bacteroidetes became Bacteroidota, after Bacteroides.
5. Does a high F/B ratio cause weight gain?
   Current evidence does not show that. Early mouse and small human studies linked the two, but later re-analyses of larger human datasets did not find a consistent association, and researchers have not shown which comes first.
6. What other bacteria live in the gut besides Firmicutes and Bacteroidetes?
   Smaller groups include Actinobacteria, mainly Bifidobacterium; Proteobacteria, which include Escherichia coli; and Verrucomicrobia, which include Akkermansia muciniphila. Their shares are small, but some of their species are among the most studied gut bacteria.
7. Can a gut microbiome test measure Firmicutes and Bacteroidetes accurately?
   Tests measure their relative amounts, but the numbers depend on the lab's DNA extraction and sequencing methods. That makes repeat results from the same test more comparable than results from different labs.

Glossary

• Butyrate: A short-chain fatty acid made by certain colon bacteria, many of them Firmicutes. It is the main fuel for the cells lining the colon.
• Enterotype: A pattern of gut bacteria dominated by one genus, such as Bacteroides or Prevotella. Enterotypes are linked to long-term diet.
• F/B ratio: The amount of Firmicutes in a stool sample divided by the amount of Bacteroidetes.
• Germ-free mice: Mice raised with no microbes, used to test what gut bacteria do when they are added.
• Genus: The level of classification above species, shown as the first word of a bacterium's name, as in Bacteroides fragilis.
• Gut microbiome: The community of microbes living in your digestive tract, along with their genes.
• Inflammatory bowel disease (IBD): Long-term inflammation of the digestive tract, mainly Crohn's disease and ulcerative colitis.
• Phylum: One of the broadest levels used to classify bacteria. Firmicutes and Bacteroidetes are phyla, each containing many genera.
• Propionate: A short-chain fatty acid made mainly by Bacteroidetes from carbohydrates in the gut.
• Short-chain fatty acids: Compounds that gut bacteria make when they ferment dietary fibre, mainly acetate, propionate and butyrate.
• Strain: A genetically distinct type within a species, usually shown as a code after the species name. Findings for one strain do not automatically apply to another.

About the Author

Yuzu Tay is a science communicator trained in biomedical engineering and leads marketing and communications at AMILI. Her focus is on translating and sharing real science to help people make better decisions about their health, staying close to what the evidence does and does not show. Her work spans cancer companion diagnostics, liver fibrosis and gut health.

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