You open your gut report and see a name you know from a food-poisoning headline. Your stomach drops. Is that thing supposed to be in there?
Usually, yes. Here is how scientists decide when a gut bacterium is actually a problem, and what a stool test can and can’t tell you.
Short answer
A gut bacterium is rarely “bad” just because it is present. Strain, amount, location and the person carrying it decide whether it causes trouble.
Most E. coli are part of a healthy gut. Most infants carry C. difficile with no ill effects. The same species can be harmless in one place and a problem in another. A microbiome test shows who is there and in what proportion. It does not diagnose infection or disease.
This is general wellness information, not medical advice. It does not diagnose anything. If your gut symptoms worry you, talk with a qualified healthcare provider.
Flore data
What we see in our own microbiome reports
We looked at our own data first. Among 7,299 people with de-identified reports and symptom records:
- E. coli was flagged above our reference range for 15.8% of people (1,153 of 7,299). Among people who reported frequent diarrhea it was 19.5% (224 of 1,148). Among people with no digestive complaints it was 13.6% (146 of 1,070).
- Klebsiella pneumoniae was flagged for 11.2% of people reporting frequent diarrhea (129 of 1,148) and 7.1% of people with no digestive complaints (76 of 1,070).
- Age shows up too. Klebsiella pneumoniae was flagged for 16.5% of people 65 and older (87 of 526) and 5.9% of people 30 to 49 (133 of 2,242). E. coli: 24.9% (131 of 526) versus 14.7% (330 of 2,242).
Read that both ways. Most people with frequent diarrhea did not have these flags. And about 1 in 7 people with no digestive complaints had a high E. coli flag anyway. A “bad” name on a report links to symptoms on average. It doesn’t decide them for any one person. That is the whole point of this article.
How we ran these numbers: Flore has analyzed 23,000+ microbiome reports. This section uses the 7,299 people whose de-identified records include both report flags and the symptoms they told us about. A “flag” means a microbe was above our reference range in that person’s stool report. It is not a diagnosis. Symptoms are self-reported. “No digestive complaints” means people who reported other concerns (such as mood, skin or energy) but none of our digestive tags. These are people who came to Flore, not a random sample of the public. Groups under 20 people are never reported. Every figure is an association, not proof of cause.
“Bad bacteria” is usually a context problem
Your gut holds tens of trillions of bacteria. A widely cited estimate puts the total for an adult at about 38 trillion, most of them in the colon (Sender, Fuchs and Milo, 2016). Healthy people differ a lot in which microbes they carry. The Human Microbiome Project found that even healthy individuals “differ remarkably in the microbes” that live in the gut (HMP Consortium, 2012).
So there is no single list of good and bad names. Scientists ask four questions instead.
1. Which strain is it?
A species name covers a lot of ground. The CDC puts it plainly: “Most E. coli are harmless and are part of a healthy intestinal tract.” They help digest food, make vitamins and guard against harmful germs. But “some E. coli can make people sick with diarrhea, urinary tract infections, pneumonia, sepsis, and other illnesses” (CDC).
Bacteroides fragilis is another example. It is a common gut resident. Only certain strains, called enterotoxigenic B. fragilis, make a toxin, and those toxin-making strains are linked to diarrheal disease (Sears, 2009). Same species. Different strain. Different story.
2. How much of it is there?
Presence and overgrowth are different things. A microbe that sits at a low level in a balanced community can become a problem when something clears out its competition.
C. difficile shows this well. About 60% to 70% of healthy newborns and infants carry it, and they “show no ill effects” (Jangi and Lamont, 2010). In adults, the CDC notes that most C. diff infections “occur while or soon after taking antibiotics,” which can knock back the bacteria that normally keep it in check (CDC). Researchers now study colonization and infection as separate states (Crobach et al., 2018).
3. Where is it?
Location matters as much as identity. E. coli belongs in the colon. Some types of the same species cause urinary tract infections (CDC). Inside the gut, the colon holds dense communities, while the upper small intestine normally holds far fewer bacteria. When colon-type bacteria build up in the small intestine, that is called small intestinal bacterial overgrowth, or SIBO (ACG, 2020). We cover that in SIBO: when normal gut bacteria end up in the wrong place.
4. Who is carrying it?
The host counts. Researchers use the word pathobiont for “potentially pathogenic symbionts”: microbes found in most healthy people that may contribute to inflammation when genetics, immune function or environment shift (Chow, Tang and Mazmanian, 2011). Age, recent antibiotics, diet and immune health all change how a gut community behaves.
Put the four questions together and you get the honest answer to “is this one bad?” It depends. For the flip side, read Can bad bacteria be good?.
What a stool test can tell you, and what it can’t
It can show which microbes are in your sample and how they compare to each other. That is useful for seeing the shape of your gut community: what is abundant, what is scarce and how it shifts over time.
It can’t diagnose an infection or a disease. A few limits to keep in mind:
- The numbers are relative. Sequencing reports each microbe as a share of the total. If one group grows, others can look smaller even when they did not change (Gloor et al., 2017).
- Stool mostly reflects the colon. That is where most gut bacteria live (Sender, Fuchs and Milo, 2016). It is not a SIBO test.
- A name is not a strain. Toxin-making and harmless strains can share a species name, and a general microbiome test is not built to sort them for a diagnosis.
For more on reading a report, see how to read your microbiome test results and how an at-home microbiome test works. If you want the list version, we also wrote up 10 common microbes with a bad reputation.
Where Flore fits
Flore doesn’t sort your gut into heroes and villains. Flore reads the whole community from an at-home gut microbiome test, then builds a personalized probiotic-and-prebiotic formula from that picture. You retest, and the next formula is built from the new read. The decision is grounded in your data, not a scary name on a report. It is wellness insight, not a diagnosis. If a result worries you, bring it to your healthcare provider.
Flore (formerly Sun Genomics) builds personalized probiotics from your own gut microbiome. Your sample is read by a CLIA-certified partner lab. Flore builds and makes your formula from the results. That work spans 10 years, 18,700+ people served, 23,000+ microbiome reports analyzed and 40,000+ personalized formulations.
Educational, structure/function information. Not medical advice, and not intended to diagnose, treat, cure or prevent any disease.
Frequently asked questions
How do you know if a gut bacterium is bad?
Ask four questions. Which strain is it? How much of it is there? Where in the body is it? And what is going on with the person carrying it? Most E. coli are harmless and part of a healthy gut, while some types cause illness (CDC). Researchers call resident microbes that can cause trouble only under certain conditions “pathobionts.” A name on a report is a starting point, not a verdict.
What is a pathobiont?
A pathobiont is a normal resident of the gut microbiota that has the potential to cause problems when conditions change, for example in a host with a weakened or misdirected immune response. The term was coined to separate these resident microbes from infectious agents you pick up from outside (Chow, Tang and Mazmanian, 2011).
Can a stool test tell me I have an infection?
Not a wellness microbiome test. It shows which microbes are in your sample and in what proportion. It does not diagnose infection or disease. Sequencing results are relative: each microbe is a share of the total, so one going up can make others look like they went down (Gloor et al., 2017). If you think you have an infection, see a clinician.
Is it bad if my test shows C. diff or E. coli?
Not on its own. About 60% to 70% of healthy newborns and infants carry C. difficile with no ill effects (Jangi and Lamont, 2010), and most E. coli are harmless (CDC). Whether a microbe matters depends on the strain, the amount, the location and the person. Bring any result that worries you to your healthcare provider.
Does a stool test show what is in my small intestine?
Mostly no. The colon holds the large majority of the body’s bacteria (Sender, Fuchs and Milo, 2016), so stool mainly reflects that community. Overgrowth in the small intestine, called SIBO, is assessed with breath tests or a small-bowel sample, not a stool test.
How common are “bad bacteria” flags in people without gut symptoms?
Common enough that one flag means little on its own. In Flore’s de-identified data on 7,299 people, E. coli was flagged above reference for 13.6% of people with no digestive complaints (146 of 1,070) and 19.5% of people reporting frequent diarrhea (224 of 1,148). That is an association, not a cause, and most people with frequent diarrhea did not have the flag.
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Sources
- Flore internal analysis of de-identified, aggregated report flags and self-reported symptoms, 7,299 people, November 2026. Groups under 20 not reported. Associations only.
- CDC: About E. coli (Escherichia coli)
- Sears CL. Enterotoxigenic Bacteroides fragilis: a rogue among symbiotes. Clin Microbiol Rev. 2009;22(2):349-369. PMID 19366918
- Jangi S, Lamont JT. Asymptomatic colonization by Clostridium difficile in infants: implications for disease in later life. J Pediatr Gastroenterol Nutr. 2010;51(1):2-7. PMID 20512057
- CDC: About C. diff (May 2026)
- Crobach MJT, et al. Understanding Clostridium difficile colonization. Clin Microbiol Rev. 2018;31(2):e00021-17. PMID 29540433
- Chow J, Tang H, Mazmanian SK. Pathobionts of the gastrointestinal microbiota and inflammatory disease. Curr Opin Immunol. 2011;23(4):473-480. PMID 21856139
- Human Microbiome Project Consortium. Structure, function and diversity of the healthy human microbiome. Nature. 2012;486(7402):207-214. PMID 22699609
- Sender R, Fuchs S, Milo R. Revised estimates for the number of human and bacteria cells in the body. PLoS Biol. 2016;14(8):e1002533. PMID 27541692
- Gloor GB, et al. Microbiome datasets are compositional: and this is not optional. Front Microbiol. 2017;8:2224. PMID 29187837
- Pimentel M, Saad RJ, Long MD, Rao SSC. ACG Clinical Guideline: Small Intestinal Bacterial Overgrowth. Am J Gastroenterol. 2020;115(2):165-178. PMID 32023228
Flore was formerly Sun Genomics. This article is general wellness information and is not medical advice, diagnosis or treatment. Statements have not been evaluated by the FDA. Talk to a qualified healthcare provider about your health.