Alistipes

Most gut bacteria get sorted into good or bad. This genus refuses the filing system. The same bacterial group runs higher in people with major depression and some colorectal cancer studies, and lower in studies of fatty liver, metabolic syndrome, and chronic constipation.

That flip is the point. The number only says something when you read it against the rest of your picture, and when you watch it move over time.

The Organism Itself

This is a genus of bacteria, one of the ordinary residents of a healthy adult colon. It cannot survive in oxygen, and it belongs to Bacteroidetes, a large bacterial group found in most people's guts.

The test doesn't grow the bacteria in a dish. It copies a stretch of bacterial DNA over and over until there's enough to read, a lab technique called PCR, and then reports how much of your sample's bacterial DNA belongs to this genus.

Two things follow from that. The result is a share, not a headcount, so if another group blooms, this one's percentage can fall even when the actual number of cells hasn't budged. And it's reported at the genus level, which averages together species that can behave very differently.

Depression and the Gut-Brain Link

The most repeated human finding is in depression. In a study of 213 adults that combined stool sequencing, fecal metabolite measurement, and brain imaging, this genus was more abundant in people with major depressive disorder than in people without it. An earlier study of 76 adults pointed the same way.

There's a plausible mechanism. These bacteria break down an amino acid called tryptophan into compounds known as indoles. Your body also uses tryptophan to build serotonin, one of the brain's mood chemicals. Researchers are testing whether bacteria using up tryptophan helps explain the mood signal.

None of this proves the bacteria cause low mood. These studies looked at people at a single moment, so depression changing the gut fits the data just as well as the gut changing mood.

Colorectal Cancer

Stool studies have linked this genus with more advanced colorectal cancer. In a study of 192 people, Alistipes and several other bacterial groups were enriched in stage III-IV disease, and the authors used the broader bacterial pattern to distinguish earlier from later stages.

The strongest outcome data comes from a surgical cohort. Among 333 people who gave a stool sample before colorectal cancer surgery, a multi-marker bacterial score that included Alistipes-assigned markers picked out a group with about twice the risk of the cancer progressing afterward. That score predicted progression better than CEA, the blood protein often tracked after colorectal surgery.

Two things temper that. The signal came from a species-level panel of several markers, not from a genus-level number on its own. And some cancer papers measured bacteria in tumor or nearby tissue rather than stool, which is a different sample from a different place.

The Same Bacteria, the Opposite Signal

Now the reversal. A meta-analysis pooling gut microbiome data from thousands of people found this genus enriched in healthy people compared with people who had active inflammatory bowel disease or a C. difficile infection. In a 72-person study that included 53 people with ulcerative colitis and 19 controls, people who couldn't tolerate the standard ulcerative colitis drug mesalamine had lower levels of mucoprotective bacteria including this genus in the lining of the lower small intestine.

Sources: Liu et al. (2025); Yang et al. (2023); de Meij et al. (2016).

So which is it? Neither. This isn't a good-number, bad-number marker. It's a context marker, and three things decide what a given level means: which species make up the total, whether the sample came from stool or from the gut lining, and what the rest of your labs and symptoms say. A genus-level stool result averages over species that push in different directions, which is exactly why the same number can rise in one disease and fall in another.

Fatty Liver and Metabolic Health

Here the direction is consistently down. In 106 adults, people with fatty liver disease carried less of this genus. The lower it ran, the worse their blood sugar and liver enzymes looked. ALT and GGT rise in blood when liver or bile-duct cells are under strain.

The pattern repeats across metabolic conditions. In 1,342 adults, one species was linked with healthier lifestyle scores and lower metabolic syndrome risk. In 99 children, levels were lower with obesity, alongside a shifted profile of short-chain fatty acids. These are fuel molecules your gut bacteria make when they ferment fiber, and your colon cells run on them.

A study of 151 overweight adults at high cardiometabolic risk found something sharper: the relationship between this genus and blood lipid particles depended on whether the person had metabolic syndrome or lupus. Same bacterial group, same blood test, different meaning depending on the underlying condition.

Kidney Function

A 2025 Mendelian randomization analysis used Chinese microbiome and kidney-disease genetic datasets, then checked East Asian validation datasets, to ask whether genetic tendency toward higher levels of this genus tracked with chronic kidney disease risk. It did. That design is harder to fool than a plain correlation, because inherited variants are fixed long before kidney disease develops, but this is still a hypothesis-generating genetics result rather than a clinical kidney test.

Cancer Immunotherapy Response

One more reversal is worth knowing. In a study with 75 lung cancer patients and 31 controls, 50 of the patients received immune checkpoint therapy. Higher levels of one species predicted lasting clinical benefit. A genus that looks unhelpful in colorectal cancer can look helpful when the goal is waking the immune system up. Nobody should be adjusting cancer treatment on this basis yet. Direction alone means very little.

Why One Reading Is Not Enough

Gut bacteria are not a stable trait. When researchers sampled 20 people daily, day-to-day shifts in many bacterial genera were as large as the differences between one person and another. In a year-long study of 75 adults, a meaningful share of the total variation came from the same people changing over time.

Stool is the steadier end of the spectrum. Across four body sites tracked over years, the gut and mouth held together better than skin or nose. Steadier is not steady, though. A single stool result is one frame of a moving picture.

Treat the first result as a baseline. The direction across repeat readings tells you more than any one of them.

This is a research-stage measurement. There are no agreed clinical cutoffs, and a number on its own shouldn't drive a medical decision. That's an argument for starting your own record now rather than skipping it. If better cutoffs arrive, you'll have your own history to read them against.

When Results Can Be Misleading

• Day-to-day swing: this genus can move meaningfully within the same person over days. One unusual reading, on its own, is more likely to be noise than news.
• Which lab ran it: PCR starts with short DNA starting points called primers. Different primer sets can under-read or miss some gut bacteria, including this genus. Numbers from different labs or methods are not interchangeable.
• Stool versus gut lining: stool reflects what's passing through your colon. Some cancer and colitis evidence was measured in tissue brushed or sampled from the gut wall, and the two don't always agree.
• Recent antibiotics or an acute gut infection: antibiotics knock down normal gut residents and let opportunists expand. Wait several weeks after finishing a course, and don't sample during a bout of diarrhea.

Stool also carries substances that can block the DNA copying reaction itself, which is why labs dilute samples before running them. That's a lab-side problem, but it's one more reason two runs of the same sample can land in slightly different places.

What to Do With an Out-of-Pattern Result

Don't treat the genus. Read it together with everything else on the report.

If this runs high and you have persistent low mood or unexplained bowel symptoms, the useful next step is the rest of the panel: fecal calprotectin for inflammation in the gut wall, the fiber-fermenting bacteria that make short-chain fatty acids, and blood inflammation markers. An inflammatory pattern plus ongoing symptoms is a reason to see a gastroenterologist, not a reason to buy a supplement.

If this runs low alongside abnormal liver enzymes, high triglycerides, or a rising waist, you're looking at the metabolic side of the story. That combination is worth checking with a full liver panel and, when the pattern persists, liver imaging with someone who manages metabolic liver disease.

One line shouldn't blur: whatever this number does, it doesn't screen for colon cancer. Stool blood testing and colonoscopy do that. This result neither substitutes for them nor buys you time before them.