Campylobacter Upsaliensis

If you have diarrhea that will not settle and your stool culture came back negative, that negative result may mean less than you think. This organism is one of the gut bacteria standard culture is built to miss, because the antibiotics added to the culture plate to suppress other stool bacteria can also kill it.

This test looks for the bacterium's DNA directly in stool, so it does not depend on the organism surviving the trip to the lab or growing on a selective plate. That is the whole reason it exists.

Why Culture Misses It

C. upsaliensis (Campylobacter upsaliensis) is a heat-tolerant Campylobacter species. It is closely related to C. jejuni, the better-known species behind many undercooked-chicken food poisoning cases.

Standard Campylobacter culture uses plates loaded with cephalosporin antibiotics to hold back the rest of the stool flora. This species is sensitive to those same antibiotics, so it often never grows.

The scale of the gap is documented. In a multi-center study of 1,552 stool specimens sent for routine testing, culture missed 13 of 47 true-positive Campylobacter specimens. All four C. upsaliensis positives were missed by culture, and those four made up 8.5% of confirmed Campylobacter positives in that cohort. In a separate survey of 3,738 diarrheal specimens, PCR found 11 cases while routine labs cultured only two.

So a negative stool culture does not rule this organism out. That is the single most useful thing to know about this test.

What a Positive Result Means

A positive result means this bacterium's DNA is present in your stool. Whether it is the cause of your symptoms is a separate question, and the answer depends on your clinical picture.

The case for it being a real pathogen is strong in specific settings. It has been confirmed as the cause of persistent bloody diarrhea using filtration culture, PCR, and sequencing together. In HIV-infected people it has been associated with prolonged mild to moderate diarrhea. In children in low-resource settings it turns up in acute diarrhea, often alongside other Campylobacter species rather than alone.

The case against reading every positive as a diagnosis is also strong. In a Canadian population survey comparing people with diarrhea and healthy controls, this species and other non-jejuni Campylobacters showed no statistically significant association with acute disease. In pediatric cohorts in low-resource settings, non-jejuni, non-coli Campylobacter species were also common in healthy control infants.

Reconciling Those Two Pictures

Both findings are true. This is a context-dependent organism, not a good-result or bad-result marker. Detection alone tells you the bacterium is present. Detection plus persistent diarrhea, blood in the stool, or a compromised immune system is what makes it clinically meaningful.

The practical rule: a positive result in someone with no gut symptoms is a finding, not a diagnosis. A positive result in someone with weeks of diarrhea and a negative culture is a more plausible lead. Treat the test as one input into a clinical picture, never as a verdict on its own.

Persistent and Bloody Diarrhea

The clearest documented role for this organism in otherwise healthy people is diarrhea that does not resolve on the usual timeline. Case work has traced persistent bloody diarrhea directly to it, confirmed by three independent methods, in situations where routine workup had come up empty.

In immunocompetent people the illness is usually self-limiting: fever, cramping, and watery or inflammatory diarrhea that clears on its own. The value of identifying it is often less about needing treatment and more about closing an open question and stopping an unfocused hunt for other causes.

Immune Deficiency and Invasive Disease

In people with impaired antibody production, the picture changes sharply. X-linked agammaglobulinemia is an inherited condition in which the body cannot make normal antibodies. Case reports in people with this condition have linked the organism to persistent bacteremia, cellulitis, focal ileocolitis, painful skin nodules, and joint inflammation after infection.

Those reports were not just stool PCR findings. They involved stool culture, blood findings, plasma microbial DNA, and clinical relapse together. That distinction matters: a stool positive in someone with antibody deficiency carries more weight when fever, rash, arthritis, blood infection, or persistent diarrhea are also present.

If you have a known antibody deficiency, a hematologic condition, or advanced HIV, a positive result here should be treated as a real clinical event rather than an incidental finding, and it warrants specialist involvement.

Co-Infection Can Change the Read

A positive result carries more weight if it is the only pathogen found. If it appears alongside several other organisms, the question shifts from naming the organism to sorting out what the full exposure picture says about food, water, travel, or animal contact.

This is especially true in high-exposure pediatric settings. Shotgun sequencing of fecal samples from Peruvian children under two found that over 65% had multiple Campylobacter species in the same stool sample. That finding comes from children in an endemic setting, so it should not be pasted onto every adult stool result. It does explain why a broad panel result sometimes needs sorting, not just reading one positive line.

Where It Comes From

Dogs and cats are the main reservoir, especially puppies and kittens. For Campylobacter as a genus, poultry is the classic source. For this species, the stronger signal is pets.

This is best understood as an exposure organism, not a resident marker of a healthy gut. Recent contact with a new or young pet is worth reporting alongside the result.

When Results Can Be Misleading

The single biggest interpretive trap is the one already covered: a negative culture does not overturn a positive PCR. The reverse is also true. PCR detects DNA whether or not the bacterium is alive, so a positive can persist after an infection has already resolved.

• Genus-level cross-reactivity: broad assays that target shared 16S ribosomal DNA can flag a related Campylobacter sequence without proving this exact species. A species-specific assay or sequencing settles it.
• Panel coverage gaps: many commercial gastrointestinal panels only look for C. jejuni and C. coli. A negative from one of those panels says nothing about this species.
• PCR inhibition and low load: stool can contain substances that block the PCR reaction. A low bacterial load can also push the signal below the detection limit, producing a false negative.
• Delayed processing: DNA can degrade if the sample sits too long in the wrong transport medium before it reaches the lab.

Rapid antigen tests are a separate problem. Some can react with non-jejuni Campylobacter species, including this one, so a culture-negative antigen result is not automatically false. But some rapid antigen positives are false positives: one quality improvement review of 372 specimens found 84% of positive rapid Campylobacter antigen results were not confirmed by molecular testing, with C. difficile and norovirus among the usual culprits.

How Much Bacteria There Is

Some qPCR reports include a cycle threshold. This is the number of amplification rounds needed before DNA is detected. Fewer rounds usually means more bacterial DNA in the sample.

Studies of Campylobacter as a group, mostly not this exact species, find that lower cycle thresholds are more likely to match culture-positive samples. That does not create a universal cutoff for C. upsaliensis. A high cycle threshold can mean low-level carriage, leftover DNA after an infection, or a sample near the assay's detection limit.

What to Do With an Unexpected Result

Start by pairing the result with your actual symptoms. If you are asymptomatic and this turned up on a broad panel, the most likely explanation is transient carriage from a pet or food exposure, and no action is needed beyond hand hygiene around animals.

If you have persistent diarrhea, fecal calprotectin can show whether the gut is inflamed. C. difficile testing can rule out a treatable colitis that can look identical. A broader multiplex stool panel is worth running if it was not part of the original order, since co-infection is common enough in some settings that finding one organism does not always finish the question.

If you want antibiotic susceptibility testing, ask specifically for non-selective filtration culture or antibiotic-free low-oxygen culture conditions. Ordinary Campylobacter culture often will not recover this species, and a lab that is not told to use a specialized method may report a negative that does not answer this question.

Three situations call for specialist input rather than watchful waiting: blood in the stool with a positive result, any known immune deficiency, and symptoms that persist after treatment. In those cases an infectious disease physician or gastroenterologist should shape the next step, because the organism's behavior in compromised hosts is different and the antibiotic choice is not straightforward.