Aeromonas Species

If you've had persistent diarrhea and an initial stool panel found no cause, Aeromonas is one possibility that may not have been checked. Some widely used panels don't report it. Its frequency varies by location, age, and testing method, so findings from one region don't establish how often it causes diarrhea everywhere.

A positive needs context. Aeromonas can be found in people who feel well, and many diarrheal illnesses associated with it settle without targeted treatment. Detection alone doesn't establish that these bacteria caused your symptoms.

What This Test Detects

Aeromonas is a group of bacteria found in fresh and brackish water, fish, shellfish, and other foods. Contaminated food or water can introduce them into your gut. Laboratory studies show that some strains produce toxins that damage cells, although their ability to cause illness in people varies. Species recovered from human intestinal infections include A. caviae, A. veronii, A. hydrophila, and A. dhakensis.

This test looks for Aeromonas DNA in a stool sample using PCR. PCR copies selected stretches of genetic material until they can be detected. A result reported as Aeromonas species does not identify the particular species, establish toxin production, or show which antibiotics would work.

The reporting unit is CFU/g stool. CFU stands for colony-forming units. In culture, this unit describes bacteria capable of growing into colonies. PCR detects DNA without growing bacteria, so a result labeled CFU/g depends on the laboratory's calibration and must not be treated as a direct count of living organisms.

Stool PCR can contribute to an investigation of diarrhea. The clinical use of a quantitative Aeromonas count is less established: there is no validated universal count that separates harmless carriage from disease, grades severity, or defines a health target.

What the Diarrhea Studies Show

In about 90,000 stool samples submitted by people with gastrointestinal symptoms in Sydney, a commercial PCR assay detected Aeromonas about 5.5 times as often as culture. Some culture-positive samples were PCR-negative too. This comparison involved a specific commercial assay and does not establish the sensitivity or accuracy of every Aeromonas PCR test.

In an earlier Australian dataset of more than 341,000 samples, Aeromonas was the second most frequently detected of the five bacterial groups studied. It was the most frequently detected group in infants aged 6 to 18 months. These were laboratory detections in people with symptoms; the study did not establish that Aeromonas caused every illness.

Case-control studies provide a different kind of evidence. In Pakistani children, those with moderate-to-severe diarrhea had about twice the odds of carrying A. caviae in infancy and about three times the odds at ages two to four compared with healthy controls. That study identified species from cultured stool isolates using DNA sequencing. It supports an association with illness, but does not translate into a risk estimate for a generic stool PCR result.

Carrying It Without Being Sick

Aeromonas has been recovered from healthy people as well as people with diarrhea. Estimates of carriage vary with the population and testing method. Older culture-based estimates should not be used as the expected positivity rate for a sensitive PCR assay.

In one human challenge experiment, only 2 of 57 healthy volunteers developed diarrhea after swallowing strains then identified as A. hydrophila. That finding applies to the strains and volunteers studied. It does not rule out disease from other strains or in more vulnerable people.

Spontaneous recovery is also common in clinical reports. A retrospective review presented as a conference abstract found that at least half of untreated people with a positive stool culture became asymptomatic without an active antibiotic. The small study and incomplete clinical records limit how precisely it can predict your course.

Other infections can complicate interpretation. In a small Beijing study, roughly two thirds of the 23 people whose stools yielded Aeromonas by culture had another intestinal pathogen detected. A positive result therefore needs to be considered with the other findings in your sample.

Bacterial Load and Gut Inflammation

The Sydney study assessed bacterial load through the PCR copying-cycle signal. Within that assay, fewer cycles generally indicated more starting DNA. Higher estimated loads were associated with a greater chance of recovering Aeromonas by culture and with higher fecal calprotectin. Calprotectin is a protein released by immune cells and used as a marker of intestinal inflammation.

Those were group-level associations from a different reporting method. They don't establish a CFU/g threshold for this test. Calprotectin can rise with intestinal infection or inflammatory bowel disease, so a high value cannot identify Aeromonas as the cause.

Ulcerative Colitis and Crohn's Disease

Ulcerative colitis is a long-term inflammatory disease of the colon. A 2025 study found Aeromonas more often in stool from people with ulcerative colitis than in healthy controls and investigated a variant that produces the toxin aerolysin. The research involved specialized testing of the organism and its toxin; this general stool PCR result does not establish that you carry that variant.

Cell experiments showed that aerolysin could kill macrophages. These immune cells help protect the intestinal lining. The variant worsened colitis in susceptible mouse models. The human findings remain an association and do not establish that Aeromonas causes ulcerative colitis or that eliminating a positive stool result will treat it.

A separate study of cultured stool isolates found A. veronii more often among people with inflammatory bowel disease and associated it with more severe illness. Infection and an inflammatory bowel disease flare can coexist, and their symptoms can overlap. A positive stool result alone cannot decide which is responsible for worsening symptoms.

When the Immune System Is Weakened

In one Iranian study of children with diarrhea and weakened immune systems, most of whom were receiving chemotherapy for blood cancers, stool PCR detected Aeromonas in about 15%. This was a selected hospital population, not an estimate for everyone taking an immune-suppressing medicine. A pancreas-kidney transplant case report also described prolonged diarrhea with repeated positive stool cultures.

Aeromonas can cause infections beyond the intestine, especially in people with serious underlying illness such as cirrhosis or cancer. Studies of these infections examine blood, bile, wounds, and other specimens, which are different from this stool test. In a Japanese hospital cohort, 10% died within 30 days; most infections involved the liver or bile ducts. That mortality figure cannot be applied to someone with an isolated positive stool PCR.

Species identification can matter in invasive infection. Studies of bloodstream isolates have associated A. dhakensis with worse outcomes than some other species. A general Aeromonas stool result does not establish the species or predict that the bacteria will spread.

Antibiotic Resistance

Resistance varies by species and strain. Aeromonas often carries genes for enzymes that break down penicillin-type antibiotics. Some stool isolates in the Beijing study also carried genes associated with resistance to colistin. Research comparing bacterial genomes with drug testing shows that a resistance gene does not always predict the observed response.

This PCR result does not provide antibiotic susceptibility results. Growing the organism in culture allows the laboratory to test which drugs inhibit it. A negative culture can occur even when PCR detects DNA, so the two methods provide different information.

When Results Can Be Misleading

• DNA and living bacteria: PCR can detect DNA from living or dead organisms. A positive result can reflect carriage or residual DNA as well as active infection. Failure to grow the organism in culture does not prove that the DNA came from dead bacteria.
• Differences between assays: Tests target different DNA sequences and may detect species or strains with different sensitivity. A negative result on a panel that does not report Aeromonas provides no information about it.
• Sample and laboratory effects: Small amounts of target DNA, substances in stool that inhibit PCR, contamination, and collection or storage problems can affect results. Laboratory controls reduce these risks but do not make every result definitive.
• Recent antibiotics: Treatment can reduce viable bacteria while DNA remains detectable. A result obtained during or after treatment can be harder to interpret.
• Other causes of diarrhea: Another pathogen, a medicine, or inflammatory bowel disease may explain symptoms despite a positive result.

Counts from different laboratories or PCR methods are not directly interchangeable, even when both reports use CFU/g stool. A change between samples has no validated meaning as a measure of recovery or future disease risk on its own.