Salmonella

Fever, cramping and diarrhea can come from several different infections. A stool culture can identify Salmonella as a possible cause. A positive culture confirms that live bacteria were recovered from your sample, though whether they explain your symptoms depends on the rest of your illness.

This is an established diagnostic test with a qualitative result: Salmonella was isolated or it wasn't. It doesn't report a bacterial concentration. Growing the organism also provides material for further testing, including identifying its type and checking which antibiotics work against it.

What the Test Looks For

Salmonella includes different types of bacteria with different patterns of illness. Most foodborne diarrheal infections are caused by nontyphoidal Salmonella. Salmonella Typhi and Paratyphi cause typhoid and paratyphoid fever, which are illnesses involving the bloodstream and require a different approach to diagnosis and treatment. An initial report that says Salmonella doesn't necessarily identify which type was found.

Cell-culture and animal experiments explain how these bacteria can enter the intestinal lining. Salmonella injects proteins into intestinal cells that change their shape and promote bacterial entry. Some bacteria then survive inside those cells. In mouse experiments, the resulting inflammation also produces chemicals that help Salmonella obtain energy and grow among the other gut bacteria. These experiments explain possible mechanisms of infection; a stool result doesn't measure those processes in your intestine.

Acute Gastroenteritis

The usual illness involves diarrhea, abdominal cramps and fever. Nausea and vomiting can occur. Diarrhea may contain blood or mucus, but Salmonella can also cause watery diarrhea without either. Symptoms usually last four to seven days, although some infections last longer and bowel habits can take additional time to settle.

The symptoms overlap with infections caused by Campylobacter, Shigella and other organisms. Their presence or absence cannot reliably identify Salmonella. A positive culture is most informative when it fits the timing and pattern of your illness.

Infection Beyond the Intestine

Salmonella sometimes spreads beyond the intestine into the blood or other normally sterile parts of the body. CDC estimates that invasive infections occur in about 8% of people with laboratory-confirmed salmonellosis. That figure describes diagnosed infections, which include more severe illness than infections that never reach medical attention. It isn't the probability that an ordinary episode of diarrhea will progress to a bloodstream infection.

Infants, older adults and people with weakened immune systems have a higher risk of invasive disease. Atherosclerosis, certain heart or joint conditions, and sickle cell disease can also matter. Complications can include infection of bone, joints or blood vessel walls.

Stool culture has limited value for excluding bloodstream infection. In a study of hospitalized children in the Democratic Republic of the Congo, about two thirds of those with Salmonella in their blood had a negative stool culture. Stool collection generally occurred after blood collection and during antibiotic treatment, which could reduce recovery from stool. The study supports a practical distinction: a negative stool culture cannot exclude invasive infection.

Blood culture is the main diagnostic test for suspected typhoid or paratyphoid fever. Stool culture is less sensitive early in those illnesses. Evidence from blood cultures describes infection in a different part of the body and cannot be used to interpret a stool result as proof of bloodstream infection.

Culture and PCR Can Give Different Answers

Culture requires living bacteria to grow. PCR detects bacterial DNA. These are different methods, and neither detects every infection. Culture recovery depends on the number of living bacteria in the sample, transport conditions and the growth methods used. PCR performance depends on the particular assay and can be affected by substances in stool that interfere with DNA detection.

PCR isn't uniformly more sensitive for Salmonella. In an Australian comparison of 90,291 stool samples, culture detected more Salmonella-positive samples than the particular commercial PCR assay studied. PCR missed about 23% of culture-positive samples. Some samples were positive only by PCR, so the disagreement went in both directions.

A PCR-positive, culture-negative result can reflect bacterial DNA remaining after organisms have died, but it can also reflect living bacteria that culture failed to recover. That combination alone doesn't establish whether you are infectious. A positive culture provides direct evidence that viable bacteria were present in the sample.

A recovered culture isolate permits antibiotic susceptibility testing and further identification for outbreak investigations. A routine positive PCR result usually provides neither a live isolate nor a complete assessment of antibiotic susceptibility.

Antibiotic Resistance

Resistance varies by bacterial type, location and travel history. In one pediatric-focused hospital study in Shenzhen, about three quarters of tested stool isolates resisted ampicillin. That was a local hospital finding, not a worldwide resistance rate.

Susceptibility testing checks the recovered bacteria against antibiotics. It is especially useful when infection is severe or has spread beyond the intestine. A drug that worked against an earlier infection or another person's strain may not work against yours.

Carriage After Symptoms Stop

You can continue passing Salmonella in stool after you feel better. In a Norwegian study, 24% of participants still had a closely matching strain recovered five weeks after their initial positive sample. Only about a quarter of eligible reported cases participated, so that percentage should not be treated as everyone's expected recovery course. Young children were more likely to have prolonged shedding.

People without recent symptoms can also carry Salmonella. Community studies have found substantial differences between locations, and those estimates depend partly on how samples were collected. A positive result without symptoms establishes carriage, but it doesn't by itself predict future illness.

Shedding can be intermittent. A community study in the Democratic Republic of the Congo found substantially more carriers when participants supplied three samples than when only their first sample was considered. This finding concerns detecting carriage in that population. It doesn't establish that everyone with acute diarrhea needs three cultures.

What the Colon Cancer Research Shows

A Dutch registry study followed 14,264 adults with laboratory-confirmed Salmonella infection for a median of seven years. Most infections were intestinal, though the cohort also included bloodstream and other infections. Among people infected before age 60, subsequent colon cancer incidence was about 50% higher than expected from population rates.

Within that younger-at-infection group, the association was concentrated in the ascending and transverse colon. Incidence in those regions was about twice the expected rate, and close to three times the expected rate after Salmonella Enteritidis infection. These were subgroup associations; the increase for the full cohort was not statistically conclusive.

Later studies haven't consistently reproduced that pattern. A larger Danish study found no increase in total colon cancer risk beyond the first year after infection. A Michigan study also found no increase in colorectal cancer across its full cohort, although an exploratory age subgroup had higher incidence. Differences in infection reporting, underlying health and cancer detection complicate these comparisons.

Experiments in cells and animals suggest that Salmonella can alter signals controlling cell growth. Human observational studies haven't established that a prior infection causes colon cancer, or that changing screening because of that infection improves outcomes. A current stool culture is not a validated measure of future colon cancer risk.

When Results Can Be Misleading

• Antibiotic exposure: treatment can reduce the number of living bacteria available to grow, making culture negative even when infection has not been fully cleared.
• Intermittent or low-level shedding: a negative sample can occur between periods of detectable shedding, especially during recovery.
• Collection and transport problems: unsuitable containers, contamination or delayed processing can reduce the reliability of culture.
• Carriage without symptoms: a positive culture confirms viable Salmonella in stool, but another condition may account for your current symptoms.
• Comparing different methods: a PCR result and a culture result answer different questions. Switching between them can create an apparent change without establishing clearance or recurrence.
• Illness outside the intestine: a stool culture cannot rule out infection in the blood or another body site.