What is being tested?
Escherichia coli (E. coli) bacteria commonly occur in nature and are a necessary component of the digestive process. Most strains of E. coli are harmless, but disease-causing (pathogenic) E. coli can cause inflammation of the stomach and intestines (gastroenteritis). Laboratory tests can detect the presence of pathogenic E. coli that produce Shiga toxins.
Multiple subtypes of E. coli cause diarrheal illness, and they are classified by how they cause the disease. For example, some invade the lining of the intestines, causing inflammation, while others produce toxins.
E. coli that produce poisons called Shiga toxins are generally the only type of E. coli that are tested for in clinical settings from stool specimens. The Shiga toxins associated with these infections are so called because they are related to the toxins produced by another type of disease-causing bacteria, Shigella. Shiga toxin-producing E. coli (STEC) may also be called verocytotoxic E. coli (VTEC) or enterohemorrhagic E. coli (EHEC).
A strain of STEC called O157:H7 is the STEC strain responsible for most gastrointestinal illness outbreaks in the U.S. However, non-O157 strains of STEC are gaining recognition, in part due to increased testing for them by clinical laboratories. For example, a 2011 outbreak of E. coli O104:H4, a non-O157 STEC, was associated with travel to Germany and resulted in 32 deaths related to contaminated sprouts. According to the Centers for Disease Control and Prevention, STEC O157 causes about 36% of STEC infections in the U.S., while non-O157 STEC cause the rest.
Outbreaks have been linked to the consumption of contaminated food, including undercooked ground beef, unpasteurized juice, unpasteurized milk, and raw produce such as leafy greens and alfalfa sprouts. STEC may also be transmitted through contaminated water, contact with farm animals or their environment, and from person to person. Even ingesting small numbers of E. coli can cause an infection.
In addition to symptoms of nausea, severe abdominal cramps, watery diarrhea, fatigue, or possible vomiting and low-grade fever, STEC infections are often associated with bloody stools and, less commonly, can lead to serious complications, specifically hemolytic uremic syndrome (HUS). HUS is a result of the toxin entering the blood and destroying red blood cells (hemolysis). It can lead to kidney failure (uremia or the build up of nitrogen wastes in the blood) and can be life-threatening. Signs and symptoms include decreased frequency of urination (evidence of uremia), fatigue, and pale skin due to hemolytic anemia. HUS usually develops about a week after the onset of diarrhea.
About 5-10% of people who are diagnosed with an O157 STEC infection develop HUS. Children, the elderly, and persons with weakened immune systems are at greatest risk. However, most healthy persons recover from a STEC infection within a week and do not develop HUS. Non-O157 Shiga toxin-producing E. coli can cause the same symptoms and complications and likely account for 20-50% of STEC infections in the U.S. annually. Different testing techniques are required to identify O157 and non-O157 Shiga toxin-producing E. coli.
How is the sample collected for testing?
A fresh liquid or unformed stool sample is collected in a clean, dry container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should be taken to the laboratory immediately or refrigerated and taken to the lab as soon as possible. Some laboratories provide transport media to support the survival of the organism from the time of collection until delivery to the laboratory. STEC becomes difficult to detect in the stool after one week of illness, so the timing of sample collection relative to the onset of illness is important.
How is it used?
These tests are used to detect the presence of Escherichia coli (E. coli) that produce Shiga toxin and to help diagnose an infection of the digestive tract due to these bacteria. E. coli bacteria are part of healthy digestive systems in humans and other mammals, but there are strains of E. coli that produce poisons, called Shiga toxins. In addition to severe diarrhea, Shiga toxin-producing E. coli (STEC) can cause hemolytic uremic syndrome (HUS), a serious illness that may lead to kidney failure and even death if not treated properly. Tests for STEC are used to make an accurate diagnosis and help guide treatment.
These tests may also be used to help recognize and track suspected outbreaks of STEC. Infections are often linked to the consumption of contaminated food or water, contact with farm animals or their environment, or person-to-person contact. E. coli O157:H7 is the strain that is most common in foodborne E. coli outbreaks in the U.S. However, there are non-O157 strains of STEC that can also cause severe diarrhea and HUS, such as E. coli O104:H4.
It is important that STEC infections be diagnosed quickly to prevent the bacteria from spreading throughout the community and so that interventions can be made, if necessary, to prevent HUS. Tests for STEC include:
- Stool culture: Cultures of stool samples use special nutrient media that selectively allow pathogens to grow while inhibiting growth of bacteria that are normally present in the digestive tract (normal flora). Once a pathogen grows in culture, other tests are performed to identify it. A stool sample may also be cultured for other pathogens, such as Salmonella, Shigella, and Campylobacter. O157:H7 STEC can be distinguished from other pathogens, including other types of E. coli, in culture because the bacteria have a distinct appearance when they are grown on a specific culture medium. Non-O157 STEC cannot be detected with the standard stool culture; they require special testing. Detection of non-O157 STEC is not usually performed at clinical laboratories but may be performed at public health laboratories. The Centers for Disease Control and Prevention (CDC) recommends that a culture for STEC be performed on all stools submitted for culture from people with acute, community-acquired diarrhea. Community-acquired diarrhea is in contrast to diarrhea acquired in a healthcare setting, such as a hospital.
- Toxin test: This test is used to detect the Shiga toxin directly using enzyme immunoassay (EIA). Stool cultures detect O157 STEC but do not detect non-O157 STEC. Therefore, the CDC recommends that EIA to detect Shiga toxins be used in conjunction with stool cultures. Detection of non-O157 STEC is important as it likely accounts for 20-50% of U.S. STEC infections annually. Enzyme immunoassay only identifies the presence of Shiga toxins and does not determine which strain of E. coli is producing the toxin.
- Genetic tests: PCR for Shiga toxin 1 gene (stx1) and Shiga toxin 2 gene (stx2) are rapid molecular tests that can be used to confirm the presence of Shiga toxin.
- Pulsed-field Gel Electrophoresis (PFGE): This method is used by public health laboratories to identify subtypes of E. coli that are suspected in an outbreak. It creates a DNA “fingerprint” of the bacteria detected that is entered into a national database to be compared to other fingerprints. If matches are made, it may indicate occurrences of disease caused by the same strain of bacteria. In this way, public health labs can quickly evaluate the cause of an outbreak even though the illnesses might occur in different geographic areas.
When is it ordered?
These tests may be ordered when a person has diarrhea and the healthcare practitioner suspects an infection of Shiga toxin-producing E. coli.
Some signs and symptoms include:
- Diarrhea that is severe or acute and persists for more than a few days
- Bloody stool
- Severe abdominal pain, cramping and/or bloating
These tests are often ordered when a community-wide E. coli outbreak is suspected, for example, when several people who have eaten the same food from the same source have similar signs and symptoms.
What does the test result mean?
Stool culture results are frequently reported out with the name of the pathogenic bacteria that was detected. If Shiga toxin-producing Escherichia coli (STEC) is detected (positive culture), it means STEC is the cause of the person’s symptoms.
A negative stool culture for STEC means that Escherichia coli O157:H7 was not present or was not present in sufficient numbers to be detected. The culture results may indicate that a pathogen other than E. coli is causing the symptoms. These could include the bacterial pathogens Salmonella, Shigella, and Campylobacter, viral pathogens, or parasites.
Toxin test by EIA
A negative result for enzyme immunoassay (EIA) for Shiga toxin suggests that the toxin is not present.
A positive result for enzyme immunoassay (EIA) for Shiga toxins suggests that the toxins are present in the stool and that further testing should be done to grow and identify the E. coli producing the toxins.
A negative result for a genetic test suggests that STEC was not present. A positive result suggests that STEC was present.
Is there anything else I should know?
Non-pathogenic E. coli are a normal part of a healthy human digestive system. However, E. coli infections in parts of the body other than the gastrointestinal system can cause illness. E. coli is responsible for the majority of urinary tract infections and can also cause neonatal meningitis, among other infections. These E.coli infections are detected by culturing a sample from the infected area.
How are Shiga toxin-producing E. coli infections of the digestive tract treated?
The recommended treatment for O157 Shiga toxin-producing E. coli (STEC) infection that has not progressed to hemolytic uremic syndrome (HUS) is supportive care (rest and rehydration). Those who are at an increased risk of developing HUS, such as children or the elderly, will be monitored closely. If they develop HUS, hospitalization will be required.
Infections of non-O157 Shiga toxin-producing E. coli are also treated with rest and rehydration, and they are often resolved without any additional intervention.
Why aren't antibiotics a recommended treatment for an infection with Shiga toxin-producing E. coli ?
If you are infected with Shiga toxin-producing E. coli, antibiotics may increase your risk for developing hemolytic uremic syndrome (HUS).
Why shouldn't I take anti-diarrheal medication if I have a Shiga toxin-producing E.coli infection of the gastrointestinal system?
Anti-diarrheal medicines may worsen or prolong your illness because they delay the removal of E. coli from your gastrointestinal tract by inhibiting the normal movement of food and fluids through the GI tract.
What else can cause acute diarrhea?
Other bacteria that can cause acute diarrhea include:
- Salmonella, often found in raw eggs, raw poultry, and in pet reptiles
- Shigella, from fecally-contaminated food and water
- Campylobacter, from raw or undercooked poultry
- Clostridium difficile, may become overgrown in the gut after antibiotic use
Parasites can also cause diarrhea. They are found in lakes and streams throughout the world and may also contaminate swimming pools, hot tubs, and community water supplies. The most common single-celled parasites responsible for gastrointestinal illness in the U.S. are Giardia lamblia (giardia), Entamoeba histolytica (E. histolytica), and Cryptosporidium parvum (crypto).
Rotavirus is the most common cause of severe diarrhea among children. Other viruses that cause diarrhea include Norwalk, noroviruses (also called Norwalk-like viruses), adenoviruses, calciviruses, cytomegalovirus (CMV), and HIV.