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 health care 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.
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.
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 STEC.
Stool culture A negative stool culture for Shiga toxin-producing Escherichia coli (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. A positive culture for STEC means that Shiga toxin-producing E. coli (likely Escherichia coli O157:H7) were detected.
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.
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.
This article was last reviewed on January 5, 2012. | This article was last modified on May 14, 2015.
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