Shiga toxin-producing Escherichia coli

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 diarrhoea, Shiga toxin-producing E. coli (STEC) can cause haemolytic 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 UK. However, there are non-O157 strains of STEC that can also cause severe diarrhoea 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. UK Health Security Agency recommends that all diarrhoeal stools are screened for the presence of E. coli O157 (STEC).
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. 
• 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. This test is mostly replaced by PCR test.
• PCR test – since 2013 many local laboratories are testing the samples by a PCR test. PCR detects the genetic material of the bacteria, for example the genes that produces toxins (stx1 & stx2). Some PCR methods, especially those in the reference laboratory may look for additional genetic material. This test is extremely sensitive. The PCR test is usually followed up by culture or further tests in the reference laboratory
• Typing methods – Public health or the reference laboratory may perform many other tests which may help to manage an outbreak. These tests try to find out if STEC obtained from two different people matches with each other. If STECs from a suspected outbreak matches with each other an outbreak could be confirmed. These tests are called typing methods. Some examples are - Phage typing, Pulsed Field Gel Electrophoresis (PFGE), Multilocus Sequence Typing (MLST), and Whole Genome Sequencing (WGS).

These tests may be requested when a person has diarrhoea and the doctor suspects an infection of Shiga toxin-producing E. coli Some signs and symptoms include:

• Diarrhoea that is severe or acute and persists for more than a few days
• Bloody stool
• Fever
• Severe abdominal pain, cramping and/or bloating

Doctors or public health teams may request the test when a person presents with signs or symptoms of Haemolytic Uraemic syndrome (HUS).

These tests are often requested 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. Public health (UKHSA) may request a test for someone who had close contact with a patient with a STEC infection or HUS.

Stool culture
A positive culture for STEC means that Shiga toxin-producing E. coli (likely Escherichia coli O157:H7) was detected and it is probably responsible for your signs and symptoms.
A negative culture suggest that the E coli O157:H7 is most likely not present. Unfortunately, the culture in a local laboratory is often unable to detect other types of STEC, which are called non-O157 STECs. If your signs or symptoms suggest that a STEC infection is possible, your local laboratory would refer the sample to a reference laboratory for further testing.

Polymerase Chain reaction (PCR)
Many local laboratories now test the sample by PCR first. The PCR usually detects the gene responsible for toxin production or some other genetic material of the bacteria. It is an extremely sensitive test.

PCR is able to detect both E coli O157 and non-O157 STECs. The local laboratory usually performs culture on those samples which are positive to check if this is a E coli O157, which is the commonest type in the UK. They will also send this sample to a reference laboratory for further testing.

A negative PCR may suggest that STEC infection is most likely absent, but if your signs and symptoms are strongly suggestive of a STEC infection or HUS, your sample could be send to the reference laboratory to confirm.

E coli O157 and other STECs are extremely infectious as only a small number of bacteria (<100) is required to cause infection. It is known to cause outbreaks.

Your local laboratory and clinicians must inform public health about any suspected or confirmed STEC infection or HUS. Public health may contact you to collect more information necessary to contain the infection.

Most people must isolate until 48 hours after they have stopped vomiting and having diarrhoea. However, some people, like children or those who prepare or serve unwrapped food, may need to isolate longer, if advised to do so by the public health.

The recommended treatment for O157 Shiga toxin-producing E. coli (STEC) infection that has not progressed to haemolytic 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, hospitalisation 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.