E. coli O157 and Big STEC Testing

Pathogenic E. coli strains are classified into six pathotypes. Shiga toxin-producing E. coli (STEC) is one of most common pathogens associated with foodborne outbreaks. The bacteria can form toxins to cause disease. E. coli O157 is the most important STEC serogroup in foods and E. coli O157:H7 is the most common serotype in foodborne infections.

Figure 1. Electron micrograph of E. coli O157 and related contaminated foods

E. coli O157:H7 has caused a number of major foodborne outbreaks in many countries including the USA, the UK, and other European countries. The pathogenic E. coli can be transmitted by the contaminated foods and water, or through the contact with persons or animals. Common foods that can be contaminated are typically associated with undercooked meat (especially minced beef and salamis) and unpasteurized dairy products as well as fresh produce such as leafy greens and sprouted seeds. Cattle are the main reservoir for the pathogenic E. coli and their feces can also contaminate water and environment that are used for other agricultural crops. Other non-O157 STEC serogroups that often cause human illness include "big six" serogroups in meat products which are called big STEC. E. coli O157 and STEC are very sensitive to high temperature and can be killed with proper cooking process.

Pathogenic E. coli can cause disease including diarrhea or other illnesses outside of the intestinal tract. The elderly and very young children are more likely to be infected and develop severe illness than others. The symptoms of STEC infections vary for each person and usually include severe stomach cramps, vomiting, and diarrhea. Most people can recover within a week. Some infections caused by pathogenic E. coli are mild while other symptoms can be severe.

There have been standards and regulations established to control the incidence of contamination of E. coli O157 and big STEC. The presence of STEC in 25 g sample is not acceptable for any ready-to-eat foods. The infective dose of STEC to cause foodborne illness is dependent on the virulence factors and serotype of the strain. For E. coli O157:H7, the level to cause people sick (infective dose) is usually very low (approximate 10 - 100 cells).

E. coli O157 and Big STEC Testing of Food at Alfa Chemistry

The standard methods (EN/ISO and FDA BAM) that have been widely used to detect E. coli O157 and other STEC are culture-based methods using general microbiological testing supplies and molecular biology based methods using real-time PCR assay (preferred). PCR assay targets Shiga toxin antigens or genetic determinants. The Stx produced by E. coli O157:H7 is an identification marker detected by PCR. The stx1 and stx 2 markers in the real-time PCR assays enable the screening and detection of other STEC strains. An enrichment step using general medium is needed to recover any pathogenic E. coli strains prior to the real-time PCR. After the overnight enrichment, samples that are found probable positive require a cultural confirmation step. The confirmation test is performed using biochemical identification technique.

Alfa Chemistry offer accurate and validated testing platforms for the detection and confirmation of E. coli O157and big STEC in various food samples to meet customers' needs. We also have experience working on different standards and regulations.

Platform: Real-time PCR assay

Limit of detection: Qualitative test per sample size (negative/positive)

Sample type: Solid and liquid food samples

Sample size: 25 g