Research report

Whole Genome Sequence Typing and Analysis of Non-O157 STEC

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Shiga Toxin-producing E. coli (STEC) are a group of bacteria that reside in intestines of ruminant animals, particularly cattle. They can cause serious illness in people who become exposed, either directly through contact with animal faeces in the environment, or through water or food that has become contaminated.

STEC infection is most commonly caused by a strain called E. coli O157, which has been responsible for a number of high profile foodborne outbreaks in Scotland. Non-O157 STEC are E. coli strains which also produce Shiga toxin, and are thought to account for around one third of STEC cases in Scotland. However, as non-O157 strains can be more difficult to detect, less is known about the severity of illness they can cause.

In order to improve our understanding of the importance of non-O157 STEC, FSS commissioned the Scottish E. coli O157/STEC Reference Laboratory (SERL) to undertake Whole Genome Sequencing on its historical collection of patient isolates to examine the genetic profile of organisms that have been implicated in illness in Scotland. This included looking for the presence of different genes which are thought to be associated with more severe disease, genes which may confer antimicrobial resistance (AMR) and the genetic relatedness of the different strains.

A total of 522 non-O157 STECs were identified over a 16 year period and 88 different serotypes were found. The most commonly identified serotypes were E. coli O26:H11 and E. coli O103:H2. The SERL also found diversity in relation to the genes that are considered to be important in the ability of these strains to cause illness, such as the subtype of stx genes and the presence of attachment genes such as eae. The findings indicated that non-O157 strains carrying a wide range of different stx types as well as those lacking the eae gene are capable of causing human illness, demonstrating that these markers cannot be used in isolation to determine pathogenicity.  

This study is also being used to support further research being undertaken by Health Protection Scotland to compare the genetic profile of the non-O157 STECs in clinical cases in Scotland with the symptoms that have been reported by these patients. This will provide valuable insight into the severity of illness caused by non-O157 STEC strains, helping us move towards a more detailed, molecular approach for assessing the risks associated with STEC detection in food.