Researchers in Scotland have increased knowledge of non-O157 Shiga toxin-producing E. coli (STEC) in the country as part of work on severity of illness caused by these strains.

Food Standards Scotland commissioned a study to improve understanding of non-O157 STEC. It provides a resource for surveillance and during future outbreaks of infection.

Non-O157 STEC accounts for 30 percent of all STEC isolated in Scotland but no outbreaks with more than five cases have been identified in the country. Non-O157 STEC are E. coli strains which also produce Shiga toxin (stx). However, as non-O157 strains can be more difficult to detect, less is known about severity of illnesses caused by them. Shiga toxins are one of the key virulence factors, which cause diarrhea, hemorrhagic colitis and Hemolytic Uremic Syndrome (HUS).

A recent European Food Safety Authority (EFSA) opinion found all Shiga toxin-producing E. coli strains are pathogenic and potentially associated with severe illness.

Serotypes found
The Scottish E. coli O157/STEC Reference Laboratory (SERL) is responsible for the identification and typing of E. coli O157 and other STEC. In 2017, the SERL introduced Whole Genome Sequencing (WGS) as the main typing method for this work.

SERL whole genome sequenced historical patient isolates to examine the genetic profile of organisms implicated in illness in Scotland. This included looking for different genes thought to be associated with more severe disease, genes which may confer antimicrobial resistance (AMR) and genetic relatedness of the different strains.

A total of 522 non-O157 STECs were identified in a 16 year period and 88 different serotypes were found. The most common serotypes were E. coli O26:H11 and E. coli O103:H2. These are also the main non-O157 STEC serotypes in Europe and the United States. E. coli O145:H28 was the third most common serotype detected and has been responsible for small Scottish outbreaks.

E. coli O26:H11 accounted for 27 percent, or 141 of 522, of all serotypes observed from February 2002 to February 2018. The next most common serotypes were O103:H2 with 49 and O145:H28 with 45. There were 47 serotypes found only once.

The top 5 serotypes account for 53 percent of all STEC analyzed in the study. E. coli O26 was detected every year since 2002; E. coli O103:H2 was found every year since 2005 and E. coli O145:H28 was identified every year since 2007.

Genes linked to severe illness
Findings indicate non-O157 strains carrying a range of different stx types and those lacking the eae (intimin) gene are capable of causing illness, showing these markers cannot be used in isolation to determine pathogenicity.

A total of 21 different stx subtype profiles were detected. Stx1a only was the most common subtype profile with 176 of 522 or 33.7 percent, followed by stx2a only at 78 and stx2a stx1a with 60. Almost 8 percent of the Scottish non-O157 STEC collection carried the stx2f variant.

Of the 176 non-O157 STEC harboring stx1a, 48.3 percent were O26:H11 and 26.7 percent were O103:H2. Of 78 isolates harboring stx2a only, 52.5 percent or 41 of them were O145:H28.

A total of 89 virulence genes (excluding the stx genes) were detected among the isolates.

Aminoglycoside resistance genes were found in 85 of 92 isolates showing resistance. Sulphonamide resistance genes were found in 75 of 92 isolates. Tetracycline resistance genes occurred in 45 isolates. There were no genes associated with resistance to colistin. Of 92 isolates carrying resistance genes 63 were multi-drug resistant, so had resistance to three or more antibiotic classes.

A second phase of the project will compare the predicted disease potential with actual symptoms of each patient to determine whether a molecular risk assessment approach might support decisions on public health interventions for STEC infection in the future.

The study is supporting research by Health Protection Scotland to compare the genetic profile of non-O157 STECs in clinical cases in the country with symptoms reported by these patients. This will provide insight into severity of illness caused by non-O157 STEC strains, helping authorities have a more detailed, molecular approach for assessing risks associated with STEC detection in food.