STEC, EHEC, or E. coli O157?
Changes in STEC detection
methods bring new challenges
“Shiga toxin-producing Escherichia coli (STEC),” “Enterohemorrhagic E. coli (EHEC),” and “E. coli O157” are often used interchangeably when discussed as threats to food establishments. Are
these three targets really the same? The answer is complicated because it can be both yes and no.
STEC strains are categorized by two characteristics:
their ability to produce verotoxins or Shiga toxins (stx)
and their ability to adhere to cell membranes. While
there are several stx variants, two have been associated
as the causative agents for human illness: stx1 and stx2.
Likewise, STEC has multiple ways to adhere to cell
membranes. The most common pathway is associated
with a set of genes, eae, that produce intimin, an outer-membrane adhesin essential for attachment. Not all
STEC strains contain eae genes, as evidenced by the
2011 E. coli O104 STEC sprouts outbreak in Europe
that resulted in 31 deaths. This strain was eae-negative
and used alternative pathways for membrane adherence.
EHEC is a subset of STEC strains that can cause
hemorrhagic colitis (HC). HC and the strains that cause
it are of primary concern to food establishments because
the disease can progress to hemolytic uremic syndrome
(HUS), a potentially fatal illness. When discussing
EHEC and STEC, it is important to remember that all
EHEC strains can be considered STEC strains, but not
all STEC strains are EHEC strains—only
those that cause HC.
E. coli O157:H7 is the leading serogroup associated with HUS. An E. coli
O157 strain that produces stx, adheres
to cell membranes, and causes HUS
would be classified as both an STEC
and an EHEC. A nontoxin-producing
E. coli O157 would be considered neither an STEC nor an EHEC. Thus,
the classification of these three terms
remains open-ended and warrants that
food establishments have a strong understanding of each.
Challenges for STEC Detection
E. coli O157 has long been considered an adulterant in foods, and detection methods have been used in the
food industry for nearly 20 years. STEC
strains are estimated to cause over
250,000 illnesses annually, with around
one-third resulting from O157. But there
are many more STEC strains than just
O157. Six additional serogroups—O26,
O45, O103, O111, O121, and O145—
have been identified as the source of
70 percent of the non-O157-related
illnesses and have been identified as
adulterants by the U.S. Department of
Agriculture Food Safety and Inspection
Service (USDA-FSIS). In June 2012,
USDA-FSIS implemented requirements
for the expansion of STEC testing in
raw beef trimmings, targeting these six
serogroups, and made public a new reference standard for analysis.
The U.S. was not the only country
to increase testing requirements. As a
result of the deadly sprout outbreak,
the European Union (EU), in 2013, issued new microbiological testing criteria
for sprouts. Those recommendations
included additional testing for STEC.
In the years since, several EU countries
have also increased the testing for STEC
strains in beef and dairy products. All
testing in the EU would be conducted
using the ISO TS 13136 STEC testing
By Patrick Bird, M.Sc.