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Shiga Toxin-producing E. coli, Diarrhea, Hemolytic Uremic Syndrome, and Sprouts
By Stan Deresinski, MD, FACP, FIDSA, Clinical Professor of Medicine, Stanford University, Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.
In May of this year, German public health authorities reported a significant increase in the number of patients with diarrhea caused by a Shiga toxin-producing Escherichia coli (STEC), as well as of cases of hemolytic uremic syndrome (HUS).1 Between May 2, and June 14, 2011, 818 cases of HUS and 3,332 STEC cases were reported from European Union/European Economic Area Member States; 36 patients died. The proportion of those with STEC infection who developed HUS is much larger than reported in previous outbreaks. Also unusual is that the majority of cases occurred in adults, a finding quite different from the usual target of young children by STEC, and two-thirds of cases were in females. More than 95% of cases were reported from Germany and most of the remainder had recently traveled to northern Germany. Additional cases have been identified in Switzerland, the United States, and Canada, and also after travel, in most cases to Germany.
The culprit has been identified as STEC serogroup O104:H4, rather than the much more common serogroup 0157:H7, a factor which may have led to a delay in the recognition of the outbreak. While investigations continue, classical epidemiology has identified the likely contaminated food product as bean sprouts, including fenugreek mung beans, lentils, adzuki beans, and alfalfa. Thus, a "recipe-based restaurant cohort study" found that customers who had eaten sprouts had an 8.6-fold increased risk of diarrhea and/or HUS compared to those who had not eaten sprouts.2 Furthermore, 100% of those who became ill had eaten sprouts. The implicated sprouts have been traced to a farm in Lower Saxony near Hamburg.
The outbreak serotype 0104:H4 strain has been extensively characterized and been found to have some unusual characteristics, including being a hybrid with enteroaggregative E. coli (EAggEC). In fact, among E. coli sequences to which it has been compared, the genome of the outbreak strain is most closely related to an EAggEC isolated in 2002, mostly differing by the addition of stx, the gene encoding Shiga toxin (also known as Vero toxin), and antibiotic resistance genes. Its hybrid nature means that it can most accurately be designated as enteroaggregative, Shiga toxin/verotoxin-producing E. coli (EAggEC STEC/VTEC).
EAggEC is a type of E. coli that causes diarrhea, but it does not secrete either the heat-stable or heat-labile toxins that characterize enteroxigenic E. coli and that adhere to the surface of cells in culture in characteristic aggregations that resemble stacked bricks. The adherent bacteria produce various diarrheogenic toxins. This typical aggregative growth is the result of the presence of aggregative adherence fimbriae (AAF), whose expression is regulated by a plasmid-borne gene, aggR. EAggEC infection most often is associated with persistent watery diarrhea, particularly in infants in developing countries, as well as in HIV individuals.3
The outbreak strain is multidrug resistant, carrying not only the common TEM-1 b-lactamase, but also CTX-M-15. It is resistant to cephalosporins, penicillins, and b-lactam/b-lactamase inhibitor combinations, as well as to streptomycin, nalidixic acid, tetracycline, and trimethoprim/sulfamethoxazole.
The organism is, by definition, characterized as an STEC by virtue of its possession of genes encoding Shiga toxin, in this case Shiga toxin 1 subtype 2a the gene encoding Shiga toxin 2 is absent. Shiga toxin 1 differs from "true" Shiga toxin (of Shigella) by 1-7 amino acid differences, while Shiga toxin 2 shares approximately 60% homology to Shiga toxin 1.
In contrast to E. coli 0157:H7, the outbreak 0104:H4 strain does not ferment sorbitol. Since lack of sorbitol fermentation is the most commonly used characteristic in screening for STEC, this may have led to a failure of identification of the organism in some clinical laboratories. This is the reason why the Centers for Disease Control and Prevention (CDC) in 2009 recommended the following: "All stools submitted for testing from patients with acute community-acquired diarrhea (i.e., for detection of the enteric pathogens Salmonella, Shigella, and Campylobacter) should be cultured for O157 STEC on selective and differential agar. These stools should be simultaneously assayed for non-O157 STEC with a test that detects the Shiga toxins or the genes encoding these toxins. All O157 STEC isolates should be forwarded as soon as possible to a state or local public health laboratory for confirmation and additional molecular characterization (i.e., PFGE analysis and virulence gene characterization). Detection of STEC or Shiga toxin should be reported promptly to the treating physician, to the public health laboratory for confirmation, isolation, and subsequent testing of the organism, and to the appropriate public health authorities for case investigation. Specimens or enrichment broths in which Shiga toxin or STEC are detected, but from which O157 STEC are not recovered, should be forwarded as soon as possible to a state or local public health laboratory."4 It is clear, however, that not many clinical microbiology laboratories in the United States follow this recommendation and thus the danger of delayed recognition of an outbreak remains a risk. Of note is that screening for serotype 0104:H4 can be performed by agglutination with K9 capsular antigen antiserum since the antigens of both are identical.1
It has been suggested that the multidrug resistance of the outbreak strain is irrelevant since antibiotic therapy (as well as administration of antimotility agents) is believed by many to be contraindicated in these infections, because of some evidence indicating an associated increased risk of HUS. This is thought to be the result of mobilization of phage and resultant increased production of Shiga toxin caused by antibiotic activation of the bacterial SOS system. Treatment of HUS has been only supportive, including hemodialysis and plasma exchange, but other possibilities are being explored. Shiga toxin activates the complement system and this is believed to play a key role in the development of HUS. The monoclonal C5 antibody, eculizumab, which inhibits terminal complement complex formation has met primary endpoints in a Phase 2 study in children with atypical HUS, which is associated with genetically determined chronic uncontrolled complement activation. Recently, eculizumab has been administered to several patients with STEC-associated HUS with perceived benefit.5 Separately, urtoxazumab, a humanized monoclonal antibody directed against the Shiga-like toxin 2 was safe when administered to healthy adults and children with STEC infection in a Phase 1 trial.6
This STEC outbreak is one of the largest ever described anywhere in the world and is associated with an unusually high incidence of HUS. It has resulted from the capture of a bacteriophage encoding Shiga toxin by a typical strain of EAggEC, resulting in an organism that is hypervirulent and may also have the ability to persist in the gastrointestinal tract. The lack of sorbitol fermentation by the outbreak strain means it is not detected by many clinical microbiology laboratories that do not test directly for the toxin or its gene. The lesson is that clinicians and clinical laboratories in the United States need to be alert to the detection of STEC and STEC-EAggEAC hybrids of all stripes.