**2. Physiopathology of ETEC and EHEC infections**

### **2.1. Epidemiological data**

ETEC are a significant cause of watery diarrhea in developing countries where sanita‐ tion and clean water remain scarce and a main cause of traveler's diarrhea [2]. In contrast, EHEC are a major public health concern of developed countries [3] (**Figure 1**). Hence, ETEC are among the top four pathogens causing moderate to severe diarrhea among children in Africa and South Asia, while EHEC are the third most common zoonotic pathogen in Europe associated with large food poisoning outbreaks in EU, the USA, Canada, and Japan. The most common serogroups implicated in outbreaks and sporadic cases are O6, O78, O8, O128, and O153 for ETEC and O157:H7, O26:H11, O45:H2, O103:H2, O111:H8, O121:H19, and O145:H28 for EHEC.

Enterotoxigenic and Enterohemorrhagic *Escherichia coli*: Survival and Modulation of Virulence in the Human... http://dx.doi.org/10.5772/intechopen.68309 5

**1. Introduction**

oped world.

**2.1. Epidemiological data**

and O145:H28 for EHEC.

Since its identification in 1885, *Escherichia coli* (*E. coli*) has become one of the most com‐ prehensively studied bacterial species. While *E. coli* is widely found in the environment and foods and is an important member of the commensal microbiota of mammals, some strains have evolved to include pathogenic mechanisms that cause significant diseases in humans and animals. In humans, *E. coli* strains can cause diverse enteric/diarrheagenic or extra‐intestinal infections by means of virulence factors that affect a wide range of cellular processes. Pathogenic *E. coli* associated with gastrointestinal illness have been divided into eight pathotypes based on their virulence profiles: (i) enteropathogenic *E. coli* (EPEC), (ii) enterohaemorrhagic *E. coli* (EHEC), (iii) enterotoxigenic *E. coli* (ETEC), (iv) enteroinvasive *E. coli* (EIEC), (v) enteroaggregative *E. coli* (EAEC), (vi) diffusely adherent *E. coli* (DAEC), (vii) adherent invasive *E. coli* (AIEC) and (viii) Shiga toxin‐producing enteroaggregative *E. coli* (STEAEC) [1]. This chapter will cover only two of them: ETEC and EHEC, which show opposite trends during their pathogenic processes. Even if in both cases human infections are primarily acquired through consumption of contaminated food products or drinking water, ETEC is a major cause of infantile diarrhea in developing countries, while EHEC is one of the main *E. coli* pathotypes associated with food poisoning outbreaks in the devel‐

4 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

To cause human illness, pathogenic enteric *E. coli* must not only survive the passage through the human gastrointestinal (GI) tract but also accomplish their pathogenic process by a com‐ plex and coordinated multistage strategy, including adherence to the host intestine and toxin/ virulence protein production. The current chapter will provide a state of the art of ETEC and EHEC physiopathology, then focus on pathogen survival in the human digestive tract and regulation of virulence determinants by GI cues. As studies on humans are ethically incon‐ ceivable and small animal models do not recapitulate human pathogenesis, we will introduce the potential of dynamic *in vitro* digestion systems for increasing our understanding of ETEC

ETEC are a significant cause of watery diarrhea in developing countries where sanita‐ tion and clean water remain scarce and a main cause of traveler's diarrhea [2]. In contrast, EHEC are a major public health concern of developed countries [3] (**Figure 1**). Hence, ETEC are among the top four pathogens causing moderate to severe diarrhea among children in Africa and South Asia, while EHEC are the third most common zoonotic pathogen in Europe associated with large food poisoning outbreaks in EU, the USA, Canada, and Japan. The most common serogroups implicated in outbreaks and sporadic cases are O6, O78, O8, O128, and O153 for ETEC and O157:H7, O26:H11, O45:H2, O103:H2, O111:H8, O121:H19,

and EHEC pathogenesis in a physiologically relevant GI environment.

**2. Physiopathology of ETEC and EHEC infections**

**Figure 1.** ETEC and EHEC pathogenesis including epidemiological data on the infections and at‐risk populations, reservoir, mode of transmission and virulence factors of the pathogen, and clinical signs are described. A/E: Attaching and effacing; CFTR: cystic fibrosis transmembrane regulator; GC‐C: guanylyl cyclase C; GM1: monosialoganglioside receptor; LT: heat‐labile enterotoxins; ST: heat‐stable enterotoxins; Stx: Shiga toxin.

ETEC cause approximately 280 million episodes of diarrhea worldwide, leading to hundreds of thousands of deaths per year [4]. With regard to EHEC, it is estimated that the pathogen is responsible for 2,801,000 acute illnesses, 3890 cases of haemolytic and uremic syndrome (HUS), 270 cases of permanent end‐stage renal disease, and 230 deaths worldwide [3]. For both pathogens, infants less than 5 years old are a high‐risk population. ETEC are respon‐ sible for 20–25% of diarrhea in young children, mostly in low‐income countries, and up to 40% of traveler's diarrhea [5]. In developing countries, children suffer from diarrhea attacks 7–8 times a year, with a peak incidence occurring between 6 and 18 months, and ETEC strains are responsible for one of each three attacks [6, 7]. In such countries, ETEC infections have then shown to play a significant part in the complex association between malnutrition and repeated bouts of diarrheal illness among young children. The impact of EHEC is also greater in infants and children, compared to other ages with 42% of cases of HUS and 29% of deaths occurring in children between the ages of 0 and 4 years [3].

While the lack of ongoing monitoring systems makes it difficult to understand ETEC pathogenesis worldwide, dedicated surveillance systems of human EHEC infections have been developed in most of the industrialized areas of the world [8]. In Europe, the surveillance of EHEC infections is embedded in the Food and Waterborne Diseases and Zoonoses (FWD) surveillance system coor‐ dinated by the European Center for Disease Prevention and Control (ECDC). FWD is a passive surveillance system, collecting data on EHEC infections including laboratory‐confirmed cases, probable cases, and possible cases. Cases of HUS are specifically recorded through a network of pediatric nephrologists and infection‐control practitioners on the basis of clinical diagnosis.

### **2.2. Reservoir and route of transmission**

Both ETEC and EHEC infections are typically acquired through the ingestion of contami‐ nated food or water (**Figure 1**). However, a major difference between ETEC and EHEC is that ETEC only have a human reservoir of infection while EHEC are zoonotic pathogens [2, 9]. The main source of ETEC infection is contaminated water, such as surface water and drinking water (especially for weaning food) suffering from a lack of adequate sanitation and sewage facilities [2]. Nevertheless, a variety of food items including vegetables and herbs imported from endemic countries have also been recently implicated in uncommon sporadic cases or outbreaks in industrialized countries. Ruminants, especially cattle, are a natural reservoir of EHEC, and hence entry into the food chain through fecal contamination. Food (mainly under‐ cooked beef products, unpasteurized milk, and vegetable) and water are the principal sources of human contamination with EHEC. Person‐to‐person transmission of EHEC may signifi‐ cantly contribute to outbreaks from a primary source, whereas this mode of transmission is not likely under most circumstances for ETEC infection.

neurological complications [14]. The use of antimicrobials is also problematic during ETEC infection since an etiologic diagnosis cannot be made rapidly, mainly in childhood diarrhea [2]. Fluoroquinolones are shown to be effective during ETEC traveler's diarrhea [15] but should be used with caution due to the rise of antimicrobial resistance worldwide and the risk of side effects. For both pathogens, antimotility agents can be prescribed but need to be carefully administered as they can prolong the residence time of bacteria or their toxins in the intestine. In this context, alternative prophylactic or therapeutic strategies are currently under develop‐ ment for ETEC and EHEC. Vaccines against the pathogens are still not commercially avail‐ able, although vaccine strategies have been developed and used with variable success in animal models and/or humans [16, 17]. However, Dukoral®, a vaccine commercialized for *Vibrio cholerae*, can be prescribed to prevent traveler's diarrhea due to ETEC. Global alterna‐ tive approaches involving dietary supplementation or probiotics have also been considered for both ETEC [18, 19] and EHEC [20, 21], with various levels of evidence from *in vitro* and *in vivo* studies. Other therapeutic options targeting a specific step in bacterial pathogenesis have been developed, mainly for EHEC, such as the use of agents that link toxins or block their binding at the cell surface [13] or antibodies that inhibit the terminal complement com‐

Enterotoxigenic and Enterohemorrhagic *Escherichia coli*: Survival and Modulation of Virulence in the Human...

http://dx.doi.org/10.5772/intechopen.68309

7

After ingestion by humans, ETEC and EHEC pursue a strategy of infection involving coloni‐ zation of the intestinal mucosal surface and production of toxins. The main sites of coloniza‐ tion differ between the two pathogens: from the upper jejunum to the ileum for ETEC [23, 24] and terminal ileum and colon for EHEC [25–27]. Notably, EHEC show a preferential tropism to the follicle‐associated epithelium (FAE) of small intestinal Peyer's patches [25, 28], which has not been described for ETEC. Even if for both pathogens toxins are clearly identified as their main virulence factor, bacterial pathogenesis is not limited to toxin‐mediated effects, and a combination of virulence traits is required to make ETEC and EHEC strains fully patho‐ genic to humans. This part describes the main virulence factors that have been identified for

After being ingested, the pathogens must first breach the acidic barrier of the human stomach to reach their intestinal niche. It is well described that *E. coli* strains have intricate acid resis‐ tance (AR) systems that enable their survival in the harsh gastric environment, the glutamate‐ dependent AR system providing the highest level of acid protection [29]. Such acid resistance is a critical virulence trait of the infection, especially for EHEC for which the infectious dose

ETEC adhere to the intestinal epithelium by means of several colonization factors (CFs). More than 25 CFs that are antigenically and structurally diverse, have been identified in ETEC and

plex formation [22].

**2.4. Virulence factors**

ETEC and EHEC.

*2.4.1. Acid resistance*

is typically very low.

*2.4.2. Colonization factors*

The infective dose widely differs between ETEC and EHEC. It fluctuates between 108 and 1010 cells for ETEC in adults, but vulnerable populations such as infants may be susceptible to infection at lower doses [7, 10]. The infective dose for EHEC is recognized to be much lower: less than 50 to a few hundred organisms are usually sufficient to lead to the clinical signs [11].

### **2.3. Clinics and treatments**

ETEC or EHEC show similar clinical pictures at the beginning of infections: watery diar‐ rhea leading to rapid dehydration, usually associated with nausea, vomiting, and abdominal cramps [2, 11]. With regard to ETEC, following an incubation period of 10–72 hours, the dura‐ tion of illness is typically 3–5 days, and resolved usually without antimicrobial treatment, even though symptoms can persist for 2–3 weeks. ETEC infections are generally self‐limited and cannot be distinguished from Cholera on clinical grounds. Symptoms are much more severe in children from developing countries where diarrhea and malnutrition combine to form a vicious cycle leading to declining health status and death. Unlike ETEC, EHEC infections may evolve toward extra‐digestive complications. EHEC infections typically progress from watery to bloody diarrhea and resolve within a week or 10 days in the majority of infected individuals. Nevertheless, in 5–7% of cases, the infection may lead to life‐threatening compli‐ cations, namely HUS and thrombotic thrombocytopenic purpura (TTP), and death [11, 12]. HUS is characterized as a triad of acute kidney failure, microangiopathic hemolytic anemia, and thrombocytopenia, and remains the most common cause of acute renal failure in children in the EU and US. The elderly mostly develop TTP, which differs from HUS because of neu‐ rological symptoms including lethargy, severe headache, convulsions, and encephalopathy.

Currently, treatment for ETEC and EHEC infections consists primarily of supportive ther‐ apy, with oral rehydration to prevent dehydration and loss of electrolytes. For EHEC, gen‐ eral supportive measures also include peritoneal dialysis or hemodialysis and management of anemia with transfusion of whole blood or packed red cells [13]. Conventional antibiotic treatment is generally not recommended for EHEC‐infected patients as it increases HUS or neurological complications [14]. The use of antimicrobials is also problematic during ETEC infection since an etiologic diagnosis cannot be made rapidly, mainly in childhood diarrhea [2]. Fluoroquinolones are shown to be effective during ETEC traveler's diarrhea [15] but should be used with caution due to the rise of antimicrobial resistance worldwide and the risk of side effects. For both pathogens, antimotility agents can be prescribed but need to be carefully administered as they can prolong the residence time of bacteria or their toxins in the intestine.

In this context, alternative prophylactic or therapeutic strategies are currently under develop‐ ment for ETEC and EHEC. Vaccines against the pathogens are still not commercially avail‐ able, although vaccine strategies have been developed and used with variable success in animal models and/or humans [16, 17]. However, Dukoral®, a vaccine commercialized for *Vibrio cholerae*, can be prescribed to prevent traveler's diarrhea due to ETEC. Global alterna‐ tive approaches involving dietary supplementation or probiotics have also been considered for both ETEC [18, 19] and EHEC [20, 21], with various levels of evidence from *in vitro* and *in vivo* studies. Other therapeutic options targeting a specific step in bacterial pathogenesis have been developed, mainly for EHEC, such as the use of agents that link toxins or block their binding at the cell surface [13] or antibodies that inhibit the terminal complement com‐ plex formation [22].

## **2.4. Virulence factors**

and 1010

**2.2. Reservoir and route of transmission**

not likely under most circumstances for ETEC infection.

**2.3. Clinics and treatments**

Both ETEC and EHEC infections are typically acquired through the ingestion of contami‐ nated food or water (**Figure 1**). However, a major difference between ETEC and EHEC is that ETEC only have a human reservoir of infection while EHEC are zoonotic pathogens [2, 9]. The main source of ETEC infection is contaminated water, such as surface water and drinking water (especially for weaning food) suffering from a lack of adequate sanitation and sewage facilities [2]. Nevertheless, a variety of food items including vegetables and herbs imported from endemic countries have also been recently implicated in uncommon sporadic cases or outbreaks in industrialized countries. Ruminants, especially cattle, are a natural reservoir of EHEC, and hence entry into the food chain through fecal contamination. Food (mainly under‐ cooked beef products, unpasteurized milk, and vegetable) and water are the principal sources of human contamination with EHEC. Person‐to‐person transmission of EHEC may signifi‐ cantly contribute to outbreaks from a primary source, whereas this mode of transmission is

6 *Escherichia coli* Escherichia coli - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications - Recent Advances on Physiology, Pathogenesis and Biotechnological Applications

The infective dose widely differs between ETEC and EHEC. It fluctuates between 108

cells for ETEC in adults, but vulnerable populations such as infants may be susceptible to infection at lower doses [7, 10]. The infective dose for EHEC is recognized to be much lower: less than 50 to a few hundred organisms are usually sufficient to lead to the clinical signs [11].

ETEC or EHEC show similar clinical pictures at the beginning of infections: watery diar‐ rhea leading to rapid dehydration, usually associated with nausea, vomiting, and abdominal cramps [2, 11]. With regard to ETEC, following an incubation period of 10–72 hours, the dura‐ tion of illness is typically 3–5 days, and resolved usually without antimicrobial treatment, even though symptoms can persist for 2–3 weeks. ETEC infections are generally self‐limited and cannot be distinguished from Cholera on clinical grounds. Symptoms are much more severe in children from developing countries where diarrhea and malnutrition combine to form a vicious cycle leading to declining health status and death. Unlike ETEC, EHEC infections may evolve toward extra‐digestive complications. EHEC infections typically progress from watery to bloody diarrhea and resolve within a week or 10 days in the majority of infected individuals. Nevertheless, in 5–7% of cases, the infection may lead to life‐threatening compli‐ cations, namely HUS and thrombotic thrombocytopenic purpura (TTP), and death [11, 12]. HUS is characterized as a triad of acute kidney failure, microangiopathic hemolytic anemia, and thrombocytopenia, and remains the most common cause of acute renal failure in children in the EU and US. The elderly mostly develop TTP, which differs from HUS because of neu‐ rological symptoms including lethargy, severe headache, convulsions, and encephalopathy. Currently, treatment for ETEC and EHEC infections consists primarily of supportive ther‐ apy, with oral rehydration to prevent dehydration and loss of electrolytes. For EHEC, gen‐ eral supportive measures also include peritoneal dialysis or hemodialysis and management of anemia with transfusion of whole blood or packed red cells [13]. Conventional antibiotic treatment is generally not recommended for EHEC‐infected patients as it increases HUS or After ingestion by humans, ETEC and EHEC pursue a strategy of infection involving coloni‐ zation of the intestinal mucosal surface and production of toxins. The main sites of coloniza‐ tion differ between the two pathogens: from the upper jejunum to the ileum for ETEC [23, 24] and terminal ileum and colon for EHEC [25–27]. Notably, EHEC show a preferential tropism to the follicle‐associated epithelium (FAE) of small intestinal Peyer's patches [25, 28], which has not been described for ETEC. Even if for both pathogens toxins are clearly identified as their main virulence factor, bacterial pathogenesis is not limited to toxin‐mediated effects, and a combination of virulence traits is required to make ETEC and EHEC strains fully patho‐ genic to humans. This part describes the main virulence factors that have been identified for ETEC and EHEC.

### *2.4.1. Acid resistance*

After being ingested, the pathogens must first breach the acidic barrier of the human stomach to reach their intestinal niche. It is well described that *E. coli* strains have intricate acid resis‐ tance (AR) systems that enable their survival in the harsh gastric environment, the glutamate‐ dependent AR system providing the highest level of acid protection [29]. Such acid resistance is a critical virulence trait of the infection, especially for EHEC for which the infectious dose is typically very low.

### *2.4.2. Colonization factors*

ETEC adhere to the intestinal epithelium by means of several colonization factors (CFs). More than 25 CFs that are antigenically and structurally diverse, have been identified in ETEC and include fimbrial and fimbrillar structures. Among them, seven are generally more prevalent than others: CFA/I (colonization factor antigen) and CS1 to CS6 (*coli* surface antigen) [30]. Most CF receptors have not been yet identified, but CFs are thought to bind to glycopro‐ tein conjugates in mucus fraction from the small intestine and on the surface of host cells. Non‐fimbrial adhesins such as TibA, a glycosylated autotransporter; Tia, an outer membrane protein; and EtpA, which acts as a molecular bridge binding host cell receptors to the tips of ETEC flagella, have also been implicated in the pathogenesis [31].

Shiga toxins (Stx) are produced by EHEC in the lumen of the intestine, and then cross the epithelial barrier by poorly described mechanisms to eventually reach their target organs [38]. Two toxin families encoded in the genomes of lysogenic lambdoid phages are produced by the bacteria, namely Stx1 and Stx2, the latter being associated with the most severe complica‐ tions [39]. Stx contain two major structural subunits, A and B [40]. The B subunit binds to the toxin cellular receptor, globotriaosylceramide‐3 (Gb3), expressed on host microvascular endo‐ thelial cell surfaces (kidney, intestine, and brain). This explains the life‐threatening complica‐ tions associated with EHEC infections. The A subunit exhibits an RNA N‐glycosidase activity

Enterotoxigenic and Enterohemorrhagic *Escherichia coli*: Survival and Modulation of Virulence in the Human...

http://dx.doi.org/10.5772/intechopen.68309

9

Bacterial survival in the human GI tract is a key parameter in ETEC and EHEC physiopathol‐ ogy. Nevertheless, how pathogens can survive in the human digestive environment remains largely unknown as studies in humans are impossible. For regulatory, ethical, technical, and cost reasons, artificial digestive systems are increasingly used as an alternative to *in vivo* stud‐ ies in humans. Until now, almost no data are available for ETEC under human digestive simulated conditions while a number of studies have assessed the survival of EHEC during

Masters et al. [41] have shown that after exposure to pH 2, ETEC became undetectable by plate counting after 2 hours. A recent study using flow cytometry analysis indicated that there was no significant difference in the percentage of live bacteria when ETEC were subjected to pH 5 or pH 7 [42]. Only one study has investigated the impact of 30 g/L bile on the survival of ETEC *in vitro*. Despite the known bactericidal effect of bile in the intestine, growth curves for ETEC in Luria Bertani (LB) media and LB‐bile showed similar slopes during the exponential

With regard to EHEC, most of the studies have been carried out, like for ETEC, using simpli‐ fied *in vitro* approaches integrating a limited number of digestive parameters, such as acid pH or bile salts [44, 45]. Even if the pathogen is considered as acid resistant, large variations in survival rates have been obtained for *E. coli* O157:H7 in acidified culture media or in simu‐ lated gastric fluid [45, 46]. This wide range of response may be explained by differences in culture conditions, bacterial strains, and pH values used to simulate the gastric phase. Other more recent studies have evaluated the survival of EHEC strains by using dynamic multi‐ compartmental *in vitro* models that closely mimic the gastric, small intestinal, and colonic human digestive conditions. In the TNO GastroIntestinal model (TIM), which simulates the stomach and the three segments of the human small intestine, it has been shown that EHEC survival was affected in the stomach and duodenum (when ingested within a food matrix but not with a glass of water), while bacterial growth was observed at the end of digestion in the jejunum and ileum [47–49]. This growth renewal in the distal parts of the small intes‐ tine was probably linked to the occurrence of less stringent conditions, such as neutral pH and lower concentrations of bile salts due to their reabsorption (as occurred *in vivo*). EHEC survival in the TIM model was found to be strain/serotype dependent [48] and influenced

against the 28S rRNA, resulting in inhibition of protein synthesis and cell death.

**3. Bacterial survival in the human digestive tract**

human *in vitro* digestion.

growth phase [43].

EHEC colonization involves attaching and effacing (A/E) lesions on the enterocytes, which are characterized by ultra‐structural changes, including loss of microvilli and intimate attach‐ ment of the bacterium to the cell surface [32]. Genes encoding A/E lesion formation are local‐ ized on a pathogenicity island, the locus for enterocyte effacement (LEE), which encodes a bacterial type III secretion system (T3SS). Colonization is mainly mediated by the primary adhesin, namely intimin (encoded by *eae* gene), but other putative adherence factors have been described, such as long polar fimbriae—Lpf—or curli [33]. A number of other non‐fim‐ brial EHEC adhesins have been implicated in adhesion including the plasmid‐encoded *toxB*, the chromosomally encoded adhesins Iha, Cah, and OmpA [32, 33].

Mucin‐degrading enzymes, which allow temporary access to intestinal cell membrane and promote bacterial adhesion have been recently identified in both ETEC and EHEC. In ETEC, YghJ, a mucin‐binding metalloprotease [34] and EatA, a member of serine protease autotrans‐ porters of the Enterobacteriaceae (SPATE) family [35] have been described. In EHEC, one protein has been shown to have mucinase activity: StcE, an extracellular zinc metalloprotease which specifically recognizes α‐O‐glycan‐containing substrates [36].

### *2.4.3. Secretion of toxins*

Toxins are considered as the main virulence factor for both ETEC and EHEC as they are responsible for the main clinical symptoms and/or systemic complications. In ETEC, secre‐ tory diarrheas are mediated through the action of heat‐stable (ST) and/or heat‐labile (LT) enterotoxins.

ETEC strains are able to secrete either one or two toxins (LT and/or ST), but it has been shown that LT toxin is less likely to cause disease than ST or LT/ST ETEC toxins [7]. LT toxins encoded by the *eltAB* gene are similar in structure and function to Cholera toxin by sharing 80% homol‐ ogy. LT shows an AB5 configuration with a catalytically active LTA subunit and a pentameric ring of LTB subunits responsible for binding and internalization [37]. LT are mainly secreted associated with outer membrane vesicles (OMVs) and bind irreversibly to monoganglioside (GM1) on the host cell. LT leads to an increase in cAMP that induces cystic fibrosis transmem‐ brane regulator (CFTR) phosphorylation, eliciting massive fluid loss and watery diarrhea. In addition to causing diarrhea, LT plays multiple roles in modulating host cell function and providing a competitive advantage for ETEC adherence to cultured intestinal epithelial cells. ST toxins encoded by the *estAB* gene are small cysteine‐rich peptides which mimic the human hormone guanylin. They are divided into two structural and antigenically distinct groups: STa and STb which reversibly bind to guanylyl cyclase C (GC‐C) and sulphatide, respectively [37], leading to CFTR activation and diarrhea.

Shiga toxins (Stx) are produced by EHEC in the lumen of the intestine, and then cross the epithelial barrier by poorly described mechanisms to eventually reach their target organs [38]. Two toxin families encoded in the genomes of lysogenic lambdoid phages are produced by the bacteria, namely Stx1 and Stx2, the latter being associated with the most severe complica‐ tions [39]. Stx contain two major structural subunits, A and B [40]. The B subunit binds to the toxin cellular receptor, globotriaosylceramide‐3 (Gb3), expressed on host microvascular endo‐ thelial cell surfaces (kidney, intestine, and brain). This explains the life‐threatening complica‐ tions associated with EHEC infections. The A subunit exhibits an RNA N‐glycosidase activity against the 28S rRNA, resulting in inhibition of protein synthesis and cell death.
