Escherichia coli is part of the normal flora of the colon in humans and other animals but can be pathogenic both within and outside of the GI tract. [Note: The differences in the degree of virulence of various E. coli strains is correlated with the acquisition of plasmids, integrated prophages, and pathogenicity islands.] E. coli has fimbriae or pili that are important for adherence to host mucosal surfaces, and different strains of the organism may be motile or nonmotile. Most strains can ferment lactose (that is, they are Lac⁺) in contrast to the major intestinal pathogens, Salmonella and Shigella, which cannot ferment lactose (that is, they are Lac ^–). E. coli produces both acid and gas during fermentation of carbohydrates.

A. Structure and physiology

 E. coli shares many properties with the other Enterobacteriaceae. They are all facultative anaerobes , they all ferment glucose, and they all can generate energy by aerobic or anaerobic respiration (using nitrate, nitrite, or fumarate as terminal electron acceptors). They all lack cytochrome c oxidase (that is, they are oxidase negative). Typing strains is based on differences in three structural antigens: O, H, and K (Figure 1). The O antigens (somatic or cell wall antigens) are found on the polysaccharide portion of the LPS. These antigens are heat stable and may be shared among different Enterobacteriaceae genera. O antigens are commonly used to serologically type many of the enteric gram-negative rods. The H antigens are associated with flagella, and, therefore, only flagellated (motile) Enterobacteriaceae such as E. coli have H antigen. The K antigens are located within the polysaccharide capsules. Among E. coli species, there are many serologically distinct O, H, and K anti gens, and specific serotypes are associated with particular diseases. For example, a serotype of E. coli possessing O157 and H7 (designated O157:H7) causes a severe form of hemorrhagic colitis.

Fig1. Electron micrograph of Escherichia coli showing virulence factors.

B. Clinical significance: intestinal disease

Transmission of intestinal disease is commonly by the fecal–oral route, with contaminated food and water serving as vehicles for transmission. At least five types of intestinal infections that differ in pathogenic mechanisms have been identified (Figure 2): enterotoxigenic (ETEC), enteropathogenic (EPEC), enterohemorrhagic (EHEC), enteroinvasive (EIEC), and enteroaggregative (EAEC). E. coli all are basically the same organism, differing only by the acquisition of specific pathogenic traits. EHEC E. coli infection should be suspected in all patients with acute bloody diarrhea, particularly if associated with abdominal tenderness and absence of fever.

Fig2. Characteristics of intestinal infections caused by Eschericia coli. Fluoroquinolones are commonly used in adults for traveler’s diarrhea but are not recommended for children. 1Rehydration and correction of electrolyte abnormalities are essential for all diarrheal illnesses. 2Rifaximin is approved for the treatment of diarrhea caused by noninvasive strains of E. coli in patients age12 years and older. Rifaximin is a nonabsorable, gastrointestinal-selective, oral antibiotic.

1. Enterotoxigenic E. coli: ETEC are a common cause of traveler’s diarrhea. Transmission occurs through food and water contaminated with human waste or by person-to-person contact. ETEC colonize the small intestine (pili facilitate the binding of the organism to the intestinal mucosa). In a process mediated by enterotoxins, ETEC cause prolonged hypersecretion of chloride ions and water by the intestinal mucosal cells, while inhibiting the reabsorption of sodium. The gut becomes full of fluid, resulting in significant watery diarrhea that continues over a period of several days. Enterotoxins include a heat-stable toxin (ST) that works by causing an elevation in cellular cyclic guanosine monophosphate (cGMP) levels, whereas a heat-labile toxin (LT) causes elevated cyclic adenosine monophosphate (cAMP) (Figure 3). [Note: LT is essentially identical to cholera toxin .]

Fig3. The action of Escherichia coli LT (heat-labile toxin). [Note: ST (heat-stable toxin) activates guanylate cyclase, causing production of cyclic guanosine monophosphate (cGMP) that also causes secretion.]

 2. Enteropathogenic E. coli: EPEC are an important cause of diarrhea in infants, especially in locations with poor sanitation. The newborn becomes infected perinatally. The EPEC attach to mucosal cells in the small intestine by use of bundle-forming pili (BfpA). Characteristic lesions in the small intestine called attaching and effacing lesions (A/E), in addition to destruction of the microvilli, are caused by injection of effector proteins into the host cell by way of a type III secretion system (T3SS). EPEC cells are presented at the apex of pedestals elicited by dramatic cytoskeletal rearrangements, induced by the T3SS effectors. EPEC are not invasive and, thus, do not cause bloody diarrhea. Toxins are not elaborated by EPEC strains. Watery diarrhea results, which, on rare occasions, may become chronic.

3. Enterohemorrhagic E. coli :EHEC bind to cells in the large intestine via BfpA and, similar to EPEC, produce  A/E lesions. However, in addition ,EHEC produce one of two exotoxins (Shiga-like toxins 1 or 2), resulting in a severe form of copious, bloody diarrhea (hemorrhagic colitis) in the absence of mucosal invasion or inflammation. Serotype O157:H7 is the most common strain of E. coli that produce Shiga-like toxins. This strain is also associated with outbreaks of a potentially life-threatening, acute renal failure (hemolytic uremic syndrome, or HUS) characterized by fever, acute renal failure, microangiopathic hemolytic anemia and thrombocytopenia in children younger than ages5 to 10years. The primary reservoir of EHEC is cattle. Therefore, the possibility of infection can be greatly decreased by thoroughly cooking ground beef and pasteurizing milk.

 4. Enteroinvasive E. coli :EIEC cause a dysentery-like syndrome with fever and bloody stools. Plasmid-encoded virulence factors are nearly identical to those of Shigellaspecies. These virulence factors allow the invasion of epithelial cells (Ipa) and intercellular spread by use of actin-based motility. In addition, EIEC strains produce a hemolysin (HlyA).

5. Enteroaggregative E. coli: EAEC also cause traveler’s diarrhea and persistent diarrhea in young children. Adherence to the small intestine is mediated by aggregative adherence fimbriae. The adherent rods resemble stacked bricks and result in shortening of microvilli. EAEC strains produce a heat-stable toxin that is plasmid encoded. An outbreak of E. coli infections in Germany in 2011, resulting in many cases of HUS and several deaths, was caused by a hybrid strain. The causative agent was an EAEC strain that had acquired the phage-encoded gene to produce Shiga-like toxin 2. The resulting strain was capable of tight adher ence to the small intestine in addition to toxin production, which resulted in HUS.

 C. Clinical significance: extraintestinal disease

 The source of infection for extraintestinal disease is frequently the patient's own flora, in which the individual’s own E. coli is nonpathogenic in the intestine. However, it causes disease in that individual when the organism is found, for example, in the bladder or bloodstream (normally sterile sites).

 1. Urinary tract infection: E. coli is the most common cause of urinary tract infection (UTI), including cystitis and pyelonephritis. Women are particularly at risk for infection. Uncomplicated cystitis (the most commonly encountered UTI) is caused by uropathogenic strains of E. coli, characterized by P fimbriae (an adherence factor) and, commonly, hemolysin, colicin V, and resistance to the bactericidal activity of serum complement. Complicated UTI (pyelonephritis) may occur in settings of obstructed urinary flow, which may be caused by nonuro pathogenic strains.

 2. Neonatal meningitis: E. coli is a major cause of this disease occurring within the first month of life. The K1 capsular antigen, which is chemically identical to the polysaccharide capsule of group B Neisseria meningitidis, is particularly associated with such infections.

 3. Nosocomial (hospital-acquired) infections: These include sepsis/bacteremia, endotoxic shock, and pneumonia.

D. Laboratory identification

 1. Intestinal disease: Because E. coli is normally part of the intestinal flora, detection in stool cultures of disease-causing strains is generally difficult. EIEC strains often do not ferment lactose and may be detected on media such as MacConkey agar . EHEC, unlike most other strains of E. coli, ferment sorbitol slowly, if at all, and may be detected on MacConkey sorbitol agar. Current molecular techniques, such as polymerase chain reaction, may be employed to identify E. coli strains producing Shiga-like toxins.

2. Extraintestinal disease: Isolation of E. coli from normally sterile body sites (for example, the bladder or cerebrospinal fluid) is diagnostically significant. Specimens may be cultured on MacConkey agar. Strains of E. coli can be further characterized on the basis of serologic tests.

 E. Prevention and treatment

 Intestinal disease can best be prevented by care in selection, preparation, and consumption of food and water. Maintenance of fluid and electrolyte balance is of primary importance in treatment. Antibiotics may shorten duration of symptoms, but resistance is nevertheless widespread. Extraintestinal diseases require antibiotic treatment (Figure 4). Antibiotic sensitivity testing of isolates is necessary to determine the appropriate choice of drugs.

Fig4. Summary of Escherichia species. 1 Indicates first-line drugs.

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