escherichia coli

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    • Escherichia coli
      • part of the normal flora of the colon in humans
      • has fimbriae or pili that are important for adherence to host mucosal surfaces
      • Most strains can ferment lactose (that is, they are Lac+) in contrast to the major intestinal pathogens, (MCQ)
        • Salmonella and Shigella cannot ferment lactose (that is, they are Lac–) (MCQ)
      • E. coli produces both acid and gas during fermentation of carbohydrates.
      • E. coli shares many properties with the other Enterobacteriaceae. (MCQ)
        • all are facultative anaerobes
        • all ferment glucose
        • all can generate energy by aerobic or anaerobic respiration (using nitrate, nitrite, or fumarate as terminal electron acceptors).
        • 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
      • O antigens (somatic or cell wall antigens)
        • found on the polysaccharide portion of the LPS(MCQ)
        • heat stable
        • shared among different Enterobacteriaceae genera
        • commonly used to serologically type many of the enteric gram-negative rods. (MCQ)
      • H antigens
        • are associated with flagella
        • only flagellated (motile) Enterobacteriaceae such as E. coli have H antigen. (MCQ)
      • K antigens
        • Located within the polysaccharide capsules. (MCQ)
      • A serotype of E. coli possessing O157 and H7 (designated O157:H7) causes a severe form of hemorrhagic colitis (MCQ)
    • Clinical significance: intestinal disease
      • Transmission of intestinal disease is commonly by the fecal–oral route
      • contaminated food and water serving as vehicles for transmission.
      • five types of intestinal infections
        • entero- toxigenic (ETEC)
        • entero pathogenic (EPEC)
        • entero hemorrhagic (EHEC)
        • entero invasive (EIEC),
        • entero aggregative (EAEC)
      • EHEC E. coli infection should be suspected in all patients with acute bloody diarrhea, particularly if associated with abdominal tenderness and absence of fever. (A Very important MCQ)
      • Enterotoxigenic E. coli:
        • a common cause of traveler’s diarrhea. (MCQ)
        • Transmission occurs through
          • food and water contaminated with human waste
          • by person-to-person contact.
        • colonize the small intestine
        • pili facilitate the binding of the organism to the intestinal mucosa
        • enterotoxins
          • cause prolonged hypersecretion of chloride ions and water by the intestinal mucosal cells
          • inhibit the reabsorption of sodium. (MCQ)
          • result in significant watery diarrhea
          • heat-stable toxin (ST) (MCQ)
            • works by causing an elevation in cellular cGMP  levels
          • heat-labile toxin (LT) (MCQ)
            • causes elevated cAMP
            • LT is essentially identical to cholera toxin
      • Enteropathogenic E. coli:
        • an important cause of diarrhea in infants(MCQ)
        • newborn becomes infected perinatally
        • occurs especially in locations with poor sanitation
        • attach to mucosal cells in the small intestine by use of bundle-forming pili (BfpA).
        • Characteristic lesions in the small intestine
          • attaching and effacing lesions (A/E) (MCQ)
          • destruction of the microvilli
          • caused by injection of effector proteins into the host cell by way of a type III secretion system (T3SS).
        • EPEC are not invasive and, thus, do not cause bloody diarrhea. (MCQ)
        • Toxins are not elaborated by EPEC strains.
        • Watery diarrhea results
      • Enterohemorrhagic E. coli:
        • produce one of two exotoxins (Shiga-like toxins 1 or 2),
        • result in a severe form of copious, bloody diarrhea (MCQ) (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) (MCQ)
          • characterized by fever, acute renal failure, microangiopathic hemolytic anemia and throm- bocytopenia
          • occur in children younger than ages 5 to 10 years
        • The primary reservoir of EHEC is cattle
          • possibility of infection can be greatly decreased by thoroughly cooking ground beef and pasteurizing milk. (MCQ)
      • Enteroinvasive E. coli
        • cause a dysentery-like syndrome with fever and bloody stools (MCQ)
        • Plasmid-encoded virulence factors are nearly identical to those of Shigella species(MCQ)
        • These virulence factors allow the
          • invasion of epithelial cells (Ipa) (MCQ)
          • intercellular spread by use of actin-based motility
        • In addition, EIEC strains produce a hemolysin (HlyA) (MCQ)
      • 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(MCQ)
      • The adherent rods resemble stacked bricks (MCQ)
      • result in shortening of microvilli
      • EAEC strains produce a heat-stable toxin that is plasmid encoded.
    • 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 non- pathogenic in the intestine
      • it causes disease in that individual when the organism is found, for example, in the bladder or bloodstream (normally sterile sites).
      • Urinary tract infection
        • E. coli is the most common cause of urinary tract infection (UTI), including cystitis and pyelonephritis. (MCQ)
        • Women are particularly at risk for infection.
          • Uncomplicated cystitis
            • It is the most commonly encountered UTI
            • It is caused by uropathogenic strains of E. coli, characterized by (MCQ)
              • P fimbriae (an adherence factor)
              • hemolysin, colicin V
              • resistance to the bactericidal activity of serum complement. (MCQ)
          • Complicated UTI (pyelonephritis)
            • occur in settings of obstructed urinary flow (MCQ)
            • caused by nonuropathogenic strains.
      • Neonatal meningitis:
        • E. coli is a major cause
        • The K1 capsular antigen, which is chemically identical to the polysaccharide capsule of group B Neisseria meningitidis, is particularly associated with such infections. (MCQ)
      • Nosocomial (hospital-acquired) infections:
        • sepsis/bacteremia, endotoxic shock, and pneumonia.
    • Laboratory identification
      • 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 (MCQ)
          • detected on media such as MacConkey agar
        • EHEC
          • unlike most other strains of E. coli, ferment sorbitol slowly, if at all,
          • detected on MacConkey sorbitol agar. (MCQ)
          • polymerase chain reaction is used  to identify E. coli strains producing Shiga-like toxins.
      • Extraintestinal disease:
        • Isolation of E. coli from normally sterile body sites (for example, the bladder or cerebrospinal fluid) is diagnostically significant

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