questions 5

Escherichia coli

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Topic updated on 08/13/17 10:06am

Introduction
 
  • Classification
    • bacteria
      • gram-negative bacilli
        • the enterics
          • facultative anaerobes
            • lactose fermenting
              • Escherichia
                • E. coli
Pathogenesis
  • Transmission
    • endogenous
    • fecal-oral
    • contaminated food
      • undercooked meat
  • Reservoir
    • normal colonic flora
    • cow feces
  • Molecular biology
    • normal flora acquires virulence factors
      • pili 
        • P-fimbrae virulence factor is most important for colonization of urinary tract
      • exotoxin
        • LT
        • ST
        • shiga-like
      • endotoxin
        • lipid A part of LPS
      • K capsule  
        • K-1 capsular antigen allows meningeal infection
      • iron-binding siderophore
    • molecular mechanism of virulence depends on strain (see below)
Laboratory
  • Oxidase negative
  • Most E. coli ferment sorbitol
    • except EHEC which usually does not
  • Distinguish pathogenic strains via
    • immunoassay
    • serotyping
    • DNA probes
    • PCR
Clinical Conditions
  • Diarrhea
    • severity of diarrhea depends on strain ("PITcH")
    • virulence factors are shared between VibrioE. coli, and Shigella
      • EPEC ("pediatric")
        • second most common cause of diarrhea in infants
        • adherence to M cells prevents absorption
          • no toxin produced
        • developing countries
      • EIEC ("inflammatory")
        • bacteria invade epithelial cells
          • invasion and toxin act synergistically
        • virulence factor is shared by Shigella
        • blood and WBC in stool
          • like Shigellosis
      • ETEC ("traveler") 
        • LT (heat labile) toxin
          • similar to cholera toxin
          • ↑ cAMP
        • ST (heat stable) toxin
        • secretion of Cl- and HCO3- into intestinal lumen results in rice-water stools resembling those seen in cholera
      • EHEC ("hamburger")
        • Shiga-like toxin (verotoxin)
          • inhibits protein synthesis by inhibiting 60S ribosome 
        • no invasion of epithelium
          • virulence is only toxin-mediated
        • blood in stool
          • but no WBCs because no invasion
        • hemolytic uremic syndrome (HUS)
          • O157:H7 strain associated
          • classic triad
            • anemia
            • thrombocytopenia
            • acute renal failure
          • swelling of endothelium narrows lumen leading to
            • hemolysis
            • ↓ renal blood flow
            • ↓ platelets as damage is repaired
          • O157:H7 strain does not ferment sorbitol nor does it produce glucuronidase (vs. most other E. coli strains)
      • EAEC 
        • enteroaggregative E. coli
        • has the ability to adhere to intestinal cells
        • cause of traveler's diarrhea to less-developed countries
  • UTI
    • most common cause of UTI
    • acquisition of pili allows motility up the GU tract
    • ascending infection can cause pyelonephritis
  • Neonatal meningitis
    • second most common cause (vs. group B strep)
    • acquisition of K capsule inhibits phagocytosis to allow spread to meninges 
  • Sepsis
    • most common cause of Gram-negative sepsis
    • hospitalized patients
  • Pneumonia
    • hospital acquired
    • in neonates
  • Cystitis
  • Pyelonephritis
Treatment for Common Clinical Conditions Caused by E.Coli
EPEC diarrhea Supportive
EIEC diarrhea -----
ETEC diarrhea Rehydration with TMP/SMX
EHEC diarrhea Supportive (do not use antibiotics - they may cause HUS)
UTI Fluoroquinolones or sulfonamides
Neonatal meningitis Ceftriaxone
Sepsis Fluoroquinolones or 3rd generation cephalosporins



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Qbank (2 Questions)

TAG
(M1.MC.13) A 27-year-old male suddenly develops severe abdominal cramping and bloody diarrhea. The patient reports consuming undercooked ground beef four days prior to the onset of the symptoms. Which of the following best describes the toxin-mediated mechanism of this disease process? Topic Review Topic

1. Depolymerization of actin filaments in gastrointestinal mucosal cells, leading to mucosal cell death
2. Increased pH of gastrointestinal lumen resulting in reduced mucosal absorption
3. Increased intracellular cAMP in gastrointestinal mucosal cells, resulting in decreased absorption and increased secretion in the digestive tract
4. Inhibition of elongation factor-2 (EF-2), resulting in decreased protein synthesis in gastrointestinal mucosal cells
5. Inhibition of the 60S ribosomal subunit, resulting in decreased protein synthesis in gastrointestinal mucosal cells

PREFERRED RESPONSE ▶
TAG
(M1.MC.75) A 23-year-old woman goes to a walk-in clinic while on the fourth day of her honeymoon. She is very upset saying that her honeymoon is being ruined because she is in severe pain. She states that yesterday she began to experience severe pain with urination and seems to be urinating more frequently than normal. She does admit that she has been having increased sexual intercourse with her new husband while on their honeymoon. The physician diagnoses the patient and prescribes trimethoprim-sulfamethoxazole. Which of the following virulence factors is most likely responsible for this patient's infection? Topic Review Topic

1. LPS endotoxin
2. K capsule
3. P fimbriae
4. Flagella
5. Exotoxin

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