Escherichia coli

Quick Profile

• Classification: Gram-negative rod, facultative anaerobe, lactose fermenter
• Key labs: Pink colonies on MacConkey agar (lactose fermenter), IMViC pattern (indole +, methyl red +, Voges-Proskauer −, citrate −)
• Normal flora: large intestine; pathogenic when introduced to other sites or when carrying virulence factors
• Clinical style: Multiple distinct pathotypes with very different disease mechanisms

Identification And Classification

• Organism family: Enterobacteriaceae
• Lactose fermenter (pink on MacConkey) distinguishes from Salmonella and Shigella
• Serotyping based on O antigen (somatic), H antigen (flagellar), and K antigen (capsular)

Reservoir And Transmission

• Human and animal GI tract
• UTI: ascending colonization from perineum
• Diarrheal disease: fecal-oral route, contaminated food or water
• Neonatal meningitis: vertical transmission during delivery (K1 capsule)

Virulence Factors And Pathogenesis By Pathotype

Uropathogenic E. coli (UPEC)

• Pili (P fimbriae) bind uroepithelial cells
• Most common cause of community-acquired UTI

Enterotoxigenic E. coli (ETEC)

• Heat-labile toxin (LT) and heat-stable toxin (ST) cause secretory diarrhea
• Traveler’s diarrhea; watery, non-bloody

Enterohemorrhagic E. coli (EHEC) — O157:H7

• Shiga-like toxin (verotoxin) inhibits 60S ribosome (same mechanism as Shiga toxin)
• Toxin damages renal endothelium → hemolytic uremic syndrome (HUS)
• Bloody diarrhea without fever; hamburger-associated outbreaks
• Do not give antibiotics: may increase toxin release and HUS risk

Enteroinvasive E. coli (EIEC)

• Invades colonic mucosa like Shigella
• Bloody, inflammatory diarrhea with fever

Enteropathogenic E. coli (EPEC)

• Effacement of intestinal brush border without invasion
• Watery diarrhea in infants; no toxin production

Neonatal Meningitis E. coli (K1 strain)

• K1 polysaccharide capsule confers resistance to phagocytosis and complement
• Leading gram-negative cause of neonatal meningitis

High-Yield Disease Patterns

• Cystitis: dysuria, frequency, urgency, pyuria, bacteriuria; no systemic symptoms
• Pyelonephritis: costovertebral angle tenderness, fever, flank pain, bacteremia
• Traveler’s diarrhea: watery diarrhea 1–2 days after ingestion of contaminated food
• HUS triad: microangiopathic hemolytic anemia, thrombocytopenia, acute kidney injury following bloody diarrhea

Diagnosis

• Urinalysis and urine culture for UTI (>100,000 CFU/mL)
• Stool culture for diarrheal disease; O157:H7 requires sorbitol-MacConkey agar (non-fermenter)
• Blood cultures for bacteremia and sepsis
• CSF culture and Gram stain for neonatal meningitis

Treatment And Resistance

UTI

• Uncomplicated cystitis: TMP-SMX or nitrofurantoin; fluoroquinolones reserved for complicated cases
• Pyelonephritis: fluoroquinolone or ceftriaxone; adjust based on culture

ETEC (Traveler’s Diarrhea)

• Fluid replacement; azithromycin or fluoroquinolone shortens duration
• Prevention: bismuth subsalicylate, food safety precautions

EHEC

• Supportive care only; antibiotics and anti-motility agents are contraindicated
• Monitor renal function closely for HUS

Neonatal Meningitis

• Ampicillin plus cefotaxime empirically; duration typically 21 days

For antibiotic mechanisms, see Pharmacology: Antimicrobials.

Exam Traps

• Do not give antibiotics for EHEC O157:H7; this increases Shiga toxin release and risk of HUS
• Nitrofurantoin does not achieve adequate tissue levels and cannot treat pyelonephritis
• Sorbitol-MacConkey agar: O157:H7 is a non-fermenter (colorless), unlike other E. coli strains