BACTERIA · Gram-negative rods (Enterobacteriaceae)
Escherichia coli
Escherichia coli — Gram stain
Stain: Gram stainMorphology: Gram-negative bacilli (rods)Yield: HIGHDifficulty: EASY
Image: Wikimedia Commons · File:Escherichia_coli_Gram_Stain.jpg · Dr Graham Beards (CC BY-SA 4.0)
Key facts
**Pathogenesis**: Strain-defined virulence — UPEC (P-fimbriae → UTI), ETEC (LT/ST toxins → traveler's diarrhea), EHEC O157:H7 (Shiga-like toxin → HUS, no fever), EIEC (mucosal invasion). Lipid A of LPS → septic shock. **Diagnostic clue**: Lactose-fermenter (pink on MacConkey), indole-positive, EHEC does NOT ferment sorbitol (SMAC agar). **Virulence**: Capsular K antigen, flagellar H antigen, fimbriae, exotoxins.
Boards buzzwords
- lactose fermenter
- pink on MacConkey
- metallic green sheen on EMB
- indole-positive
- P-fimbriae
- Shiga-like toxin
Associated diseases
- UTI (#1 cause)
- Neonatal meningitis (K1 capsule)
- Gram-negative sepsis
- Traveler's diarrhea (ETEC)
- Hemolytic uremic syndrome (EHEC O157:H7)
- Nosocomial pneumonia
Treatment
UTI → nitrofurantoin, TMP-SMX, or fosfomycin; sepsis → ceftriaxone, pip-tazo, or carbapenem (ESBL/CRE); EHEC HUS — supportive care only, AVOID antibiotics
Related organisms
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Spirochete
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Gram-negative diplococci