gram-positive rods
Summary
Gram-positive rods (bacilli) are bacteria that retain crystal violet stain due to a thick peptidoglycan cell wall and have a rod-shaped morphology. Key pathogenic genera include Bacillus, Clostridium, Corynebacterium, Listeria, and Actinomyces, causing diseases ranging from anthrax to tetanus to listeriosis.
Detail
Gram-positive rods are classified by spore formation and oxygen requirements, which is high-yield for boards:
SPORE-FORMING: - Bacillus anthracis (aerobic): causes anthrax (cutaneous, inhalational, GI); produces edema toxin and lethal toxin; boxcar-shaped chains - Bacillus cereus: food poisoning (reheated rice) - emetic (preformed toxin) or diarrheal form - Clostridium species (anaerobic, obligate): - C. tetani - tetanospasmin blocks glycine/GABA release → spastic paralysis, trismus - C. botulinum - preformed toxin blocks ACh release → flaccid paralysis - C. perfringens - alpha toxin (lecithinase) → gas gangrene, myonecrosis - C. difficile - pseudomembranous colitis, toxins A/B, associated with antibiotic use (clindamycin classically)
NON-SPORE-FORMING: - Corynebacterium diphtheriae - exotoxin (ADP-ribosylation, inhibits EF-2) causes pseudomembranous pharyngitis, myocarditis; club-shaped, metachromatic granules, Chinese letter arrangement - Listeria monocytogenes - facultative intracellular, motile via actin rockets; causes meningitis in neonates/elderly/immunocompromised, unpasteurized dairy, transplacental transmission causing granulomatosis infantiseptica - Actinomyces israelii - anaerobic, forms sulfur granules, causes oral/facial abscesses with draining sinus tracts, sensitive to penicillin (vs Nocardia which is partially acid-fast and treated with sulfonamides)
Clinical pearls: Spore-forming rods survive in harsh environments (soil, GI tract) making them relevant in wound infections and food-borne illness. Toxin-mediated diseases (tetanus, botulism, diphtheria) are classic USMLE topics requiring knowledge of toxin mechanism and clinical presentation. Gram stain morphology helps narrow differential rapidly in clinical microbiology.
Sources
- First Aid for the USMLE Step 1
- Sketchy Microbiology
- Levinson's Review of Medical Microbiology and Immunology
- Harrison's Principles of Internal Medicine
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