STEPTRACKERMICROBIOLOGY | GRAM-POSITIVE RODS | HIGH-YIELD REVIEW

Bacillus, Listeria & Corynebacterium

Aerobic gram-positive rods: anthrax and fried-rice poisoning, the cold-loving foodborne pathogen of pregnancy, and the toxin of diphtheria.

FOCUS ON
B anthracis: spore-forming, polypeptide capsule (D-glutamate), "medusa head" colonies; cutaneous (painless black eschar), inhalational (hemorrhagic mediastinitis, wide mediastinum). B cereus: reheated rice — emetic (preformed cereulide, 1–5 h) or diarrheal. Listeria: tumbling motility, actin rockets, grows in cold → neonates, pregnancy, elderly, immunocompromised; deli meat, soft cheese → ampicillin. C diphtheriae: club-shaped, metachromatic granules, tellurite agar; pseudomembrane, myocarditis.
ORGANISM · DISEASE
FEATURES · VIRULENCE
CLINICAL PICTURE
CLUE · TREATMENT
Cutaneous anthrax
Spores enter skin; edema factor (adenylyl cyclase) + lethal factor (protease)
Painless papule → vesicle → black eschar with surrounding edema
Wool sorters, hides · ciprofloxacin or doxycycline
Inhalational anthrax
Spores to mediastinal nodes
Flu-like illness → widened mediastinum, hemorrhagic mediastinitis, shock, meningitis
Bioterrorism · fluoroquinolone + others, antitoxin
GI anthrax
Contaminated meat
Abdominal pain, bloody diarrhea, ascites
Rare
B cereus — emetic
Spores survive cooking; preformed heat-stable cereulide
Nausea, vomiting 1–5 h after reheated fried rice
"Reheated rice syndrome"
B cereus — diarrheal
Enterotoxin made in gut
Watery diarrhea 8–18 h later (meats, sauces)
Self-limited
Listeria — neonatal
Facultative intracellular; transplacental or at birth
Granulomatosis infantiseptica, early sepsis, late meningitis
Ampicillin + gentamicin
Listeria — pregnancy
Cell-mediated immunity ↓
Mild flu-like illness → miscarriage, stillbirth, preterm labor
Avoid unpasteurized cheese, deli meats
Listeria — adults
Grows at 4 °C; actin rockets spread cell-to-cell
Meningitis/bacteremia in elderly, immunosuppressed (steroids, transplant)
Add ampicillin to empiric meningitis therapy > 50 y
Listeria — healthy hosts
Foodborne outbreak
Febrile gastroenteritis
Self-limited
Diphtheria
Exotoxin (β-prophage) ADP-ribosylates EF-2
Sore throat, gray adherent pseudomembrane (bleeds if scraped), "bull neck" lymphadenopathy, myocarditis (arrhythmia), neuropathy
Tellurite agar (black), Löffler (metachromatic granules), Elek test · antitoxin + erythromycin/penicillin, DTaP
C diphtheriae shape
Club-shaped rods in "Chinese letter" / V arrangements
—
Metachromatic granules · toxin from β-prophage, ADP-ribosylates EF-2

Mechanism — Listeria spreads without leaving cells

IngestedCold-tolerant, survives refrigeration
→
Listeriolysin OEscapes phagosome
→
Actin rocketsPushes into neighbor cells
→
Avoids antibodiesNeeds T-cell immunity

That's why pregnancy, neonates, the elderly and steroid/transplant patients are at risk.

Food poisoning by incubation

ONSET
ORGANISM
FOOD
1–6 h, vomiting
S aureus, B cereus (emetic)
Mayonnaise, custard · fried rice
8–16 h, diarrhea
C perfringens, B cereus (diarrheal)
Reheated meat
Days, fever ± diarrhea
Listeria
Deli meat, soft cheese, melon

How it's tested

COMMON CONFUSIONAnthrax eschar is painless; a spider bite or ecthyma is painful. The capsule is protein, not polysaccharide.
THE TRAPListeria is intrinsically resistant to cephalosporins — empiric meningitis therapy for neonates and the elderly must include ampicillin.
YOUR APPROACHGram ⁺ rod: spores? (Bacillus, Clostridium) · tumbling motility? (Listeria) · club shape with granules? (Corynebacterium).

If you see… think…

Painless black eschar in a wool or hide worker → cutaneous anthrax
Widened mediastinum after flu-like illness → inhalational anthrax
Vomiting 2 h after reheated fried rice → B cereus (emetic toxin)
Pregnant woman with fever after soft cheese → Listeria
Tumbling motility, grows in the fridge → Listeria
Meningitis in a 70-year-old on steroids → add ampicillin (Listeria)
Gray pseudomembrane, bull neck, unvaccinated child → diphtheria
Death in diphtheria from → myocarditis / airway obstruction
Metachromatic granules on Löffler medium → C diphtheriae
Bacterium with a polypeptide capsule → B anthracis