Corynebacterium Details - The Club‑Shaped Culprit
- Morphology: Gram +ve, non-motile, non-sporing bacilli; club-shaped (V/L, Chinese letter, palisade arrangements).
- Babes-Ernst granules (metachromatic, volutin; Albert's/Ponder's stain).
- C. diphtheriae Strains: Gravis, Mitis, Intermedius.
- Diphtheria Toxin:
- A-B exotoxin (heat-labile); tox gene from β-phage (lysogenic conversion).
- Mechanism: ADP-ribosylation of Elongation Factor 2 (EF-2) → inhibits protein synthesis.
- Pathogenesis & Complications:
- Local: Greyish-white adherent pseudomembrane (pharynx, larynx) → respiratory obstruction, "bull neck".
- Systemic (toxin-mediated): Myocarditis, neuropathy (palatal palsy), acute tubular necrosis.
- 📌 Mnemonic (ABCDEF): Airway obstruction, Bull neck, Cardiac, Deglutition, Eye, Facial/Peripheral neuropathy.
- Lab Diagnosis:
- Specimen: Throat swab.
- Stains: Albert's (granules green), Ponder's (blue-black granules). Gram stain.
- Culture: Loeffler's serum slope, Potassium Tellurite agar (PTA; black colonies).
- Toxigenicity: Elek's gel precipitation test. PCR for tox gene.
⭐ The Elek gel precipitation test is the gold standard for confirming toxigenicity of C. diphtheriae.
- Treatment: Diphtheria Antitoxin (DAT) + Antibiotics (Penicillin/Erythromycin). Airway support.
- Prevention: Active immunization with Toxoid vaccine (DPT, Tdap, Td).
Listeria Monocytogenes - The Tumbling Invader
- Morphology: Gram-positive bacillus/coccobacillus. Non-spore-forming. Catalase-positive, Oxidase-negative.
- Key Features:
- Motility: Characteristic "tumbling" motility at 20-25°C. Intracellularly, uses actin rockets (via ActA protein) for direct cell-to-cell spread.
- Growth: Facultative anaerobe. Grows at 4°C (refrigerator temp), enabling "cold enrichment" for isolation.
- Motility: Characteristic "tumbling" motility at 20-25°C. Intracellularly, uses actin rockets (via ActA protein) for direct cell-to-cell spread.
- Virulence Factors:
- Internalins (InlA, InlB): Mediate invasion into host cells.
- Listeriolysin O (LLO): Pore-forming toxin (β-hemolysin); crucial for phagosomal escape.
- ActA: Induces host actin polymerization for intracellular movement.
- Phospholipases (PlcA, PlcB): Aid phagosomal escape and cell spread.
- Pathogenesis (Facultative Intracellular):
- Clinical Syndromes: 📌 Risk Groups: Pregnant, Elderly, Neonates, Immunocompromised (PENI).
- Neonatal: Meningitis, sepsis (granulomatosis infantiseptica).
- Immunocompromised/Elderly: Meningitis, meningoencephalitis, sepsis.
- Pregnant: Flu-like illness, abortion, stillbirth.
- Healthy: Usually self-limiting febrile gastroenteritis.
⭐ Listeria is a key foodborne pathogen; notably crosses placental barrier (fetal infections) and blood-brain barrier (meningitis).
- Lab Diagnosis: Culture (blood/CSF); cold enrichment at 4°C. CAMP test positive (block-type hemolysis with S. aureus). Tumbling motility at 25°C.
- Treatment: Ampicillin is drug of choice, often with Gentamicin for synergy.
Comparative Features - Differentiating Duo
| Feature | Corynebacterium | Listeria monocytogenes |
|---|---|---|
| Morphology | GPR, club-shaped, V/L forms ("Chinese letters") | GPR, coccobacillary, short chains |
| Granules | Metachromatic (Babes-Ernst); Albert's stain | Absent |
| Motility | Non-motile | Tumbling at 22-28°C; actin "rocket tails" |
| Growth Temp (°C) | 37 | 30-37; grows at 4 (cold enrichment) |
| Key Toxin & Mech. | Diphtheria toxin (ADP-ribosylates EF-2) | Listeriolysin O (LLO), ActA (actin polymerization) |
| Key Diseases | Diphtheria (pharyngeal, cutaneous), endocarditis | Listeriosis (meningitis, sepsis), neonatal infections, gastroenteritis |
| Lab Dx Highlights | Albert's stain, Tellurite agar (black colonies) | Cold enrichment, CAMP test (+), tumbling motility, umbrella motility |
| At-risk Pops. | Unimmunized individuals | Pregnant women, neonates, elderly, immunocompromised |
| Treatment | Antitoxin + Penicillin/Erythromycin | Ampicillin ± Gentamicin |
High‑Yield Points - ⚡ Biggest Takeaways
- C. diphtheriae causes diphtheria; its exotoxin ADP-ribosylates EF-2, halting protein synthesis.
- Key signs: pharyngeal pseudomembrane, bull neck; diagnose with Albert's stain (metachromatic granules).
- Culture on Loeffler's serum slope / tellurite agar; Elek's test for toxigenicity.
- L. monocytogenes: characteristic tumbling motility (25°C) and actin rockets for intracellular spread.
- Causes meningitis/sepsis in neonates, pregnant women, immunocompromised; grows at 4°C (cold enrichment).
- Transmitted by contaminated food (e.g., unpasteurized dairy); treat with ampicillin.
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