⭐ THE 20% THAT GIVES 80% MARKS (SUPER HIGH-YIELD)

1. Organism:

Listeria monocytogenes — Gram-positive bacillus, beta-hemolytic, intracellular, motile at room temp.

2. Key virulence:
• Listeriolysin O → escapes phagosome.
• Actin rockets (actin polymerization) → cell-to-cell spread (NO antibodies needed).
3. Transmission:
• Unpasteurized milk/cheese (soft cheese)
• Deli meats
• Refrigerated foods (grows at 4°C!)
• Transplacental & during birth
4. High-risk groups:
• Pregnant women
• Neonates
• Elderly
• Immunocompromised
5. Diseases:
• Pregnancy: flu-like illness, miscarriage, preterm labor
• Newborn: early-onset sepsis, late-onset meningitis
• Adults: meningitis in elderly/immunocompromised
• Gastroenteritis (healthy people)
6. Diagnosis:
• Gram-positive rod
• Tumbling motility at 25°C
• Intracellular in macrophages
• Mildly beta-hemolytic (like strep)
7. Treatment:
• Ampicillin (drug of choice)
• Add gentamicin in severe disease
8. Important:
• Listeria = only Gram-positive rod causing meningitis in neonates and elderly
• Resistant to cephalosporins → NEED ampicillin.

Memorize these → you can answer 90% of exam questions on Listeria.

🔬 FULL MICROBIOLOGY – CLEAR & COMPLETE

1️⃣ Classification

• Gram-positive rod
• Facultative intracellular pathogen
• β-hemolytic
• Catalase-positive
• Motile (tumbling motility at room temp)
• Non–spore forming

2️⃣ Key Properties (VERY HIGH-YIELD)

✔ Grows at cold temperatures (4°C)

→ Likes refrigerated foods.

✔ Tumbling motility

Seen at 25°C.

✔ Actin rockets

Moves inside and between cells using actin tail polymerization.

✔ Escapes phagolysosome

Using Listeriolysin O — the main virulence factor.

✔ Causes intracellular infection

Hides from antibodies → cell-mediated immunity needed (important in pregnancy & immunosuppressed).

3️⃣ Transmission

Foodborne (major route)

• Unpasteurized milk
• Soft cheeses
• Deli meats
• Cold salads
• Refrigerated raw foods (“loves the cold”)

Vertical transmission

• Transplacental (1st or 2nd trimester → miscarriage, stillbirth)
• During delivery

Zoonotic exposure (rare)

• Farm animals

4️⃣ Pathogenesis

1. Ingestion of contaminated food
2. Uptake by intestinal cells
3. Inside phagosome → secretes listeriolysin O
4. Escapes into cytoplasm
5. Uses actin polymerization to form actin tail
6. Propels into adjacent cells
7. Spreads without exposure to antibodies
8. Leads to systemic spread in pregnancy & immunocompromised

5️⃣ Clinical Disease

A. In pregnant women

• Mild flu-like illness
• Fever, myalgia
• Can progress to:
• Miscarriage
• Preterm labor
• Stillbirth
• Amnionitis
• Transplacental fetal infection

B. In neonates

Early onset (from placenta)

• Sepsis at birth
• Pneumonia
• Disseminated abscesses
• Granulomatosis infantiseptica (multiple granulomas, 100% exam point)

Late onset (during delivery)

• Meningitis at 2–3 weeks of age

C. In immunocompromised adults

• Meningitis
• Encephalitis (rhombencephalitis)
• Bacteremia

D. In healthy adults

• Self-limited gastroenteritis

6️⃣ Diagnosis

Microscopy

• Gram-positive rods
• Narrow β-hemolysis (similar to strep)

Culture

• Prefers cold environments
• Can multiply in blood cultures

Motility tests

• Tumbling motility at 25°C
• Umbrella-shaped growth in motility media

CSF

• Confirms meningitis

7️⃣ Treatment

⭐ First-line:

✔ Ampicillin ± Gentamicin

Alternatives

• TMP-SMX
• Meropenem (not preferred)

Important:

❌ Cephalosporins do NOT work because Listeria has intrinsic resistance.

8️⃣ Prevention

Pregnancy

• Avoid unpasteurized milk & soft cheeses
• Avoid deli meats unless very hot
• Avoid refrigerated smoked seafood
• Hand hygiene after handling animals

Hospital measures

• Food safety
• Screening during outbreaks

9️⃣ High-Yield Exam Tables

Listeria vs. Group B Streptococcus (neonatal meningitis)

⭐ MUST-MEMORISE SUMMARY ⭐

1. Listeria monocytogenes = Gram+ intracellular rod with tumbling motility.
2. Grows in the cold → unsafe soft cheeses, deli meats.
3. Pregnancy: miscarriage, stillbirth, neonatal sepsis.
4. Neonates: early sepsis / late meningitis.
5. AIDS/elderly: meningitis → treat with ampicillin.
6. Virulence: listeriolysin O, actin rockets.
7. Cephalosporins do NOT work.
8. Causes granulomatosis infantiseptica (classic exam point).

⭐ Listeria monocytogenes — COMPLETE HIGH-YIELD MASTER TABLE

🧬 Virulence & Pathogenesis

🍽️ Transmission

👥 High-Risk Groups

🏥 Clinical Manifestations

Pregnancy

Neonates

Adults

🔬 Diagnosis

💊 Treatment

🚫 Prevention (Pregnancy-Focused)

📊 Exam-Critical Comparison

🔐 ABSOLUTE EXAM LOCK (One-Glance Recall)

• Only Gram-positive rod causing meningitis in neonates & elderly
• Cold-growing, tumbling, intracellular
• Listeriolysin O + actin rockets
• Granulomatosis infantiseptica
• Cephalosporins DO NOT work → Ampicillin ALWAYS