Vibrio and Other Gram-Negative. Bacilli

I. Vibrio

General Characteristics

✔ Gram-negative, comma-shaped bacilli
✔ Motile with a single polar flagellum (monotrichous)
✔ Oxidase-positive
✔ Facultative anaerobes
✔ Grow in alkaline conditions (pH 9.5)
✔ String test-positive

Classification of Vibrio cholerae

• Toxigenic serotypes: V. cholerae O1 and O139

• O1 Biotypes:

  • Classical

  • El Tor (more resistant, causes mild infections)

• O1 Serotypes:

  • Ogawa (A & B antigens)

  • Inaba (A & C antigens)

  • Hikojima (A, B & C antigens)

II. Determinants of Pathogenicity

1. Virulence Factors

✔ Choleragen (Enterotoxin) → causes massive fluid loss & diarrhea
✔ Flagella → aids in motility & attachment
✔ Mucinase → destroys mucus layer for adherence
✔ Endotoxin (LPS) → triggers immune response

III. Cholera (Clinical Manifestation)

Transmission

🔹 Humans are the only reservoir
🔹 Spread via contaminated food & water (4 F’s: Fingers, Food, Feces, Flies)

Pathogenesis

1⃣ Ingestion of V. cholerae
2⃣ Survives stomach acid (large infecting dose needed)
3⃣ Attaches to small intestine mucosa
4⃣ Produces choleragen toxin, leading to:

• Increased cAMP → excessive chloride & water loss

• Watery diarrhea (“rice-water stools”)

• Dehydration, electrolyte imbalance, hypovolemic shock

Symptoms

✔ Sudden severe diarrhea (~1L/hr fluid loss)
✔ Dehydration → sunken eyes, loss of skin turgor
✔ Weak pulse, muscle cramps, vomiting
✔ Choleraic facies (sunken eyes, gaunt appearance)

IV. Diagnosis of V. cholerae

🔬 Culture Media

• Thiosulfate Citrate Bile Salt Sucrose (TCBS) Agar → yellow colonies

• Tellurite Taurocholate Gelatin Agar (TTGA) → transparent colonies with cloudy halo

• Darkfield Microscopy → rapid darting motility

• String Test (+)

V. Other Pathogenic Vibrios

1. Vibrio parahaemolyticus

✔ Halophilic (salt-loving), requires 2% NaCl
✔ Found in seafood (fish, mollusks, crustaceans, oysters, squid, etc.)
✔ Causes gastroenteritis → diarrhea, abdominal pain, vomiting, mild fever
✔ Self-limiting, lasts ~3 days

🛑 Diagnosis: TCBS Agar → green colonies

2. Vibrio vulnificus

✔ Causes wound infections (from seawater exposure) and septicemia (from raw oysters)
✔ Common in warm, brackish water
✔ Can cause bacteremia & liver disease

VI. Campylobacter

General Characteristics

✔ Curved, S-shaped or “gull-wing” Gram-negative bacilli
✔ Motile, monotrichous flagellum
✔ Oxidase-positive
✔ Microaerophilic (requires 5% O₂)
✔ Does NOT ferment carbohydrates

Pathogenic Species

1⃣ Campylobacter jejuni → Causes diarrhea (enterocolitis)
2⃣ Campylobacter intestinalis → Causes bacteremia

Pathogenesis

• Fecal-oral transmission (contaminated food, water, animal products)

• Infecting dose: ~10⁴ organisms

• Invades intestinal epithelium → causes inflammation → bloody diarrhea

Symptoms

✔ Watery, foul-smelling diarrhea → progresses to bloody stools
✔ Fever, abdominal pain, nausea
✔ Self-limiting (lasts ~1 week)

🛑 Diagnosis:

• Culture on Campy-BAP, Campy-CVA at 42°C

• Gram stain → "gull-wing" shape

• Growth inhibited by nalidixic acid

VII. Helicobacter pylori

General Characteristics

✔ Curved, spiral Gram-negative rod
✔ Motile (lophotrichous flagella)
✔ Microaerophilic
✔ Oxidase, catalase, urease-positive

Pathogenesis

1⃣ H. pylori penetrates the stomach mucosa
2⃣ Produces urease → converts urea into ammonia, neutralizing stomach acid
3⃣ Inflammatory response damages gastric mucosa
4⃣ Leads to gastritis, ulcers, gastric cancer

Symptoms

✔ Recurrent upper abdominal pain
✔ Gastrointestinal bleeding (peptic ulcer disease)
✔ Nausea, bloating

🛑 Diagnosis:

• Urease breath test (radiolabeled urea)

• Endoscopy & biopsy

• Culture on Skirrow’s medium

VIII. Summary Table