A 32-year-old man from Delhi presents with acute onset bloody diarrhea, crampy abdominal pain, and fever (38.5°C) for 4 days. He reports eating undercooked chicken at a wedding 3 days ago. Stool examination shows WBCs and RBCs. Stool culture on selective media (Campy-Food Agar) yields a curved, Gram-negative rod that is oxidase-positive, catalase-positive, and grows at 42°C but not at 37°C. The organism is resistant to cephalosporin. What is the most likely diagnosis and the BEST antibiotic choice?

Explanation

## Campylobacter jejuni: Identification and Treatment ### Clinical Presentation and Epidemiology **Key Point:** Campylobacter jejuni is the most common bacterial cause of acute gastroenteritis in developed countries and is frequently acquired from poultry products. **High-Yield:** Classic features of Campylobacter gastroenteritis: - Incubation period: 2–5 days (matches this patient's 3-day exposure) - Bloody diarrhea with fecal leukocytes - Fever and crampy abdominal pain - Self-limited in immunocompetent hosts (typically 3–7 days) - Source: undercooked poultry, unpasteurized milk, contaminated water ### Microbiological Identification | Feature | Campylobacter jejuni | Salmonella | Shigella | Vibrio parahaemolyticus | | --- | --- | --- | --- | --- | | **Gram stain** | Gram-negative rod (curved, S-shaped) | Gram-negative rod (straight) | Gram-negative rod (straight) | Gram-negative rod (curved) | | **Oxidase** | **Positive** ✓ | Negative | Negative | Positive | | **Catalase** | **Positive** ✓ | Positive | Negative | Positive | | **Growth at 42°C** | **Yes** ✓ | No | No | No | | **Growth at 37°C** | **No** ✓ | Yes | Yes | Yes | | **Selective medium** | Campy-Food Agar | SS/HE agar | SS/HE agar | TCBS agar | | **Cephalosporin resistance** | **Common** ✓ | Susceptible | Susceptible | Variable | **Mnemonic:** **C. jejuni = Curved, 42°C, Oxidase-positive** — the three key identifying features. ### Antibiotic Management **Clinical Pearl:** Antibiotic choice in Campylobacter depends on severity and resistance patterns. #### First-line Antibiotics 1. **Fluoroquinolones** (ciprofloxacin, levofloxacin) - Excellent GI penetration - Oral bioavailability - Most effective for invasive disease - Resistance emerging in some regions but still preferred 2. **Macrolides** (azithromycin, erythromycin) - Alternative if fluoroquinolone resistance - Good for severe disease - Oral or IV formulations available #### Why NOT Cephalosporins? **Warning:** The stem explicitly states **cephalosporin resistance**. This is a key identifying feature of Campylobacter jejuni. Cephalosporins are NOT effective and should not be used. #### Why NOT Other Options? - **Ampicillin:** Campylobacter is intrinsically resistant to β-lactams (including ampicillin and cephalosporins) due to altered penicillin-binding proteins. - **Tetracycline:** While Vibrio parahaemolyticus is susceptible to tetracycline, this organism is not the diagnosis here (it grows at 37°C, is oxidase-positive but has different epidemiology). ### Treatment Algorithm ```mermaid flowchart TD A[Suspected Campylobacter gastroenteritis]:::outcome --> B{Severity?}:::decision B -->|Mild-moderate, immunocompetent| C[Supportive care + oral rehydration]:::action B -->|Severe or immunocompromised| D[Antibiotic therapy]:::action D --> E{Fluoroquinolone resistance?}:::decision E -->|No| F[Ciprofloxacin 500 mg BD × 5-7 days]:::action E -->|Yes| G[Azithromycin 500 mg daily × 5-7 days]:::action C --> H[Symptoms resolve in 3-7 days]:::outcome F --> H G --> H ``` ### Why This Patient Needs Antibiotics Although Campylobacter gastroenteritis is often self-limited, **early antibiotic therapy** (within 3 days of symptom onset) reduces duration of diarrhea and bacteremia, especially in patients with fever and bloody stools indicating invasive disease.