A 32-year-old man from Delhi presents with a 5-day history of profuse watery diarrhoea (8–10 stools/day), crampy abdominal pain, and fever (38.5°C). He reports eating undercooked chicken at a wedding 2 days before symptom onset. Stool culture grows a Gram-negative, curved, motile rod that is oxidase-positive and grows microaerophilically at 42°C. Which organism is most likely responsible, and what is the mechanism of diarrhoea?

Explanation

## Organism Identification **Key Point:** The combination of **curved, Gram-negative rod + oxidase-positive + microaerophilic growth at 42°C** is pathognomonic for *Campylobacter jejuni*. ## Distinguishing Features of Campylobacter | Feature | Campylobacter jejuni | Other Gram-Negatives | |---------|----------------------|---------------------| | **Morphology** | Curved/S-shaped rod | Straight rod (Vibrio, Salmonella, Shigella) | | **Oxidase** | Positive | Vibrio: +; Salmonella, Shigella: − | | **Growth temperature** | 42°C (thermophilic) | Most prefer 35–37°C | | **Atmosphere** | Microaerophilic (5% O₂) | Aerobic or facultative anaerobic | | **Motility** | Darting, corkscrew | Various | **High-Yield:** The **42°C thermophilic preference** is unique to Campylobacter among enteric pathogens and reflects its adaptation to avian and mammalian body temperatures. ## Pathogenesis of Campylobacter Diarrhoea **Clinical Pearl:** Campylobacter causes **invasive, inflammatory diarrhoea** (not secretory): 1. **Invasion:** The organism penetrates intestinal epithelium via M cells and causes mucosal ulceration. 2. **Cytokine release:** Invasion triggers IL-8, TNF-α, and IL-1β production by epithelial cells and macrophages. 3. **Inflammatory response:** Neutrophilic infiltration leads to bloody diarrhoea, cramping, and fever. 4. **Bloody stools:** Common in Campylobacter (distinguishes it from Vibrio cholerae, which causes watery diarrhoea). **Mnemonic:** **CJ = Curved + 42°C + Jejunum invasion** (Campylobacter jejuni). ## Clinical Context: Poultry Exposure **Key Point:** Campylobacter is the **most common bacterial cause of acute diarrhoea in developed countries** and is frequently isolated from poultry, unpasteurized milk, and contaminated water. The 2-day incubation period (range 2–5 days) and undercooked chicken exposure are classic. ## Comparison with Other Organisms ```mermaid flowchart TD A[Gram-negative rod diarrhoea]:::outcome --> B{Oxidase test?}:::decision B -->|Positive| C{Growth at 42°C?}:::decision B -->|Negative| D[Salmonella or Shigella]:::outcome C -->|Yes, microaerophilic| E[Campylobacter jejuni]:::action C -->|No, aerobic| F[Vibrio cholerae]:::outcome E --> G[Invasive, bloody diarrhoea]:::outcome F --> H[Secretory, watery diarrhoea]:::outcome ``` **Warning:** Do not confuse Campylobacter with Vibrio cholerae. Both are oxidase-positive, but Vibrio grows aerobically at 37°C and causes **secretory** (non-inflammatory) diarrhoea via enterotoxin-cAMP, whereas Campylobacter is microaerophilic, thermophilic, and causes **invasive** diarrhoea. [cite:Robbins 10e Ch 8; Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases Ch 220]