In a village, several people developed dysentery after consuming raw milk. On laboratory examination, gram-negative, curved rods with polymorphonuclear infiltration were found in stool samples. Which of the following is the most likely causative organism?

Explanation

## Correct Answer: C. Campylobacter jejuni Campylobacter jejuni is the most likely causative organism in this outbreak scenario. The clinical clues are: (1) **raw milk exposure** — C. jejuni is a common zoonotic pathogen in poultry and cattle, transmitted via unpasteurized dairy; (2) **gram-negative, curved rods** — this is the pathognomonic morphology of Campylobacter (comma or S-shaped appearance on Gram stain); (3) **polymorphonuclear (PMN) infiltration in stool** — indicates invasive colitis, consistent with C. jejuni's mechanism of mucosal invasion and inflammation. C. jejuni causes acute bacterial dysentery with bloody diarrhea, fever, and abdominal cramps. In India, where raw milk consumption is common in rural areas and dairy hygiene standards vary, C. jejuni is a recognized cause of foodborne gastroenteritis outbreaks. The organism is microaerophilic and thermophilic (optimal growth at 42°C), making poultry a natural reservoir. Campylobacteriosis is self-limiting but can cause severe disease in immunocompromised patients and rarely leads to Guillain-Barré syndrome. The combination of curved gram-negative rods + PMN colitis + raw milk exposure makes C. jejuni the discriminating answer. ## Why the other options are wrong **A. Clostridium perfringens** — C. perfringens is a gram-positive, anaerobic rod that causes food poisoning with watery diarrhea (not dysentery) and minimal inflammation. It does not produce curved rods on Gram stain and is not associated with PMN colitis. While it can contaminate raw milk, it does not cause invasive dysentery with bloody stools. **B. Staphylococcus aureus** — S. aureus is a gram-positive coccus (not a curved rod) that causes food poisoning via enterotoxins, resulting in watery diarrhea with rapid onset (1–6 hours) and no blood in stool. It does not cause invasive colitis with PMN infiltration. The morphology and pathology are incompatible with the clinical presentation. **D. Vibrio parahaemolyticus** — V. parahaemolyticus is a gram-negative curved rod, but it is a halophilic (salt-loving) marine organism associated with seafood consumption, not raw milk. While it can cause dysentery with PMN infiltration, the epidemiological link (raw milk, not seafood) and the absence of coastal/seafood exposure in the question make this less likely than C. jejuni in an Indian village setting. ## High-Yield Facts - **Campylobacter jejuni** is the most common bacterial cause of acute gastroenteritis in developed countries and a rising cause of foodborne illness in India. - **Gram-negative, curved (comma or S-shaped) rods** are the hallmark morphology of Campylobacter on Gram stain; requires microaerophilic culture conditions. - **Raw milk and poultry** are the primary reservoirs; C. jejuni is thermophilic (optimal growth 42°C), matching avian body temperature. - **Invasive colitis with PMN infiltration** indicates mucosal invasion; causes bloody diarrhea, fever, and tenesmus (true dysentery). - **Campylobacteriosis is self-limiting** (3–7 days) but can progress to Guillain-Barré syndrome (post-infectious autoimmune neuropathy) in 1 in 1000 cases. ## Mnemonics **CURVED gram-negative = Campylobacter** When you see **curved rods** on Gram stain + foodborne dysentery, think **C. jejuni**. Vibrio is also curved but halophilic (seafood); Campylobacter is thermophilic (poultry/milk). **Campylobacter = Poultry + Milk (not Seafood)** **C. jejuni** lives in bird gut (42°C body temp) → poultry, eggs, raw milk. **Vibrio** lives in salt water → seafood. Raw milk outbreak = Campylobacter. ## NBE Trap NBE pairs "curved gram-negative rod" with Vibrio parahaemolyticus to trap students who memorize morphology without considering epidemiology. The raw milk exposure is the discriminating clue that excludes Vibrio (which requires seafood/marine environment) and confirms C. jejuni. ## Clinical Pearl In rural India, raw milk consumption is common and often unpasteurized. C. jejuni outbreaks in villages are frequently linked to dairy cattle or poultry contact. Unlike Vibrio (coastal/seafood), Campylobacter thrives in the 42°C gut of birds and cattle, making it the expected pathogen in inland milk-borne outbreaks. _Reference: Jawetz, Melnick & Adelberg's Medical Microbiology, Ch. 23 (Campylobacter); Harrison's Principles of Internal Medicine, Ch. 158 (Bacterial Gastroenteritis)_