A 28-year-old man from rural Uttar Pradesh presents with a 10-day history of sustained fever (39.5°C), headache, and abdominal discomfort. He reports eating contaminated water from a local well 2 weeks ago. On examination, he has hepatosplenomegaly and a faint rose-coloured rash on the trunk. Blood culture yields a Gram-negative rod that ferments glucose and maltose but not lactose, produces H₂S, and is motile with peritrichous flagella. What is the most likely causative organism?

Explanation

## Diagnosis: Salmonella typhi Enteric Fever ### Clinical Presentation The patient presents with the classic triad of enteric fever: 1. **Sustained fever** (>7 days, often rising in a step-ladder pattern) 2. **Hepatosplenomegaly** (present in ~50% of cases) 3. **Rose spots** (faint maculopapular rash on trunk, pathognomonic but uncommon) **Key Point:** The 2-week incubation period after contaminated water exposure is typical for *Salmonella typhi* (range 6–30 days). ### Microbiological Identification | Feature | *S. typhi* | *S. paratyphi A* | *Shigella* | *V. cholerae* | |---------|-----------|-----------------|-----------|---------------| | **Lactose fermentation** | − | − | − | − | | **Glucose fermentation** | + | + | + | + | | **Maltose fermentation** | + | + | + | − | | **H₂S production** | + | − | − | − | | **Motility** | + (peritrichous) | + (peritrichous) | − | + (polar) | | **Blood culture positivity** | ↑↑ (early phase) | ↑ (early phase) | Rare | − | **High-Yield:** H₂S production is the **key differentiator** between *S. typhi* (positive) and *S. paratyphi A* (negative). Both are non-lactose fermenters, but H₂S production clinches *S. typhi*. **Clinical Pearl:** Blood culture is the gold standard in the first 2 weeks of illness; bone marrow culture (if available) has the highest sensitivity (~90%) even after antibiotics. ### Pathophysiology *Salmonella typhi* invades the Peyer's patches in the terminal ileum → systemic dissemination → bacteremia → seeding of reticuloendothelial organs (liver, spleen, bone marrow). The rose spots represent microabscesses in the dermis. **Mnemonic: SALTY** — *S. typhi* features: - **S**ustained fever - **A**bdominal pain (often minimal despite hepatosplenomegaly) - **L**ow relative pulse rate (bradycardia relative to fever) - **T**yphoidal state (delirium, confusion in severe cases) - **Y**ellow appearance (jaundice in some cases) ### Why This Organism? *S. typhi* is endemic in India and transmitted via faecal–oral route (contaminated water). It causes systemic infection (enteric fever), whereas *S. paratyphi A* causes a milder, shorter-duration illness and rarely produces H₂S.