A 28-year-old woman from Delhi presents with a 10-day history of sustained fever (39–40°C), headache, and abdominal discomfort. She reports constipation rather than diarrhea. On examination, she has a rose-colored rash on the trunk, hepatomegaly, and splenomegaly. Blood culture yields a gram-negative bacillus that is oxidase-negative, ferments glucose and mannitol but not lactose, and is motile with peritrichous flagella. Widal test shows O antigen titre of 1:160 and H antigen titre of 1:320. What is the most likely causative organism?

Explanation

## Diagnosis: Salmonella typhi Enteric Fever ### Clinical Presentation The vignette describes classic enteric fever with: - **Sustained fever** (not remittent) lasting >1 week - **Rose spots** (faint rose-colored maculopapular rash on trunk) - **Hepatosplenomegaly** (common in S. typhi) - **Constipation** (more typical of typhoid than paratyphoid) - **Headache and abdominal discomfort** ### Microbiological Features | Feature | S. typhi | S. paratyphi A | |---------|----------|----------------| | **Lactose fermentation** | Negative | Negative | | **Glucose fermentation** | Positive | Positive | | **Motility** | Peritrichous flagella | Peritrichous flagella | | **Oxidase test** | Negative | Negative | | **H₂S production** | Positive (TSI: black butt) | Negative or weak | **Key Point:** S. typhi is identified by: 1. Negative lactose fermentation (non-lactose fermenter) 2. Positive glucose and mannitol fermentation 3. Gram-negative bacillus with peritrichous flagella 4. Oxidase-negative ### Widal Test Interpretation **High-Yield:** - **O antigen titre 1:160** = recent or active infection (somatic antigen) - **H antigen titre 1:320** = indicates S. typhi (H antigen more elevated than O in typhoid) - A fourfold rise in paired sera is diagnostic; single high titre in endemic areas suggests active infection **Clinical Pearl:** In endemic regions like India, a single O antigen titre ≥1:160 or H antigen ≥1:320 is suggestive of enteric fever, especially with compatible clinical presentation. ### Why S. typhi Over S. paratyphi A? - **Constipation** is more characteristic of S. typhi; S. paratyphi A often causes diarrhea - **Rose spots** are more common in S. typhi - **Hepatosplenomegaly** is more pronounced in S. typhi - **H antigen titre > O antigen titre** favors S. typhi (in paratyphoid, O titre is often higher) [cite:Park 26e Ch 27]