Salmonella that causes typhoid fever is a:

Appeared in: BTSC 3 August 2025

Explanation

The bacterium that causes typhoid fever is Salmonella enterica serotype Typhi.
Microscopically, Salmonella species are classified as bacilli because they are rod-shaped.
In Gram staining, Salmonella does not retain the primary crystal violet stain and is counterstained pink/red by safranin, classifying it as gram-negative.
Therefore, the correct description is a gram-negative bacillus.

Option A: This is incorrect because Salmonella is a bacillus (rod-shaped), not a coccus (spherical).
Option C: This is incorrect because Salmonella is gram-negative (stains pink/red) and a bacillus (rod-shaped), not gram-positive (stains purple) and a coccus (spherical).
Option D: This is incorrect because Salmonella is gram-negative, not gram-positive. While it is a bacillus, its Gram stain reaction is different.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Microbiological classification of Salmonella typhi as background academic context rather than a clinical decision trigger.
Knowing the Gram stain and morphology of a pathogen is the first step in identifying it and is crucial for selecting the appropriate initial antibiotic therapy.
Typhoid fever is a major public health concern, especially in areas with poor sanitation. Understanding that it's caused by a bacterium transmitted via the fecal-oral route emphasizes the importance of safe water, food handling, and hygiene practices for prevention.
What if? If the bacterium were gram-positive, the initial choice of antibiotics would be different. For example, penicillins are often effective against gram-positive bacteria but less so against many gram-negative organisms, which might require cephalosporins or fluoroquinolones.

How to Approach the Question

First, identify the key terms in the question: 'Salmonella' and 'typhoid fever'.
This is a factual recall question. Access your knowledge about the microbiological characteristics of the Salmonella genus.
Recall the two main classification criteria for bacteria presented in the options: Gram stain reaction (positive or negative) and morphology (shape - cocci or bacillus).
Remember that Salmonella is a rod-shaped bacterium, which makes it a 'bacillus'. This eliminates the 'cocci' options.
Next, recall its Gram stain property. Salmonella is a classic example of a gram-negative bacterium.
Combine these two facts: 'gram-negative' and 'bacillus'. This leads directly to the correct option.

Concept Tested & Keywords

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E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 792-794

E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 1769-1771

Practise the full BTSC 3 August 2025

Attempt every question from this paper in a timed mock, then review the full solution for each one.