BHU NO - 2015
Applied Microbiology & Infection control
Easy
Which one of the disease is not caused by acid fast bacilli?
Appeared in: BHU NO - 2015
Explanation
- Typhoid fever is caused by the bacterium Salmonella enterica serotype Typhi.
- Salmonella typhi is a Gram-negative bacillus, meaning its cell wall lacks the high mycolic acid content required for acid-fastness.
- Therefore, it does not retain the primary stain during the acid-fast staining procedure and is considered non-acid-fast.
- Diagnosis of typhoid relies on culture (blood, stool) and serological tests (e.g., Widal test), not acid-fast staining.
Why Other Options Were Wrong
- Option A: Leprosy (Hansen's disease) is caused by Mycobacterium leprae, which is a well-established acid-fast bacillus. Its mycolic acid-rich cell wall is a key characteristic.
- Option B: Tuberculosis is caused by Mycobacterium tuberculosis. This organism is the archetypal acid-fast bacillus, and the Ziehl-Neelsen (acid-fast) stain is a primary method for its rapid detection in clinical samples like sputum.
- Option C: Nocardiosis is caused by bacteria of the genus Nocardia. These organisms are known to be 'partially' or 'weakly' acid-fast, meaning they exhibit the property, although less strongly than Mycobacteria.
Related Visual
Clinical Relevance
- Nursing practice connection: Knowing Identification of diseases based on the acid-fast property of their causative bacterial agents helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
- Staining properties are fundamental in microbiology for the rapid, presumptive identification of pathogens, which guides initial treatment choices.
- A positive acid-fast bacillus (AFB) smear from a patient's sputum is a strong indicator of tuberculosis, prompting immediate implementation of airborne infection control precautions to prevent transmission.
- What if? If a patient presents with fever and diarrhea, and their blood culture grows a Gram-negative rod, the differential diagnosis would include enteric pathogens like Salmonella, Shigella, or E. coli. An acid-fast stain would be inappropriate and yield a negative result, confirming the organism is not a mycobacterium.
How to Approach the Question
- First, identify the negative framing of the question: 'which... is NOT caused by'. This means three options will be caused by acid-fast bacilli, and one will not.
- For each disease listed in the options, recall or determine its causative microorganism.
- Recall or determine the staining characteristic of each microorganism, specifically whether it is acid-fast.
- Leprosy is caused by Mycobacterium leprae (acid-fast).
- Tuberculosis is caused by Mycobacterium tuberculosis (acid-fast).
- Nocardiasis is caused by Nocardia spp. (weakly acid-fast).
Concept Tested & Keywords
- Concept Tested: Identification of diseases based on the acid-fast property of their causative bacterial agents.
- Stem keywords: disease, acid fast bacilli
- Lead-in keywords: not caused by
- Negative lead-in flag: Question asks for the exception (NOT caused by).
Question ID
Qb8h3ZlmvmhWNSP63TZne6
Reference Book
E6 Medicine Harrison 22e Part 1 p. 1421-1423
E6 Microbiology Essentials Sastry 4e Part 2 p. 153-155
E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 381-383
Practise the full BHU NO - 2015
Attempt every question from this paper in a timed mock, then review the full solution for each one.
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