Mycobacterium tuberculosis is a?

Appeared in: NVS Staff Nurse - 2019

Explanation

M. tuberculosis is an acid-fast bacillus due to the high concentration of mycolic acid in its cell wall, which retains carbol fuchsin stain even after acid-alcohol decolorization.
It is classified as a Gram-positive bacterium based on its cell wall structure, which has a thick peptidoglycan layer and lacks an outer membrane, although it stains poorly with the Gram stain.
The bacterium is non-motile as it does not possess flagella or other organelles for movement.
It is an aerobic, rod-shaped bacterium (bacillus).

Option A: This option is incorrect because M. tuberculosis is non-motile.
Option B: This option is incorrect because M. tuberculosis is considered Gram-positive (not Gram-negative) and is non-motile.
Option C: This option is incorrect because M. tuberculosis is classified as Gram-positive, not Gram-negative.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Microbiological characteristics of Mycobacterium tuberculosis as background academic context rather than a clinical decision trigger.
The acid-fast property is the basis for the Ziehl-Neelsen (ZN) stain, a crucial and rapid diagnostic tool for the presumptive identification of TB in clinical specimens like sputum.
Understanding that M. tuberculosis is a slow-growing aerobe is vital for diagnosis (culture on Lowenstein-Jensen medium takes weeks) and explains its predilection for the oxygen-rich apices of the lungs.
What if? If the organism was Escherichia coli, it would be a Gram-negative, motile bacillus that is not acid-fast. This would require different staining techniques (standard Gram stain) and antibiotic treatments.

How to Approach the Question

First, identify the subject of the question: Mycobacterium tuberculosis.
Recall the three key microbiological descriptors provided in the options: Gram stain reaction, motility, and acid-fast property.
The name 'Mycobacterium' is a strong clue. 'Myco-' refers to the waxy mycolic acid in the cell wall, which is the basis for its defining characteristic: acid-fastness. This confirms one part of the answer.
Next, consider the Gram stain. While mycobacteria stain poorly, they are structurally Gram-positive. Eliminate the 'Gram-negative' options.
Finally, recall or deduce motility. Most pathogenic cocci and many bacilli, including M. tuberculosis, are non-motile. This eliminates the 'motile' option.
Combine the correct descriptors: Gram-positive, non-motile, and acid-fast, to select the final answer.

Concept Tested & Keywords

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E6 Medicine Harrison 22e Part 1 p. 1421-1423

E6 Pathology-Textbook of PATHOLOGYHarsh Mohan Part 1 (1-214) p. 114-116

Practise the full NVS Staff Nurse - 2019

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