What is the shape of Bacillus tuberculosis bacteria?

Appeared in: WCL Staff Nurse - 2022

Explanation

The bacterium responsible for tuberculosis, Mycobacterium tuberculosis, is classified as a bacillus.
The term 'bacillus' (plural: bacilli) is the scientific term for a rod-shaped bacterium.
Microscopically, M. tuberculosis appears as a thin, straight or slightly curved rod, measuring about 0.5 μm by 3 μm.
Its rod shape is a defining morphological characteristic used in its identification, particularly in acid-fast staining procedures.

Option A: This describes cocci bacteria, which are spherical or oval. Examples include Staphylococcus and Streptococcus.
Option B: This describes bacteria that grow in long, branching, thread-like structures. M. tuberculosis does not form filaments.
Option D: This describes bacteria that are curved or comma-shaped. M. tuberculosis is a straight or slightly curved rod, not comma-shaped.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Bacterial Morphology as background academic context rather than a clinical decision trigger.
Identifying the rod shape of M. tuberculosis is fundamental for its diagnosis via Acid-Fast Bacilli (AFB) smear from sputum, where the bacteria appear as red rods under the microscope.
A positive AFB smear result is a critical finding that prompts the immediate implementation of airborne precautions, such as placing the patient in a negative-pressure isolation room and requiring healthcare staff to wear N95 respirators, to prevent the spread of TB.
What if? If a sputum smear showed Gram-positive cocci in grape-like clusters instead of acid-fast rods, the diagnosis would shift from tuberculosis to a likely staphylococcal pneumonia. This would require different antibiotics (e.g., vancomycin instead of RIPE therapy) and infection control measures (contact precautions instead of airborne).

How to Approach the Question

This is a factual recall question testing basic knowledge of microbiology.
Identify the key terms in the question: 'Bacillus tuberculosis' and 'shape'.
Recall the primary classifications of bacteria based on their morphology. The term 'bacillus' itself is a major clue, as it directly translates to a rod-shaped organism.
Systematically evaluate each option. 'Ellipsoidal/Spherical' corresponds to cocci. 'Filamentous' refers to branching bacteria like Actinomyces. 'Vibrio' describes comma-shaped bacteria like the one causing cholera.
Based on this process of elimination and direct recall, confirm that 'Rod Shaped' is the correct description for M. tuberculosis.

Concept Tested & Keywords

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

E6 Microbiology Applied AnjuDhir 3e Part 1 p. 54-56

E6 Microbiology Essentials Sastry 4e Part 1 p. 35-37

Practise the full WCL Staff Nurse - 2022

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