Corynebacterium diphtheriae is a?

Appeared in: MNS 2024

Explanation

Corynebacterium diphtheriae is classified based on its reaction to Gram staining and its cellular shape.
It is a Gram-positive organism, which means it stains purple because its thick peptidoglycan cell wall retains the primary crystal violet stain.
Morphologically, it is a bacillus, which is a rod-shaped bacterium.
Therefore, the correct classification is Gram-positive bacilli.

Option A: This is incorrect because C. diphtheriae is rod-shaped (bacillus), not spherical (coccus).
Option B: This is incorrect because C. diphtheriae is Gram-positive (stains purple) and is a bacillus (rod-shaped), not a Gram-negative coccus.
Option D: This is incorrect because C. diphtheriae is Gram-positive, not Gram-negative. It does not lose the primary stain to take up the safranin counterstain.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Microbiological classification of Corynebacterium diphtheriae as background academic context rather than a clinical decision trigger.
Identifying C. diphtheriae as a Gram-positive bacillus is the first step in diagnosing diphtheria, a serious infection that can lead to breathing difficulties, heart problems, and nerve damage.
The organism's main virulence factor is a potent exotoxin, which is the target of the diphtheria vaccine (toxoid). Vaccination (usually as DTaP/Tdap) is crucial for prevention.
A hallmark of respiratory diphtheria is the formation of a thick, gray pseudomembrane in the throat, which can obstruct the airway. Prompt identification and treatment with antitoxin and antibiotics are critical.

How to Approach the Question

This is a factual recall question that tests your knowledge of basic microbiology.
First, identify the key organism in the question: 'Corynebacterium diphtheriae'.
Next, recall the two main characteristics used for initial bacterial classification: Gram stain reaction (positive or negative) and morphology (shape, e.g., cocci or bacilli).
Access your memory about C. diphtheriae. It is known as the Klebs-Löffler bacillus. The term 'bacillus' tells you it is rod-shaped.
Recall its Gram stain property. C. diphtheriae is a classic example of a Gram-positive bacterium.
Combine these two facts: Gram-positive and bacillus.

Concept Tested & Keywords

Q8LDdHBf-o8Tg5bOPQU210

E6 Nelson Textbook of Pediatrics(2024) — Volume 1 p. 1728-1730

E6 Medicine Harrison 22e Part 1 p. 1263-1265

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

Practise the full MNS 2024

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