A nurse is reviewing a Gram-stained slide of a bacterial sample. She observes Gram-positive cocci arranged in clusters. Which organism is most likely present?

1. A

   Staphylococcus

2. B

   Streptococcus

3. C

   Clostridium

4. D

   Bacillus

Appeared in: RML MAINS 2025

Explanation

• The question provides three key identifiers: Gram-positive staining, a spherical (cocci) shape, and an arrangement in clusters.
• Staphylococcus is a genus of bacteria that are Gram-positive cocci.
• The characteristic microscopic appearance of Staphylococcus is irregular, grape-like clusters.
• This clustering pattern occurs because staphylococci divide in multiple, random planes and the daughter cells remain attached.

Why Other Options Were Wrong

• Option B: While Streptococcus species are also Gram-positive cocci, they divide along a single plane, causing them to form chains or pairs (diplococci), not clusters.
• Option C: Clostridium species are bacilli (rod-shaped), not cocci (spherical).
• Option D: Bacillus species are also bacilli (rod-shaped), not cocci.

Related Visual

Clinical Relevance

• Nursing practice connection: Knowing Bacterial identification using Gram stain morphology and arrangement helps nurses interpret findings accurately and avoid errors in routine assessment, medication administration, and patient teaching.
• Rapid identification via Gram stain is a crucial first step in diagnosing bacterial infections, allowing clinicians to start appropriate empirical antibiotic therapy while awaiting definitive culture results.
• For instance, finding Gram-positive cocci in clusters from a wound swab strongly suggests a staphylococcal infection, guiding the choice towards antibiotics effective against Staphylococcus, such as cloxacillin or vancomycin (for suspected MRSA).
• What if? If the slide showed Gram-positive cocci arranged in chains from a throat swab, the likely organism would be Streptococcus pyogenes, and the treatment of choice would be penicillin to prevent complications like rheumatic fever.

How to Approach the Question

• First, identify the three key descriptors provided in the clinical finding: 1) Gram stain result (Gram-positive), 2) bacterial shape (cocci), and 3) cellular arrangement (clusters).
• Systematically evaluate each option against these three characteristics.
• Eliminate options that do not match the shape. Both Clostridium and Bacillus are bacilli (rods), not cocci, so they can be ruled out.
• Differentiate between the remaining cocci based on their arrangement. Streptococcus forms chains, while Staphylococcus forms clusters.
• Conclude that Staphylococcus is the only organism that matches all three descriptors: Gram-positive, cocci, and clusters.

Concept Tested & Keywords

• Concept Tested: Bacterial identification using Gram stain morphology and arrangement
• Stem keywords: Gram-stained slide, Gram-positive cocci, clusters
• Lead-in keywords: most likely present
• Negative lead-in flag: false

Question ID

Q-gUD4rBMG38qo_8SADArO

Copy ID

Reference Book

E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 367-369

E6 Microbiology Essentials Sastry 4e Part 1 p. 83-85