Diagnostic morphology: Bipolar (safety-pin) staining is classically characteristic of which bacterial pathogen?

Introduction / Context:
Bipolar staining, also called safety-pin appearance, refers to intense staining of the poles of certain bacilli with relative clearing in the center on specific stains (e.g., Wayson or methylene blue). Recognizing this feature helps in rapid presumptive identification of key pathogens.

Given Data / Assumptions:

• The question asks for the classic organism associated with bipolar staining.
• Context is laboratory microscopy of bacterial smears.
• We consider characteristic morphology rather than culture traits.

Concept / Approach:
Yersinia pestis, the agent of plague, is well known for bipolar staining on Wayson stain, producing a safety-pin look. Although some other organisms may occasionally show polar accentuation, the teaching association is strongest for Y. pestis. Proteus mirabilis and Pseudomonas aeruginosa do not characteristically show this feature.

Step-by-Step Solution:
Recall hallmark microscopic features linked to high-yield pathogens. Match “safety-pin” morphology with Yersinia pestis. Exclude Proteus and Pseudomonas based on typical Gram-negative rod morphology lacking bipolar staining. Select Yersinia pestis.

Verification / Alternative check:
Textbook images of smears from buboes or blood demonstrate the polar staining of Y. pestis, reinforcing the classical teaching point.

Why Other Options Are Wrong:

• Proteus mirabilis: Motile swarmer; not associated with bipolar staining.
• Pseudomonas aeruginosa: Slender rods, oxidase-positive; no safety-pin hallmark.
• All of these / Klebsiella: Do not match the distinctive staining pattern classically.

Common Pitfalls:
Confusing bipolar staining with bipolar spore location (endospores) or with bipolar flagella; these are unrelated concepts.

Final Answer:
Yersinia pestis.