Bacterial colony morphology — Which organism classically produces colonies with a characteristic "Medusa head" appearance on solid media?

Introduction / Context:
Colony morphology offers rapid, low-cost clues during bacterial identification. Certain pathogens form striking colony patterns that, while not definitive alone, aid in presumptive diagnosis alongside other tests.

Given Data / Assumptions:

• The term "Medusa head" refers to irregular, filamentous colony edges with a ground-glass surface.
• We consider classic, textbook morphologies on appropriate media (e.g., blood agar).
• Other listed organisms have different hallmark features.

Concept / Approach:
Bacillus anthracis produces non-hemolytic, gray-white colonies with an irregular, hair-like edge termed "Medusa head." This appearance reflects filamentous outgrowths of the colony. While Proteus mirabilis is known for swarming motility, it does not produce the Medusa head morphology typical of B. anthracis. Pseudomonas aeruginosa produces pigmented, often greenish colonies with grape-like odor, and Clostridium tetani is typically identified by anaerobic culture and characteristic cell morphology rather than Medusa head colonies.

Step-by-Step Solution:

Verification / Alternative check:
Additional presumptive clues for B. anthracis include lack of hemolysis, non-motility, capsule demonstration by McFadyean's stain, and gamma phage susceptibility.

Why Other Options Are Wrong:

• Proteus mirabilis: swarming concentric rings, not Medusa head margins.
• Clostridium tetani: obligate anaerobe; colony morphology not described as Medusa head.
• Pseudomonas aeruginosa: pigment production and fruity odor are key, not Medusa head.

Common Pitfalls:
Over-reliance on colony appearance alone; always correlate with Gram stain, motility, hemolysis, and specific tests.

Final Answer:
Bacillus anthracis