Legionella pneumophila is classified as which type of organism?

• A. Gram-negative bacillus
• B. Gram-positive cocci
• C. Gram-positive bacillus
• D. Gram-negative cocci

Answer: (A)

Legionella pneumophila is understood by two key features: how it stains and its shape. It has a thin peptidoglycan layer and an outer membrane, which means it does not retain the crystal violet stain well and appears pink after the counterstain—this is characteristic of Gram-negative bacteria. Its shape is rod-like, not spherical, so it is described as a bacillus. Put together, Legionella pneumophila is a Gram-negative bacillus. It is not Gram-positive, nor is it a cocci, which explains why the other shapes and staining patterns do not fit.