Which word describes a category of bacteria, based on the shape?

• A. Coccus
• B. Bacillus
• C. Spirillum
• D. Vibrio

Answer: (B)

Bacteria are commonly described by their shape, which creates simple categories you’ll see in microbiology. The rod-shaped category is called bacillus (or bacilli in plural). This term directly names the rod-like form, making it the best pick when the question asks for a category defined by shape. Other terms describe different shapes—cocci for spherical, spirillum for spiral, vibrio for curved/comma-shaped—but bacillus specifically identifies the rod-shaped category.