Bacterial shapes — common term for helically curved rods In standard morphological terminology, what is the common name for bacteria that appear as helically curved rods under the microscope?

Introduction / Context:
Bacterial morphology aids preliminary identification. Among common shapes, cocci are spherical, bacilli are straight rods, spirilla are rigid helically curved rods, and spirochetes are flexible helical organisms with periplasmic flagella. Distinguishing spirilla from spirochetes helps avoid taxonomic confusion and informs motility expectations.

Given Data / Assumptions:

• “Helically curved rods” indicates a spiral shape.
• Rigidity versus flexibility differentiates spirilla from spirochetes.
• “Pleomorphic” describes variability, not a single defined shape.

Concept / Approach:
Match descriptive terms with standard categories. Spirilla (for example, Spirillum volutans) possess external flagella and rigid helical forms; spirochetes (for example, Treponema, Borrelia) have internal axial filaments, are more flexible, and move with corkscrew motion. The prompt’s wording best aligns with “spirilla.”

Step-by-Step Solution:
Identify “helically curved rods.”Distinguish rigid (spirilla) from flexible (spirochetes).Exclude cocci (spherical), bacilli (straight rods), and pleomorphic (variable forms).Select “Spirilla.”

Verification / Alternative check:
Atlas images show spiral rigid rods labeled “spirilla,” whereas spirochetes appear as thin, highly flexible helices visible with dark-field or special stains.

Why Other Options Are Wrong:

• Cocci: not rod-like.
• Pleomorphic: describes variability across shapes.
• Bacilli: straight rods, not helical.
• Spirochetes: flexible helices with unique motility; not simply “helically curved rods.”

Common Pitfalls:
Using “spirochete” as a catch-all for any spiral bacterium; morphology and flagellar arrangement matter.

Final Answer:
Spirilla (helically curved rigid rods)