Identify a spiral-shaped bacteria.

Correct — D. Spirillum is the bacterial form with a rigid spiral, or helical, shape — the name and the geometry share a root.

Shape is the oldest classification in microbiology and it is purely descriptive: round, rod, comma, spiral. The four options are that list, and option (d) is its spiral entry, which is why the item is answerable on vocabulary alone.

Why the others are wrong

• (a)
  Bacillus — Bacillus means rod-shaped, from the Latin for a small staff. Bacilli are drawn as straight cylinders, with no turn or coil anywhere in the shape.
• (b)
  Vibrio — Vibrio is comma-shaped: a short rod with a single curve. One bend is not a helix, and that distinction is exactly what the question turns on.
• (c)
  Coccus — Coccus means spherical. Cocci are the round bacteria that sit in pairs, chains or clusters, which is the shape furthest from a spiral.

Bacteria are sorted by shape before anything else, and the four names in this question are that list: coccus (spherical), bacillus (rod), vibrio (comma) and spirillum (spiral).

These are shape words, not genus names, so unrelated organisms share them. Spirilla are rigid helices driven by flagella at their ends; the flexible helical bacteria are named separately as spirochaetes.

SSC asks this as pure vocabulary, so the four-way shape list is the whole preparation.

Key facts

• Spirillum is the spiral or helical bacterial shape.
• Coccus is spherical, bacillus is rod-shaped and vibrio is comma-shaped.
• Spirochaetes are the flexible helical bacteria, distinct from the rigid spirilla.
• All bacteria are prokaryotes and have no membrane-bound nucleus.

Common traps

• Choosing vibrio because a comma looks curved — a comma has one bend, a spiral has turns.
• Treating these four names as genus names instead of shape names.
• Assuming every spiral bacterium is a spirochaete.

No directly related past PYQ was found.