Name the three basic bacterial shapes and provide an example for each.

• A. Coccus — Staphylococcus aureus; Bacillus/rod — Escherichia coli; Spirillum — Spirillum minus.
• B. Coccus — Vibrio cholerae; Bacillus — Streptococcus pyogenes; Spiral — Escherichia coli.
• C. Coccus — Helicobacter pylori; Rod — Staphylococcus aureus; Spiral — Bacillus cereus.
• D. Coccus — Escherichia coli; Rod — Vibrio vulnificus; Spiral — Listeria monocytogenes.

Answer: (A)

Bacteria are commonly categorized by shape into three basic forms: cocci (spherical), bacilli (rod-shaped), and spirilla (spiral-shaped). Staphylococcus aureus is a cocci, typically found in grape-like clusters that highlight the round shape. Escherichia coli is a bacillus, a classic rod-shaped bacterium. Spirillum minus is a spiral-shaped bacterium, illustrating the third form.

These three shapes are the simplest way to visualize bacterial morphology, though some bacteria can vary or appear curved rather than strictly spherical, rod, or spiral. Other options mix shapes with organisms that don’t align with these basic categories—for example, a curved rod listed as spiral or a cocci paired with a rod—so they don’t fit the standard trio.