Bacteria are classified by shape into three primary groups: cocci, bacilli, and spiral-shaped. The cocci are round, the bacilli are rods, and the spiral-shaped bacteria are either rigid (spirilla) or flexible (spirochetes).

Cocci (or coccus for a single cell) are round cells, sometimes slightly flattened when they are adjacent to one another. Bacilli (or bacillus for a single cell) are rod-shaped bacteria. Spirilla (or spirillum for a single cell) are curved bacteria which can range from a gently curved shape to a corkscrew-like spiral.

The spiral bacteria are spirilla (singular form is spirillum) and the rod-shaped bacteria are bacilli (singular form is bacillus). The coccus (plural form is cocci) bacteria are spherical and are usually found in clusters or chains. Bacilli have a large surface area that helps them take in nutrients, but they may dry out easily.