Bacilli (rod-shaped), spirilli (spiral-shaped), and cocci (spherical-shaped) are the major classifications of bacteria by shape.

Classification of bacteria by shape provides scientists and pathologists with a common language to talk about the bacteria, as well as a way to identify different species of bacteria.

Most bacteria exist in one of three shapes, spherical bacteria, known as cocci, include common pathogens such as Staphylococcus aureus causing skin infections. Rod-shaped bacteria, like Escherichia coli, are known as bacilli. Finally, spiral-shaped bacteria, known as spirilli, include such species as Helicobacter pylori, which causes gastric ulcers.

Cocci, bacilli, and spirilli. Prokaryotes fall into three basic categories based on their shape, visualized here using scanning electron microscopy: (a) cocci, or spherical (in a pair); (b) bacilli, or rod-shaped; and (c) spirilli, or spiral-shaped. 

Key Points

• Bacilli are rod-shaped bacteria; Escherichia coli which cause diarrhea are a typical example of bacilli.

• Spirilli are spiral-shaped bacteria; Helicobacter pylori which cause gastric ulcers are a typical example of spirilli

• Cocci are spherical-shaped bacteria; Staphylococcus aureus which cause skin infections are a typical example of cocci.

Key Terms

pathologists: physicians who specialize in the identification and characterization of disease

cocci: spherical bacteria

bacilli: rod-shaped bacteria

spirilli: spiral-shaped bacteria

prokaryote: an organism whose cell (or cells) are characterized by the absence of a nucleus or any other membrane-bound organelles