1. Cocci: These are spherical or round bacteria. They can occur individually, in pairs (diplococci), in chains (streptococci), in clusters (staphylococci), or in cubes (sarcinae).

2. Bacilli: These are rod-shaped bacteria. They can be short and plump or long and slender. Some bacilli are curved or slightly bent.

3. Spirilla: These are spiral-shaped bacteria. They can be rigid and corkscrew-shaped (spirilla) or flexible and wavy (spirochetes).