- Round bacteria called cocci (singular: coccus), cylindrical, capsule-shaped ones known as bacilli (singular: bacillus)
- Spiral bacteria, aptly called spirilla (singular: spirillum).
The shapes and configurations of bacteria are often reflected in their names. For example, the milk-curdling Lactobacillus acidophilus are bacilli, and pneumonia-causing S. pneumoniae are a chain of cocci. Some bacteria take other shapes, such as stalked, square or star.