A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape. It is one of the three distinct bacterial shapes, the other two being bacillus (rod-shaped) and spiral-shaped cells.
Cocci may occur as single cells or remain attached following cell division. Those that remain attached can be classified based on cellular arrangement:
- Diplococci are pairs of cocci (e.g. Streptococcus pneumoniae and Neisseria gonorrhoeae)
- Streptococci are chains of cocci (e.g. Streptococcus pyogenes).
- Staphylococci are irregular (grape-like) clusters of cocci (e.g. Staphylococcus aureus).
- Tetrads are clusters of four cocci arranged within the same plane (e.g. Micrococcus sp.).
- Sarcina is a genus of bacteria that are found in cuboidal arrangements of eight cocci (e.g. Sarcina ventriculi).