Coccus is the scientific term for bacteria that are spherically-shaped or slightly oval. The plural term is cocci. Many species of bacteria are cocci, including both beneficial and harmful species.

With diameters averaging 0.5 to 1.0 micrometers, cocci are the smallest bacteria. Their spherical shape also gives them the largest surface area in relation to their size, which enables them to more efficiently take up nutrients from the environment. On the other hand, the greater surface area can also mean that a coccus is more susceptible to absorbing harmful substances.

A single coccus bacterium is called a monococcus. When bacteria are reproducing, monococci are formed only when a new cell splits completely from its parent cell. Some monococci are usually present in any species of cocci bacteria, but it is quite common for cocci to form groups of two or more cells.

When cocci bacteria reproduce and form two cells, there is often not a complete split between the parent bacterium and the newly formed one. This leads to several different formations of bacteria, though each individual cell retains the same spherical shape. The simplest formation is known as diplococcus, in which two coccus cells remain attached to each other. Neisseria is a genus of bacteria that often forms these pairs of cells. Many species of Neisseria are found in healthy human throats, but the bacteria that cause meningitis also belong to it.

Cocci bacteria can also form larger groups, including groups of four, eight, or many more. Groups of four are known as tetrads, while groups of eight are called sarcinae. Many bacteria forming a clump is called a staphylococcus, which is also the name of a genus of bacteria which often form this arrangement. The genus Staphylococcus includes many species that are commonly found on human skin, as well as some bacteria that reside in the soil. It also contains Staphylococcus aureus, which can cause staph infections and is often resistant to antibiotics.

Chains of coccus bacteria are called streptococci. Many bacteria that form chains are found in the genus Streptococcus. This formation can also be found in Enterococcus and Lactococcus.

Common species of Streptococcus include normal throat and nose bacteria, but the bacteria that cause pneumonia, strep throat, and scarlet fever are also streptococci. Enterococcus and Lactococcus species are lactic acid bacteria, meaning that they produce lactic acid — a substance that is necessary to create fermented dairy products, such as yogurt. Lactococci, in particular, are often used in the dairy industry.