What is the term for spherical bacteria?

Spherical or nearly spherical bacteria are called cocci; a single cell is a coccus. The word describes bacterial shape, not a particular species, genus, or disease.

Cocci can belong to many different groups and may appear in distinctive arrangements depending on how the cells divide and remain attached. Diplococci occur in pairs, streptococci form chains, and staphylococci form irregular, grape-like clusters. Tetrads contain four cells in a square-like arrangement, while sarcinae form more three-dimensional packets.

The term comes through Latin from the Greek word kokkos, meaning “berry,” a reference to the rounded appearance. Shape alone does not identify a bacterium: cocci can differ in their cell walls, metabolism, habitats, and ability to cause disease. Some medically important cocci include Staphylococcus aureus, Streptococcus pyogenes, and Neisseria gonorrhoeae.

The distractors describe other bacterial forms. Bacilli are rod-shaped, vibrios are curved comma-shaped cells, and spirilla are rigid spiral cells. A coccobacillus is an intermediate form—shorter and stouter than a typical rod, but not truly spherical—so it should not automatically be classified as a coccus.