What does cocci mean?
Cocci are bacteria whose cells are roughly spherical or ovoid in shape, making them one of the three classic bacterial forms recognized in microbiology, alongside the rod-shaped bacilli and the spiral spirilla. The word is the plural of 'coccus', so a single spherical bacterium is called a coccus, while any number of them are referred to collectively as cocci. Depending on how they divide, cocci may appear singly, in pairs (diplococci), in chains (streptococci), or in grape-like clusters (staphylococci), arrangements that are diagnostically important in identifying infections. The term belongs chiefly to scientific and medical writing, where it almost always appears with qualifiers such as 'gram-positive' or 'gram-negative'. Though unfamiliar outside laboratory contexts, cocci describe organisms of enormous practical significance, ranging from harmless skin dwellers to pathogens responsible for pneumonia and strep throat.
nounCocci are spherical or near-spherical bacteria, the plural form of 'coccus'. They are one of the three principal bacterial shapes, alongside rod-shaped bacilli and spiral spirilla.
- Spherical or ovoid bacteria, especially as classified by shape in microbiology.
"The lab report identified gram-positive cocci arranged in clusters, consistent with Staphylococcus."
"Under the microscope, the cocci appeared as neat chains characteristic of Streptococcus."
"Bacteria are commonly grouped by shape: cocci are round, bacilli are rod-shaped, and spirilla are spiral."
This is already the plural form; the singular is 'coccus'. Avoid treating 'cocci' as a singular noun ('a cocci' is incorrect).
"Several species of cocci colonize human skin without causing harm."
Say 'cocci' aloud and it rhymes with 'sky' — yet these tiny spheres, some arranged like grape bunches, include microbes that live harmlessly on your skin and others that cause serious disease.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cocci
The word comes from the ancient Greek 'kokkos', meaning a grain, seed, or berry, which passed into New Latin as 'coccus' before being adopted into English microbiological vocabulary in the nineteenth century as bacteriology developed its shape-based classification. The same Greek root lies behind the Latin 'coccum', a berry-like gall used to produce scarlet dye, connecting the word distantly to the crimson-dye lineage that also gave us carmine and cochineal. The English plural follows the Latin pattern, hence 'cocci'.
Related word forms
How cocci is actually used
Strictly scientific register, used mainly in microbiology and medicine. It is an irregular plural borrowed from New Latin, so 'a cocci' (singular use) is incorrect — the singular is 'coccus'. In clinical writing it frequently appears with Gram-staining qualifiers such as 'gram-positive' or 'gram-negative'.
Easily confused with cocci
A coccus is a single spherical bacterium, while cocci is simply its plural, referring to multiple such bacteria.
'Cocky' is an adjective meaning arrogantly self-confident, whereas 'cocci' is a scientific noun denoting round-shaped bacteria.