![Staphylococcus aureus Gram](https://upload.wikimedia.org/wikipedia/commons/0/08/Staphylococcus_aureus_Gram.jpg)
A coccus (plural cocci) is any
bacterium
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
or
archaeon
Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaebact ...
that has a spherical, ovoid, or generally round shape.
Bacteria are categorized based on their shapes into three classes: cocci (spherical-shaped),
bacillus
''Bacillus'' (Latin "stick") is a genus of Gram-positive, rod-shaped bacteria, a member of the phylum ''Bacillota'', with 266 named species. The term is also used to describe the shape (rod) of other so-shaped bacteria; and the plural ''Bacilli ...
(rod-shaped) and spiral (
of which there are two types: spirillum and spirochete).
Coccus refers to the shape of the bacteria, and can contain multiple genera, such as staphylococci or streptococci. Cocci can grow in pairs, chains, or clusters, depending on their orientation and attachment during cell division. In contrast to many bacilli-shaped bacteria, most cocci bacteria do not have flagella and are non-motile.
Cocci is an English
loanword
A loanword (also loan word or loan-word) is a word at least partly assimilated from one language (the donor language) into another language. This is in contrast to cognates, which are words in two or more languages that are similar because th ...
of a
modern or neo-Latin noun, which in turn stems from the
Greek
Greek may refer to:
Greece
Anything of, from, or related to Greece, a country in Southern Europe:
*Greeks, an ethnic group.
*Greek language, a branch of the Indo-European language family.
**Proto-Greek language, the assumed last common ancestor ...
masculine noun () meaning 'berry'.
Structure
The
cell wall
A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
structure for cocci may vary between gram-positive (thick peptidoglycan layers) and gram-negative (thin peptidoglycan layers). While living in their host organism, cocci can be pathogenic (e.g., streptococcus), commensal, or symbiotic.
Gram-positive Cocci
The gram-positive cocci are a large group of loosely bacteria with similar morphology. All are spherical or nearly so, but they vary considerably in size. Members of some genera are identifiable by the way cells are attached to one another: in pockets, in chains, or grape-like clusters. These arrangements reflect patterns of cell division and that cells stick together. ''Sarcina'' cells, for example, are arranged in cubical pockets because cell division alternates regularly among the three perpendicular planes. ''Streptococcus'' spp. resemble a string of beads because division always occurs in the same plane. Some of these strings, for example, ''S. pneumoniae,'' are only two cells long. They are called ''diplococci.'' Species of ''Staphylococcus'' have no regular plane of division. They form grape-like structures.
The various gram-positive cocci differ physiologically and by habitat. ''Micrococcus'' spp. are obligate aerobes that inhabit human skin. ''Staphylococcus'' spp. also inhabit human skin, but they are facultative anaerobes. They ferment sugars, producing lactic acid as an end product. Many of these species produce carotenoid pigments, which color their colonies yellow or orange. ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...
'' is a major human pathogen. It can infect almost any tissue in the body, frequently the skin. It often causes nosocomial (hospital-acquired) infections.
Arrangements
Cocci may occur as single cells or remain attached following cell division. Those that remain attached can be classified based on cellular arrangement:
*
Diplococci
A diplococcus (plural diplococci) is a round bacterium (a coccus) that typically occurs in the form of two joined cells.
Types
Examples of gram-negative diplococci are '' Neisseria spp.'' and ''Moraxella catarrhalis.'' Examples of gram-positiv ...
are pairs of cocci (e.g. ''
Streptococcus pneumoniae
''Streptococcus pneumoniae'', or pneumococcus, is a Gram-positive, spherical bacteria, alpha-hemolytic (under aerobic conditions) or beta-hemolytic (under anaerobic conditions), aerotolerant anaerobic member of the genus Streptococcus. They are ...
'' and ''
Neisseria gonorrhoeae
''Neisseria gonorrhoeae'', also known as ''gonococcus'' (singular), or ''gonococci'' (plural), is a species of Gram-negative diplococci bacteria isolated by Albert Ludwig Sigesmund Neisser, Albert Neisser in 1879. It causes the sexually transmit ...
'')
*
Streptococci
''Streptococcus'' is a genus of gram-positive ' (plural ) or spherical bacteria that belongs to the family Streptococcaceae, within the order Lactobacillales (lactic acid bacteria), in the phylum Bacillota. Cell division in streptococci occurs ...
are chains of cocci (e.g. ''
Streptococcus pyogenes
''Streptococcus pyogenes'' is a species of Gram-positive, aerotolerant bacteria in the genus ''Streptococcus''. These bacteria are extracellular, and made up of non-motile and non-sporing cocci (round cells) that tend to link in chains. They are ...
'').
*
Staphylococci
''Staphylococcus'' is a genus of Gram-positive bacteria in the family Staphylococcaceae from the order Bacillales. Under the microscope, they appear spherical (cocci), and form in grape-like clusters. ''Staphylococcus'' species are facultative ...
are irregular (grape-like) clusters of cocci (e.g. ''
Staphylococcus aureus
''Staphylococcus aureus'' is a Gram-positive spherically shaped bacterium, a member of the Bacillota, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive ...
'').
*
Tetrads
Tetrad ('group of 4') or tetrade may refer to:
* Tetrad (area), an area 2 km x 2 km square
* Tetrad (astronomy), four total lunar eclipses within two years
* Tetrad (chromosomal formation)
* Tetrad (general relativity), or frame field
** Tetrad fo ...
are clusters of four cocci arranged within the same plane (e.g. ''
Micrococcus
''Micrococcus'' (mi’ krō kŏk’ Əs) is a genus of bacteria in the Micrococcaceae family. ''Micrococcus'' occurs in a wide range of environments, including water, dust, and soil. Micrococci have Gram-positive spherical cells ranging from abo ...
sp.'').
*
''Sarcina'' is a genus of bacteria that are found in cuboidal arrangements of eight cocci (e.g. ''
Sarcina ventriculi'').
References
{{Bacteria
Bacteria