Coccus
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Through Bacteria and Gram-positive bacteria
Cocci is an English loanword of a modern or Neo-Latin noun, which in turn stems from the Greek masculine noun κόκκος (cóccos) meaning 'berry'.
Structure
The cell wall 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.
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Gram-positive Cocci
The gram-positive cocci are a large group of 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 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 are pairs of cocci (e.g. Streptococcus pneumoniae and Neisseria gonorrhoeae)
- Streptococci are chains of cocci such as 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 such as Micrococcus sp.).
- Sarcina describes a pack-like cuboidal arrangement of eight cocci such as Sarcina ventriculi.