Coccus (plural cocci) can be used to describe any bacterium that has a spherical shape. It is one of the three distinct types of bacteria shapes, the other two being bacillus (rod-shaped) and spirillum (spiral-shaped) cells.
coccus, plural Cocci, in microbiology, a spherical-shaped bacterium. Many species of bacteria have characteristic arrangements that are useful in identification. Pairs of cocci are called diplococci; rows or chains of such cells are called streptococci; grapelike clusters of cells, staphylococci; packets of eight or more cells, sarcinae; and groups of four cells in a square arrangement, tetrads. These characteristic groupings occur as a result of variations in the reproduction process in bacteria. See also Staphylococcus; Streptococcus.
Bacillus is a genus of Gram-positive rod-shaped bacteria and a member of the division Firmicutes. Bacillus species can be obligate aerobes or facultative anaerobes, and test positive for the enzyme catalase. Ubiquitous in nature, Bacillus includes both free-living and pathogenic species. Under stressful environmental conditions, the cells produce oval endospores that can stay dormant for extended periods. These characteristics originally defined the genus, but not all such species are closely related, and many have been moved to other genera.
Spirochaetes (also spelled Spirochetes) belong to a phylum of distinctive Gram-negative bacteria, which have long, helically coiled (spiral-shaped) cells. Spirochetes are chemoheterotrophic in nature, with lengths between 5 and 250 µm and diameters around 0.1-0.6 µm.
Spirochaetes are distinguished from other bacterial phyla by the location of their flagella, sometimes called axial filaments, which run lengthwise between the bacterial inner membrane and outer membrane in periplasmic space. These cause a twisting motion which allows the spirochaete to move about. When reproducing, a spirochaete will undergo asexual transverse binary fission.
Most spirochaetes are free-living and anaerobic, but there are numerous exceptions.
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