Depending on the shapes, bacteria can be classified as bacillus, spirillum and coccus. Bacillus are rod-shaped cells, such as Escherichia coli cells. Coccus are spherical or round shaped cells, such as Deinococcus radiodurans. Spirillum cells are helical in shape, such as Leptospira interrogans.
Bacteria can also assume certain arrangements and are named depending on the shape. For example, the cocci (plural for coccus) that remain paired are known as diplococci. Similarly, streptococci are spherical bacteria that remain in chain like arrangement. Cocci that arrange themselves in clusters similar to grapes are termed staphylococci. Rod shaped cells that are arranged in chains are called streptobacilli. Spiral bacteria such as vibrio have only one twist in their body, while the spirilla have helical structure.
Apart from these major shapes (spherical, rod and helical) a number of other shapes such as rectangular or star-shape are also possible.
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