Morphology of Bacteria

Morphology of Bacteria

Bacteria are microscopic, single-celled organisms and they are found essentially everywhere on Earth - in soil, water, air, hot springs, deep in rock, and in and on the bodies of other living things - a distribution so universal that it is described as ubiquitous. Despite this huge diversity of habitats, bacterial cells themselves are built on just a few basic shapes. Cocci are spherical or ball-shaped; bacilli are rod-shaped; and spirilla are spiral or corkscrew-shaped. These three basic shapes may occur singly or grouped together in characteristic arrangements (in pairs, chains or clusters), and such arrangements are often used, alongside the basic shape, to describe or identify a particular bacterium.

Not every bacterium keeps a single fixed shape throughout its life, however. Bacteria that can vary in shape, rather than showing one constant, rigid form, are called pleomorphic. Rhizobium, the nitrogen-fixing bacterium associated with the root nodules of leguminous plants, is a commonly cited example of a pleomorphic bacterium.

Bacterial motility is usually provided by whip-like surface appendages called flagella, and bacteria are further classified by how many flagella they bear and how those flagella are arranged on the cell surface. A cell with no flagella at all is described as atrichous. A cell with a single flagellum at one pole is monotrichous; one with flagella at both poles is amphitrichous; one with a tuft of several flagella clustered at one pole is lophotrichous; and one with flagella distributed all over its entire surface is peritrichous. So, for example, a peritrichous bacillus is simply a rod-shaped bacterial cell that has flagella covering its whole body surface rather than being confined to one end.