Introductory Microbiology

Size, shape and arrangements of bacterial cells

Size
Bacteria are very small, most being approximately 0.5 to 1.0 micrometer in diameter. A relatively large surface through which nutrients can enter compared to small volume of cell. Hence, bacteria havehigh rate of growth and metabolism.

Shape and arrangement
The shape of the bacterium is governed by its rigid cell wall, however, exactly what attribute of this rigid material determines that a cell will have a particular shape is not yet understood. Typical bacterial cells are spherical (cocci), straight rods (Bacilli), or rods that are helically curved (spirilla), although most bacterial species have cells that are of fairly constant and characteristic shape, some have cells that are pleomorphic i.e that can exhibit a variety of shape.
Bacterial cells are usually arranged in a manner characteristics of their particular species. Although, it is rare that all the cells of a species are arranged in the same manner, it is the predominant arrangement that is the important feature.
Cocci appear in several characteristic arrangements depending on the plane of cellular division and whether the daughter cells stay together following division.
A. Diplococci - Cells divide in one plane and remain attached predominantly in pairs.
B. Streptococci- Cells divide in two planes and remain attached to form chains.e. g., Streptococcus
C. Tetrads (Tetracocci)-Cells divide in two planar and characteristically forms groups of four cells. e. g., Aerococcus
D. Staphylococci- Cells divide in three planes, in an irregular pattern, producing in bunches of cocci. E. g., Staphylococcus
E. Sarcinae- Cells divide in three planes in a regular pattern producing a cuboidal arrangement of cells.
Bacilli are not arranged in patterns as complex as those of cocci, and most occur singly or in pairs (Diplobacilli). But some species such as B. subtilis, form chains (Streptobacilli); others such as Beggiatoa and Saprospira sps, form trichomes, which are similar to chains but have a much larger area of contact between the adjacent cells.
In other Bacillus sps, such as Corynebacterium diphtheriae the cells are lined side by side like match sticks (palisade arrangement) and at angles to one another.
Still others, such as Streptomyces sps, form long, branched, multiunucleate filaments called hyphae, which collectively form mycelium.
Diagram of cocci, bacilli and spiral bacteria
Curved bacteria are usually curved with a twist. Bacteria with less than one complete twist or turn have a Vibrioid shape. Whereas those with one or more complete turns have a helical shape, Spirilla are rigid helical bacteria, whereas spirochetes are highly flexible.
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