Introduction:
One of the most important ways to describe bacteria is by their shape and arrangement.
Morphology (Shell Shape)
Bacterial cells are much smaller than eukaryotic cells and come in a variety of morphologies (shapes) and arrangements. Determining cell morphology is an important first step in identifying a bacterial species. Bacteria are classified into 3 different shapes:
(1) spheres (cocci, singular=coccus):
(2) rods (bacilli, singular=bacillus): Rods are less varied in arrangement (see below) becasue they divide only in one plane. They occur either as single cells, as a pair of cells with their ends attached, or as a chain of seeral cells (streptobacilli).
(3) spiral cells (spirilla,singular=spirillum):
Variations of these shapes include slightly curved rods (vibrios), short rods (coccobacilli) and flexible spirals (spirochetes).
Cell Arrangement:
Bacterial cells can also be categorized according to arrangement, or the way they group together. The main factors influencing the arrangement of a particular bacterial type are its pattern of division while it is growing and how the cells remain attached afterward. The greatest variety in arrangement occurs in cocci, which can be single, in pairs (diplococci) in tetrads (groups of four), in irregular clusters (as in staphylococci and micrococci), or in chains of a few to hundreds of cells (streptococci). An even more complex grouping is a cubical packet of eight, sixteen, or mroe cells called a sarcina. Because bacteria usually reproduce by splitting in two, if the divided cells remain attached, they lead to these diverse arrangements.
Cell arrangement, determined by the number of planes in which division occurs is also useful in identifying bacterial. Spirillas rarely are seen as anything other than single cells, but cocci and bacilli do form multicellular associations.
The coccus is better suited for a dry environment than a bacillus because a sphere has less surface area and thus less moisture will be lost by osmosis in a dry environment. In a moist environment, the higher surface area to volume ratio of a rod shaped bacterium will allow greater efficiency in transferring water and solutes into and out of the cell, making it possible for a rod shaped cell to have a higher metabolic rate.