Morphology (Shell Shape) and Arrangement

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):

(3) spiral cells (spirilla,singular=spirillum):

Variations of these shapes include slightly curved rods (vibrios), short rods (coccobacilli) and flexible spirals (spirochetes). 

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.