ABSTRACT
The model organism used in bacteriology, Escherichia coli, is a rod-shaped organism, about 3 µm in length and 1 µm in diameter. Many very different Bacteria have this rod-like morphology, sometimes called a bacillus. Bacilli can be found in all the taxonomic groups of the Bacteria, as well as in the Archaea. This means that, unlike the classification of higher organisms, shape is not a reliable characteristic when classifying prokaryotes (Table 1), even though it is one of the few visible differences between cells when using a light microscope. However, the shape of some microorganisms has had an influence on the naming of some prokaryotes. The Gram-positive Bacterial genus Bacillus is, of course, made up of rod-shaped species, while another Gram-positive genus, Streptococcus, is made up of species of spherical bacteria (or cocci) about 1 µm in diameter.
