Bacteria are almost always single celled, prokaryotic microscopic organisms. They are typically much smaller than eukaryotic cells because they lack lots of distinct organelles, such as nuclei. Despite being single celled, independent organisms, they generally exist in groups or colonies and essentially exist as a unified population. There are three main shapes of bacteria: bacillus, coccus, and spiral.
Bacillus, or rod shaped bacteria, are one of the main forms, and are typically 0.5 micrometers to 1.0 micrometer in width, and from 1.0 micrometer to 4.0 micrometers in length. Examples of bacilli include Bacillus anthracis, the bacterium that causes anthrax, and the well known E. coli. This shape of bacteria can also form long chains called streptobacillus. Another form is called coccobacillus, which describes a class of bacteria whose shape is somewhere between that of the coccus and the bacillus.
Another common shape of bacteria is the coccus. Cocci are spherically shaped bacteria that organize in several distinct arrangements when in groups: diplococcus, tetrad, sarcina, and staphylococcus arrangements. The diplococcus arrangement is characterized by cell division along one plane, where the bacteria will appear to form chains. Division along two planes yields a tetrad arrangement, characterized by squares of four bacteria. Division along three planes yields a sarcina arrangement, which is a cube made up of eight cells. The last arrangement occurs when the cells divide along random planes, and as one might guess, yields a random arrangement of the cells.
The last recognized form of bacteria is known as the spiral, which occurs in three distinct sub-forms. The first one is a called a vibrio, in which cells are characterized by a comma shaped rod. The second sub-form is called a spirillium. This refers to a cell that forms a thick, stiff spiral. The last sub-form is termed a spirochete, which is very closely related to the spirillium, but typically the spiral form is thinner and more flexible than the former. The spiral form can be rather large relative to the others, ranging in size from 1.0 micrometer to over 100 micrometers.
The above three forms describe how the vast majority of bacteria exist in nature, however like most rules, there do exist some exceptions, although they are much less common. Some of these include filament strains, star shapes, square shapes, and even some more complex arrangements. Some have even been termed pleomorphic, meaning they are actually variable in shape.