Cellular Morphology

Bacteria are classified by observations made with a light microscope, based on their morphology and aggregation. The morphology of a bacteria has no correlation to its reactivity to Gram staining.

Coccus
Coccus (plural cocci) refers to a spherical or nearly spherical microorganism that are on average only 1 micrometer in diameter, which makes it difficult to see under the light microscope.

Arrangements
Streptococci are chain of cocci, with an arrangement similar to that of a string of pearls.

Staphylococci are a bunch of cocci, with an arrangement similar to that of a bunch of grapes.

Diplococci are two cocci together, an example is the bacteria Neisseria.

Bacillus
Bacillus (plural bacilli), the shape of bacteria, is not to be confused with Bacillus, the genus. Bacilli are described as rod-shaped bacteria. The average bacillus it 2 micrometers long by 0.5 micrometers wide.

Arrangements
Streptobacilli are a chain of bacilli, and includes Bacillus anthracis.

Palisades are bacilli arranged next to each other, like a chain but connecting the long side of the bacilli. A notable example is Corynebacterium.

Solitary bacilli include Clostridium tetani, Escherichia coli, and Klebsiella pneumoniae

Spiral Shaped
Spiral shaped bacteria are relatively long and have at least one bend in them. These are a varied group and are usually 0.5 micrometers wide and 1 to 14 micrometers long.

Vibrio, the shape, describes a bacteria that is bent at one place. These are often confused with bacilli because the bend can be hard to see, and there is some controversy as to whether or not it is really spiral shaped. The main example of this is Vibrio cholerae.

Spirochetes describe a bacteria that is long and has a corkscrew shape, such as Treponema pallidum and Borrelia.