Shape & Arrangement of Bacterial Cells

Although bacteria are very small and simple organism, there is huge variation in their shapes.  This variation is due to the differences in genetics and ecology. The basic shapes of bacteria are coccus, bacillus, vibrio and spirilla.

Cocci

Cocci or singular coccus are spherical or ovoid shape cells. When cells divide, they can remain attached and form some arrangements. Diplococci (singular: diplococcus) are one in which cocci remains in pair after dividing as seen in Neisseria. When cocci divides repeatedly and remains attached in a chain like pattern they forms streptococci as seen in genera Enterococcus, Streptococcus etc. Some cocci remains in the group of four i.e. they forms square by dividing in two planes. They are called tetrads as seen in genus Micrococcus. Those cocci that divides in three plane and leads to the formation of cubical packet of eight cells are called sarcinae as observed in genus Sarcina. Staphylococcus divides in multiple planes and forms an irregular grapelike cluster called staphylococci.

Bacilli

Bacilli or singular bacillus are rod shape cells. Bacillus megaterium is a typical rod shape bacteria. The shape of the end of the rod varies from species to species and it can be round, flat, bifurcated or cigar shaped. Diplobacilli are one in which bacilli remains in pair after division. They can also divide repeatedly and forms chain called streptobacilli.

Vibrio

Vibrios are comma shaped cells. They basically look like a curved rod. For example, Vibrio cholerae are comma shaped Gram negative bacteria.

Spirilla

As name implies, they are spiral or helical shaped cells. They basically have rigid bodies as seen in members of genus Spirillum. Another group of spirilla are there which are also helical in shape but they have flexible body; they are known as spirochetes. Example of spirochetes include Leptospira species.

Note: Most of the bacteria maintains single shape i.e. they are monomorphic. However, some bacteria lack the single, characteristics shape and have many shapes. They are known as pleomorphic; for example Corynebacterium.