On the basis of morphology, bacteria are classified into the following three forms:

Spherical : These are coccus bacteria which are spherical in shape. The word coccus originated from the Greek word kokkos meaning berry. They are the smallest form of bacteria and are almost spherical in shape and appear in many different patterns depending on the planes in which they can be divided:

Micrococcus (Greek micros – small; kokkos – berry): These are present as a single cell, e.g., Micrococcus. These bacteria are mostly non-flagellated, grampositive, and facultative saprophytes or parasites.

Diplococci : These are present in the pair of two, e.g., Diplococcus pneumonia. A single coccus can be divided into two cocci by one plane but remain attached in pair.

Streptococci (Greek streptos – twisted) : These are present in chains, e.g., Streptococcus lactis. Although, cocci divides into one plane but remain arranged in the form of chains.

Staphylococci (Greek staphyle – bunch of grapes) : These are present in groups, e.g., Staphylococcus aureus.

Tetrads (Tetracoccus) : These are present in groups of four and can be divided into two planes.

Sarcinae : These are present in cube-like groups of eight and can be divided into three planes, e.g., Sarcina.

In figure, structures of different forms of cocci are shown:

Cylindrical or Rod-Shaped : These bacteria are rod-shaped and are also known as bacillus (meaning rod-shaped, singular bacilli). In number, the rod-shaped bacteria are more than the spherical bacteria as their ratio of the surface area to volume is greater.

These are of the following types:

Bacillus : These are single rod-shaped bacteria.

Diplobacillus : These bacteria occur in pair.

Streptobacillus : These are grouped in the form of chains, e.g., Bacillus anthracis.

In figure, different forms of bacilli are shown:

Spiral : Shape of these bacteria is never straight and one or more twists are found in them. These are sub-divided into:

Vibrio or Comma : These bacteria are comma [,] shaped, e.g., Vibrio cholera.

Spirilla : The word spirilla originated from the Greek word spira meaning coil. Shape of these bacteria is coiled or curved rod-like. These bacteria have rigid body having a helical shape (like a corkscrew), e.g., Spirillum ruprem.

Spirochetes : These bacteria use an axial filament found on the external flexible sheath for their movement, e.g., Treponema palladium.

In figure, different forms of spiral bacteria are shown:

Other than these three basic shapes, some bacteria are star-shaped (genus Stella), square, and flat (halophilic archae) of the genus Haloarcula.