Morphology deals with the study of size, shape, structure and arrangement of organism.
Shape of bacteria
Bacteria exhibit three fundamental shapes:
A. Spherical
B. Rods
C. Spiral or helical bacteria
A. Spherical bacteria :
Spherical bacterial cells are designated as cocci (cocci, singular, coccus). Some cocci (Gr., kokkos = grain or kernel) are perfect spheres others are slightly elongated or ellipsoidal in shape.
B. Rod shaped bacteria :
The cylindrical cells are termed as rods or bacilli (bacilli, singular, bacillus). The bacillus (L., bacillium = a stick or a rod) is the most common shape.
The rods show considerable variation in length/width ratio. The coccobacilli are short and wide and resemble cocci.
The rod's ends show variation in shape such as flat, rounded and cigar-shaped.
C. Spiral or Helical bacteria :
The spiral (L., spiralis = coiling) bacteria are curved rods vary in diameter, length, number of curves, rigidity and flexibility. They are of following types.
b. Spirochetes : The highly flexible helical bacteria showing series of twists are called as spirochetes. These bacteria resemble a spring.
c. Comma bacteria : The short incomplete spirals are known as common bacteria or vibrois.
Bacteria with unusual shapes :
In addition to three fundamental shapes, many others also occur: pear shaped cells, (e.g., Pasteuria); rods with square ends, (e.g., Bacillus anthracis); square cells, (e.g., Arcula); lobed spheres, (e.g., Sulfolobus); and many others.
Pleomorphism :
Most bacterial species have characteristic and constant shape. However, some organisms show changes in their shapes to some extent. This phenomenon is known as "pleomorphism". For example, Arthrobacter species are highly pleomorphic. Rhizobium is a rod shaped organism. It shows rod shapes when cultivated in laboratory. In root nodules of leguminous plants, Rhizobium cells show X, V, L and Y shapes i.e., pleomorphism. Other organisms such as Corynebacterium diphtheriae and mycoplasmas are also pleomorphic.