Morphology of Bacterial Cell

Bacteria are morphologically differentiated into various shapes based on the microscopic observations i.e. cell size, cell shape, cell staining characteristics and the manner in which the cells are arranged together. Most bacteria range from 0.2 to 2.0 µm in diameter and from 2 to 8 µm in length. However some cells having higher size range and can be up to 200 - 300 µm in length i.e. *spirochete*.

The smallest bacterial cell was observed of *Mycoplasma pneumoniae* (0.25 µm). Bacterial cells have few basic shapes:

Cocci

Cocci are usually round but can be oval, elongated, or flattened on one side. When cocci divide to reproduce, the cells can remain attached to one another and remain in pairs after divisions are called diplococci; e.g. *Diplococcus pneumoniae*.
Some cells divide and remain attached in chain like patterns are called streptococci. Example: *Streptococcus pyogenes*.
Those that divide in two planes and remain in groups of four are known as tetrads. Example: *Tetragenococcus spp.*, *Aerococcus spp.*
Those that divide in three planes and remain attached in cube like groups of eight are called sarcinae. Example: *Sarcina aurantiaca*, *Sarcena lutea*.
Those that divide in multiple planes and form grapelike clusters are called staphylococci. Example: *Staphylococcus aureus*, *Staphylococcus capitilis*.

TYPES OF BACILLUS SHAPED BACTERIA

Bacillus are the rod shaped bacteria, and after cell division they can also appear in many different forms. These are:
Most bacilli appear as single rods.
Diplobacilli appear in pairs after division. Example: *Coxiella burnetii*.
Streptobacilli occur in chains. Example: *Streptobacillus felis*.
Still others are oval and look so much like cocci that they are called Coccobacilli. Example: *Haemophilus influenzae*, *Bordetella pertussis*.

TYPES OF CURVED SHAPED BACTERIA

Spiral bacteria have one or more twists; they are never straight.
Bacteria that look like slightly curved rods (incomplete curve / twist), or Comma ( , ) shaped are called vibrios. Example: *Vibrio Cholerae*.
*Spirillum* have a helical shape, like a corkscrew, with rigid bodies and tufts of flagella. Examples: *Spirillum*, *Aquaspirillum*, *Azaspirillum*.
Yet another group of spirals are helical and flexible; they are called spirochetes. They lack flagella but move with an axial filament. Example: *Treponema pallidum*, *Leptospira interrogans*.

OTHER SHAPES

Some bacteria have pentagonic structure like stars (*Stella aquatica*).
Some bacteria are rectangular (*Haloarcula vallismortis*).
Certain bacterial cells lack a cell wall, making them flexible and variable in shape (*Pleomorphic bacteria*). Example: *Mycoplasma pneumoniae*.

TYPES OF BACTERIA ON THE BASIS OF FLAGELLA

Flagella are the complex filamentous cytoplasmic structure protruding through the cell wall. They are about 12-30 nm in diameter and 5-16 µm in length. They are responsible for bacterial motility.