What is the flagellar structure in bacteria? Explain the grouping the bacteria on the basis of absence or presence of flagella?

Hint: A lash-like body part of protozoans and eukaryotic cells which protrudes from the cell body. It aids in locomotion, but it also acts often as a sensory organelle, becoming responsive outside the cell to chemicals and temperatures.

Complete answer:
The complex cytoplasmic filamentous structure hanging down through the wall of the cell is flagella. These are unbranched, long, thread-like structures, intricately embedded in the cell envelope, mainly composed of the protein flagellin. They have a diameter of around 12-30 nm and a length of 5-16 μm. The responsibility for bacterial motility lies with them.
There are three distinct components of each flagellum: filament, hook and basal body.
Outside the cell lies the filament.
In the cell envelope, Hook is embedded.
The basal body is connected by ring-like structures to the cytoplasmic membrane.
Flagella are organelles, rather than structure, identified by feature. Flagella varies significantly. It is possible to use both prokaryotic and eukaryotic flagella for swimming, but they vary greatly in protein content, structure, and propulsion mechanisms.
On the basis of absence or presence of the flagella, the bacteria can be classified as:
Atrichos: There is no flagella in these bacteria. Corynebacterium diphtheriae is an example of Atrichous bacteria.
Monotrichous: At one end of the bacteria cell, one flagellum is attached. For example: Cholerae vibrio.
Lophotrichous: One end of the bacterial cell is connected to a bunch of flagella. E.g.: Pseudomonas.
Amphitrichous: From both ends of the bacterial cell, a bunch of flagella. Rhodospirillum rubrum is an example of amphitrichous bacteria.
Peritrichous: Around the entire bacterial cell, the flagella are uniformly distributed. Bacillus is an example of Peritrichous bacteria.

Note: Bacterial motility is the responsibility of bacterial flagella. In survival and the ability of some bacteria to cause illness, motility plays an important role. In liquids (swimming) or on surfaces (swarming), they thrust cells so that cells can shift into favourable environments.