The bacterial flagellum is a long, hairlike, helically shaped structure that acts as an organelle of locomotion in certain prokaryotic and eukaryotic cells. Bacterial Flagella protrudes from the cell body of certain bacteria and are made up of single kind of protein subunit called flagellin. A Flagellum consists of three parts, Filament, hook, and basal body.

(a) Filament or shaft:
The outermost long region of the flagellum is called filament or shaft. It is a very long structure which acts as a propeller and is made up of globular proteins, the flagellin. The flagellins are arranged in several chains that intertwine and form a helix around a hollow.

(b) Hook: The wider region at the base of the flagellum is called Hook. It is present outside the cell wall and connects filament to the motor portion of the flagellum called the basal body. The hook in Gram-positive bacteria is slightly longer than the Gram-negative bacteria.

(c) Basal body: The basal body is anchored in the cytoplasmic membrane attaches the flagellum to the cell wall and plasma membrane. It is composed of a small central rod inserted into a set of rings which are surrounded by a pair of proteins called Mot.
Gram-negative bacteria have two pairs of such rings. The proximal (outer) pair of rings: the L-ring associates with the lipopolysaccharides, the P-ring associates with peptidoglycan and the Distal ( inner) pair of rings i.e. the M ring is anchored in the plasma membrane, and the S ring is directly attached to the plasma membrane. Gram-positive bacteria have only the distal (inner) pair of these rings: the M-ring is attached to the cell membrane and the S-ring is in the peptidoglycan layer.

The numbers of flagella, as well as their distribution on the cell, varies depending on the bacterial species and is used to characterize or distinguish bacteria. Flagella arrangements may be polar (at one or both ends of the bacterium)or lateral (along with the side of the bacterium).

Different arrangements of bacterial flagella

1. Monotrichous – a single flagellum at one pole E.g. Vibrio cholera
2. Amphitrichous – single flagellum at both poles. Eg. Spirilla
3. Lophotrichous – two or more flagella at one or both poles of the cell E.g. Spirillum undula
4. Peritrichous – multiple flagella distributed over the entire cell surface E.g. E.coli
5. Atrichous – absent of flagella. E.g. Shigella

Functions of Bacterial Flagella
Many prokaryotes are motile, and move by means of flagella.

Medical Importance of Flagella

Role in Pathogenesis: The flagella of Escherichia coli and Proteus spp. Are known to play a role in uropathogenesis. Flagella help these bacteria by propelling up the urethra into the bladder and cause complicated urinary tract infections (UTI)
Roles in Organism identification
• Some bacterial species are identified in the clinical laboratory by the use of Specific antibodies against flagellar proteins. eg. Salmonella spp.
• Organisms such as Vibrio cholerae and Proteus species are easily identified by their characteristics motility pattern.

Pili and Fimbriae:
A fimbria is a proteinaceous appendage that is used to attach the bacterium to a surface. It is also referred as attachment pilus. Both fimbriae and pili are the cell surface appendages present on bacterial cell wall other than flagella. They originate from cytoplasm that protrudes outside after penetrating the peptidoglycan layer of cell wall.

Functions of Pili and Fimbriae:
There are several functions of fimbriae and pili as given below:
(a) Bacteria containing fimbriae are called fimbriate bacteria. Fimbriae have the adhesive properties which attach the organism to the natural substrate or to the other organism.
(b) Fimbriae possess antigenic properties as they act as thermolabile nonspecific agglutinogen. It can cause agglutination of blood cells such as erythrocytes, leucocytes, and epithelial cells, etc.
(c) Fimbriae affect the metabolic activity. The Fimbrae containing cells (Fim+ cells) possess a higher rate of metabolic activity than the cells devoid of fimbriae (Fim– cells). Fimbrae function as aggregation organelles that is they can form stellate aggregation on a static liquid medium.
(d) The sex pili make contact between two cells and allow the transfer of DNA between bacteria, in the process of bacterial conjugation. Since conjugative pili posses hollow core, they act as conjugation tube. The tip of pilus recognizes the female (F–) cell through which the genetic material of donor (F+) cell passes to the recipient (female) cell.