Bacterial flagella are long, thin (about 20 nm), whip-like appendages that move the bacteria towards nutrients and other attractants. Flagella are free at one end and attached to the cell at the other end. Flagellum can never be seen directly with the light microscope but only after staining with special flagella stains that increase their diameter.
Flagella are usually found in gram-negative bacilli. Gram-positive rods (e.g., Listeria species) and cocci (some Enterococcus species, Vagococcus species) also have flagella.
The long filament of flagella is composed of many subunits of a single protein, flagellin, arranged in several intertwined chains. The energy for movement, the proton motive force, is provided by ATP.
Key Information: Most of the cocci (e.g. Staphylococci, Streptococci, etc) don’t have flagella so they are non-motile.
Flagella are helical shaped structure composed of subunits of a protein called flagellin. The wider region at the base of the flagellum is called a hook. It is different in structure than that of the filament. Hook connects filament to the motor portion of the flagellum called a basal body.
The basal body is anchored in the cytoplasmic membrane and cell wall. There are presence of rings that are surrounded by a pair of proteins called Mot. These proteins actually drive the flagellar motor causing rotation of the filament. Another set of proteins called Fli proteins function as the motor switch, reversing the rotation of the flagella in response to intracellular signals.
Arrangement and Types of Bacterial Flagella
The number and location of flagella are distinctive for each genus. There are four types of flagellar arrangement.
- Monotrichous (Mono means one): Single polar flagellum e.g. Vibrio cholerae, Campylobacter spp. (polar flagella often in pairs to give a “seagull” appearance).
- Amphitrichous: Single flagellum at both ends e.g. Alcaligenes faecalis (Note: amphibians live both on land and in water).
- Lophotrichous: Tuft of flagella at one or both ends e.g. Spirilla spp
- Peritrichous (flagella in periphery): Flagella surrounding the bacterial cell. All the members of family Enterobacteriaceae, if motile have peritrichous flagella. e.g. Salmonella Typhi, Escherichia coli, Proteus spp (highly motile organism; shows swarming motility)
Functions of Bacterial Flagella
Many prokaryotes are motile, and the majority of motile prokaryotes move by means of flagella.
Medical Importance of Flagella
- Role in Pathogenesis: Escherichia coli and Proteus spp are common causes of urinary tract infections. The flagella of these bacteria help the bacteria by propelling up the urethra into the bladder.
- Roles in Organism identification
- Some species of bacteria, eg. Salmonella species are identified in the clinical laboratory by the use of specific antibodies against flagellar proteins.
- Organisms such as Vibrio cholerae (darting motility) and Proteus species (swarming growth in common culture media) are easily identified by their characteristics motility pattern.