Bacterial Flagella

Bacterial Flagella: Structure, importance and examples of flagellated bacteria

Bacterial flagella are long, thin (about 20 nm) whip-shaped appendages that move bacteria toward nutrients and other attractants. The flagella are free at one end and attached to the cell at the other end. The flagella can never be seen directly under the light microscope, but only after staining with special flagella stains that increase its diameter.

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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 interlocking chains. The energy for motion, the proton’s motive force, is provided by ATP.

Key Facts: Most cocci (eg staph, strep, etc.) do not have flagella, so they are not motile.

Flagella are helix-shaped structures made up of subunits of a protein called flagellin. The widest region at the base of the flagellum is called the hook. It is different in structure from that of the filament. The hook connects the filament to the motor part of the flagellum called the basal body.

The basal body is anchored in the cytoplasmic membrane and the cell wall. There are rings that are surrounded by a pair of proteins called Mot. These proteins actually drive the flagellar motor causing the filament to rotate. Another set of proteins called Fli proteins function as a motor switch, reversing the rotation of flagella in response to intracellular signals.

Arrangement and types of bacterial flagella

The number and location of the flagella are distinctive for each genus. There are four types of flagellar arrangement.

1. Monotrichous (Mono means one): Unique polar flagellum, p. Eg Vibrio cholerae, Campylobacter spp. (polar flagella often in pairs to give a “seagull” appearance).
2. Amphitrichous: Single flagellum at both ends, eg. Eg Alcaligenes faecalis (Note: amphibians live both on land and in water).

3. Lophotrichous: plume of flagella at one or both ends, eg. Eg Spirilla spp
4. Peritrichous (flagella on the periphery): Flagella that surround the bacterial cell. All members of the Enterobacteriaceae family, if mobile, have peritrichous flagella. eg Salmonella Typhi, Escherichia coli, Proteus spp (very mobile organism; shows swarm mobility)

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 the urethra towards the bladder.
• Functions in the identification of organisms
  • Some species of bacteria, eg. Eg Salmonella species are identified in the clinical laboratory by using specific antibodies against flagellar proteins.
  • Organisms such as Vibrio cholerae (dart motility) and Proteus species (swarming in common culture media) are easily identified by their characteristic motility pattern.

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Bacterial Flagella: Structure, importance and examples of flagellated bacteria