Flagella: Structure, Arrangement, Function

Flagella (singular, flagellum) are the locomotory structures of many prokaryotes. Most protozoa and some bacteria are motile. Protozoa use flagella, cilia, or pseudopods, whereas motile bacteria move only using flagella. The flagellum functions by rotation to push or pull the cell through a liquid medium.

Bacterial Flagella

Bacterial flagella are long, thin (about 20 nm), whip-like appendages that move the bacteria towards nutrients and other attractants. Like capsule and pili, flagella are external to the cell wall in some bacteria. 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 can be seen easily with the electron microscope.

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.

Most of the cocci (e.g. Staphylococci, Streptococci, etc.) don’t have flagella, so they are non-motile. Bacteria lacking flagella are called atrichous.

Structure

The long helical filament of bacterial flagella is composed of many subunits of a single protein, flagellin, arranged in several intertwined chains. A flagellum consists of several components and moves by rotation, much like a boat motor propeller. The base of the flagellum is structurally different from the filament.

The wider region at the base of the flagellum is called a hook. The hook connects the filament to the motor portion of the flagellum called a basal body.

The basal body is anchored in the cytoplasmic membrane and cell wall. It consists of a central rod that passes through a series of rings.

In gram-negative bacteria, three-set of rings are present in three different layers,

• L ring is anchored in the lipopolysaccharide layer.
• P ring is located in the periplasmic space and anchored in the cell wall’s peptidoglycan layer.
• MS ring is located within the cytoplasmic membrane. The smaller S-ring (stator ring) is attached to the M-ring or motor ring, forming the MS ring.
• C ring anchors the entire complex to the cell.

A series of proteins called Mot surrounds the inner pair of rings. These proteins drive the flagellar motor, causing rotation of the filament. Another set of proteins called Fli proteins functions as the motor switch, reversing the rotation of the flagella in response to intracellular signals.

The flagella of Gram-positive bacteria contain only two basal body rings; one ring is embedded in the peptidoglycan layer and another in the cell membrane.  

Arrangement and Types

Flagella are attached to bacterial cells in different places. As the number and location of flagella are distinctive for each genus, flagellar arrangements can be used to classify bacteria. There are four types of flagellar arrangement, flagella may be located at one of the cells (monotrichous flagella), flagella may be present at both ends of the cell (lophotrichous flagella), and a single flagellum may reside at both ends of the cell (amphitrichous flagella), or the entire cell surface is covered with flagella (peritrichous flagella).

Monotrichous

Single polar flagellum, e.g., Vibrio cholerae, Campylobacter spp. (polar flagella often in pairs to give a “seagull” appearance). Pseudomonas has polar monotrichous flagella.

Amphitrichous

Bacteria with two flagella, one at each end; both types are said to be polar. For example, Alcaligenes faecalis.

Lophotrichous

Bacteria with two or more flagella at one or both ends. For example, Spirillum has polar, amphitrichous flagella.

Peritrichous

Flagella surrounds the bacterial cell. All the motile bacteria of the family, Enterobacteriaceae, have peritrichous flagella. e.g., Salmonella Typhi,  Escherichia coli, Proteus spp.

Functions of Bacterial Flagella

• Organs of locomotion: Many prokaryotes are motile, and most motile prokaryotes move using 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 bacteria from the urethra into the bladder.
• Roles in Organism identification
  • Some species of bacteria, eg. Salmonella species, can be identified in the clinical laboratory using 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.

Archaeal Flagella

Flagella are also present in major species of Archaea. Major genera of methanogens, extreme halophiles, thermoacidophiles, and hyperthermophiles are capable of swimming motility. Still, their speed is comparatively less than that of bacteria, probably because of the small diameter of flagella.

Differences between Bacterial and Archaeal flagella are tabulated here:

Properties	Bacterial flagella	Archaeal flagella
Flagellar filament	Flagellar filament is made up of a single type of protein	Several different flagellin proteins are found.
Diameter of Flagella	The diameter of bacterial flagella is 15-20 nm, depending on the species.	Archaeal flagella is roughly half the diameter of bacterial flagella, measuring only 10–13 nm in width.
Source of energy for the rotation of flagella	Proton motive force	ATP

Protozoa Having Flagella

Protozoa are a heterogeneous group with three different locomotion organs: flagella, cilia, and pseudopods. Certain protozoa, such as Leishmania and Trypanosoma, have flagellated forms called promastigotes and non-flagellated forms called amastigotes. Giardia lamblia and urogenital flagellate Trichomonas vaginalis also have flagella.

The trophozoite of Giardia lamblia contains four pairs of flagella. Trichomonas vaginalis is a pear-shaped flagellated protozoan possessing five flagella, four located at its anterior portion. The fifth flagellum is incorporated within the undulating membrane of the parasite.

References and further readings

1. Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
2. Review of Medical Microbiology and Immunology, Warren E. Levinson, 15th edition