Flagella: Structure, Arrangement, Function

Last updated on June 21st, 2021

Flagella (singular, flagellum) are the locomotory structures of many prokaryotes. 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. 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.

Flagellar motion in Bacterial Cells
Flagellar motion in bacterial cells

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.


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 propeller of a boat motor. 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 filament to the motor portion of the flagellum called a basal body.

Structure of the bacterial flagella
Structure of the bacterial flagella

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 but gram-positive bacteria lack a lipopolysaccharide layer so the outer ring (L ring) is absent.

  • L ring is anchored in the lipopolysaccharide layer.
  • P ring is anchored in the peptidoglycan layer of
    the cell wall.
  • MS and C rings are located within the cytoplasmic membrane and the cytoplasm.

A series of proteins called Mot, surrounds the inner pair of rings. 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

Flagella are attached to cells in different places. As the number and location of flagella are distinctive for each genus, it can be used in the classification of bacteria. There are four types of flagellar arrangement.

  1. Monotrichous (Mono means one): Single polar flagellum e.g. Vibrio cholerae, Campylobacter spp(polar flagella often in pairs to give a “seagull” appearance).
  2. Amphitrichous: Single flagellum at both ends e.g. Alcaligenes faecalis (note: amphibians live both on land and in water).  flagellar arrangement of bacteria
  3. Lophotrichous: Tuft of flagella at one or both ends e.g. Spirilla spp
  4. Peritrichous (flagella in the periphery): Flagella surrounding the bacterial cell. All the members of the family Enterobacteriaceae, if motile have peritrichous flagella. e.g. Salmonella Typhi,  Escherichia coli, Proteus spp (highly motile organism; shows swarming motility)
Spirillum and its flagellar arrangement
Spirillum and its flagellar arrangement

Functions of Bacterial Flagella

  • Organs of locomotion: Many prokaryotes are motile, and the majority of motile prokaryotes move by means 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 bacteria from 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.

Archaeal Flagella

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

Difference between Bacterial and Archaeal Flagella is tabulated here:

PropertiesBacterial flagellaArcheal flagella
Flagellar filamentFlagellar filament is made up of single type of proteinSeveral different flagellin proteins are found.
Diameter of FlagellaThe diameter of bacterial flagella is 15-20 nm depending on the species.Archaeal flagella are roughly half the diameter of bacterial flagella, measuring only 10–13 nm in width.
Source of energy for the rotation of flagellaProton motive forceATP

Protozoa Having Flagella

Protozoa are a heterogeneous group that possesses three different organs of locomotion: 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.

Some pathogenic flagellated protozoa

The trophozoite of Giardia lamblia contains four pairs of flagella. Trichomonas vaginalis is a pear-shaped flagellated protozoan possessing five flagella, four of which are 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
About Acharya Tankeshwar 476 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.