Bacterial flagella and pili

Structure and mechanism of flagella

Flagella originate from the cytoplasmic membrane and consist of three main regions:

1. Basal body — an anchor embedded in the cell envelope. It includes a central rod and a system of stabilizing rings. Gram-positive bacteria have one pair of such rings, whereas Gram-negative bacteria have two pairs.
2. Hook — a flexible connecting segment.
3. Spiral filament — the outer, longest part formed by helically wound molecules of the flagellin protein.

Flagellar motion resembles the operation of an electric motor. The rotor is the basal body rod, which rotates the outer filament. Energy for this process is derived not from ATP hydrolysis, but from the proton motive force generated across the membrane with the participation of proton ATP synthase. During movement, multiple flagella can intertwine into a single bundle, rotating synchronously like a propeller.

Classification of bacteria by flagellation type

Depending on the number and arrangement of flagella on the surface, motile bacteria are divided into four groups:

• Monotrichous — possess a single polar flagellum (e.g., *Vibrio cholerae*).
• Lophotrichous — possess a tuft of flagella located at one pole of the cell.
• Amphitrichous — have a single flagellum or a tuft of flagella at both ends of the bacterium.
• Peritrichous — flagella are distributed over the entire surface of the microbial cell (characteristic of *Escherichia coli*, *Proteus*).

Pili: types and functions

Pili (fimbriae) are thread-like protein structures that are thinner and shorter than flagella. They consist of the pilin protein and perform a variety of tasks.

According to their functions, they are classified as:

• Adhesive — ensure the attachment of the bacterium to the host cell, serving as a major pathogenicity factor (e.g., rigid type I pili).
• Metabolic — participate in the exchange of water and nutrients.
• Sex pili (F-pili) — necessary for conjugation (DNA transfer). They are few in number (1–3 per cell), produced exclusively by donor cells containing specific plasmids, and can serve as receptors for certain bacteriophages.

There are also specific types: type IV pili (mediate twitching motility) and curli (very thin fibers for biofilm formation and microbial aggregation).

Microscopic detection methods

Due to their small thickness, flagella can be directly visualized only using an electron microscope. To observe them under a standard light microscope, special staining techniques are used—mordanting or silver impregnation. These procedures artificially thicken the flagellar filaments through the deposition of dye on their surface.

What else to study on this topic

• Mechanisms of bacterial biofilm formation
• The process of conjugation and the role of transmissible plasmids
• Serological identification of bacteria (H-antigen)
• Interaction of bacteriophages with the bacterial cell

Other topics in the section "Morphology and Classification of Microbes"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.