Structure and Mechanism of Flagellar Motility
Flagella originate from the cytoplasmic membrane and consist of three main parts:
- Basal body — an anchor embedded in the cell envelope. It includes a central rod and a system of anchoring rings. Gram-positive bacteria possess one pair of rings, while Gram-negative bacteria possess two pairs.
- Hook — a flexible connecting structure.
- Spiral filament — the outer, longest part, formed by helically wound molecules of the protein flagellin.
Flagellar movement resembles an electric motor. The rotor is the basal body rod, which rotates the outer filament. Energy is derived not from ATP hydrolysis, but from the proton motive force generated across the membrane. During movement, multiple flagella can bundle together, rotating synchronously like a propeller.
Classification of Bacteria by Flagellar Arrangement
Depending on the number and distribution of flagella on the surface, motile bacteria are divided into four groups:
- Monotrichous — have 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 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 hair-like protein appendages that are thinner and shorter than flagella. They consist of the protein pilin and perform diverse functions.
Based on function, they are classified into:
- Adhesive pili — mediate bacterial attachment to host cells, serving as a major virulence factor (e.g., rigid type I pili).
- Metabolic pili — participate in the exchange of water and nutrients.
- Sex pili (F pili) — required 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 specialized types: type IV pili (responsible for twitching motility) and curli fibers (very thin filaments involved in biofilm formation and microbial aggregation).
Microscopic Detection Methods
Due to their small thickness, flagella can be visualized directly only using an electron microscope. To observe them under a standard light microscope, special staining techniques such as mordant staining or silver impregnation are used. These procedures artificially thicken the flagellar filaments by depositing dye onto their surface.