Flagella

  Flagella

Threadlike locomotor appendages extending out from the plasma membrane and cell wall

- Slender, rigid ; 20 nm wide and 15 - 20 m long- 

- Observed using special staining techniques; detailed structure in the electron microscope

- Flagellation patterns are very useful in identifying bacteria

 

´Monotrichous (trichous means hair) - one flagellum; polar flagellum if located at an end eg., Vibrio cholerae

´Amphitrichous (amphi - “on both sides”) - a single flagellum at each pole eg., Alcaligenes faecalis

´Lophotrichous (lopho means tuft) - a cluster of flagella at one or both ends eg., Spirilla

´Peritrichous (peri means “around”) - spread fairly evenly over the whole surface of bacteria eg., E. coli

´Atrichous - no flagella eg., Staphylococcus aureus

Flagellar Ultrastructure

Flagella has three main parts:

(1) Filament - The longest and most obvious portion; extends from cell surface to the tip

(2) A basal body embedded in the cell

(3) The hook -a short, curved segment, which links the filament to its basal body and acts as a flexible coupling.

·    Some bacteria have sheaths surrounding their flagella. For example, Bdellovibrio has a membranous structure surrounding the filament. Vibrio cholerae has a lipopolysaccharide sheath

·         The filament -a hollow, rigid cylinder constructed of a single protein called flagellin, which ranges in molecular weight from 30,000 to 60,000. The filament ends with a capping protein

·         Slightly wider than the filament, the hook is made of different protein subunits.

·         The basal body -most complex part of a flagellum - In E. coli and most gram-negative bacteria, the body has four rings connected to a central rod. The outer L and P rings associate with the lipopolysaccharide and peptidoglycan layers, respectively. The inner M ring contacts the plasma membrane. There is an S ring also.

·         Gram- positive bacteria have only two basal body rings, inner M ring connected to the plasma membrane and an outer P ring to the peptidoglycan

Flagellar Synthesis

·         complex process involving at least 20 to 30 genes -genes concerned with the control of flagellar construction or function

·         Filament synthesis is an excellent example of self-assembly-structures form spontaneously through the association of their component parts without the aid of any special enzymes or other factors.

·         flagellin sub- units are transported through the filament’s hollow internal core to the tip; the subunits spontaneously aggregate under the direction of a special filament cap -the filament grows at its tip rather than at the base-

·         The information required for filament construction is present in the structure of the flagellin subunit itself.

Mechanism of Flagellar Movement

·         The filament is in the shape of a rigid helix, and the bacterium moves when this helix rotates; just like propellers on a boat

·         The direction of flagellar rotation determines the nature of bacterial movement

·       Clockwise rotation of the flagella - cell tumbles

·    Counter- clockwise rotation of the flagella (whereas the cell itself rotates slowly clockwise) - Forward movement of cell. The rotating helical flagellar filament thrusts the cell forward in a run with the flagellum trailing behind

·         Bacteria swim through rotation of their rigid flagella; presence of a motor at the base

·         A rod extends from the hook and ends in the M ring, which can rotate freely in the plasma membrane

·         It is believed that the S ring (attached to the cell wall in gram-positive cells) does not rotate

·         The P and L rings of gram-negative bacteria would act as bearings for the rotating rod.

·         The rotor portion of the motor -made of a rod, the M ring, and a C ring joined to it on the cytoplasmic side

·         Fli G protein- important in generating flagellar rotation

·         Mot A and Mot B- important proteins in motor; form a proton channel through the plasma membrane: Mot B anchors the Mot complex to cell wall peptidoglycan

·         Mot A and Fli G directly interact during flagellar rotation

·         The flagellar motor can rotate very rapidly.

·         The E. coli motor rotates 270 revolutions per second; Vibrio alginolyticus ~ 1,100 rps.

·         This rotation is driven by proton or sodium gradients in procaryotes

1. Bacterial mutants with straight flagella or abnormally long hook regions (polyhook mutants) cannot swim. 
2. When bacteria are attached to a glass slide using antibodies to filament or hook proteins, the cell body rotates rapidly about the stationary flagellum

·         Bacteria can move by mechanisms other than flagellar rotation

·         Spirochetes travel through viscous substances such as mucus or mud by flexing and spinning movements caused by a special axial filament

·         Gliding motility- cyanobacteria, myxobacteria and Cytophaga, and some mycoplasmas; No visible external structures associated with gliding motility.