IV. Flagella, motility, and taxis
A. Flagella
- flagella are responsible for motility in most bacteria
- there's a loose correlation between cell shape & the presence of flagella:
almost all spiral & curved (Ex. Vibrio cholerae)
half of all rods (Ex. Pseudomonas aeruginosa)
hardly any cocci
B. General Structure
- prok flagella are very different from euk flagella
- prok flagella are much thinner than euk flagella, & they lack the typical 9 + 2 arrangement of microtubules
- diameter thin (20 nm) & long w/ some having a length 10 X the diameter of cell (about 100 nm)
- can't be seen in the light microscope unless a special stain is applied
- the flagellar apparatus consists of several distinct proteins:
a system of rings embedded in the cell envelope (the basal body)
a hook-like structure near the cell surface
the flagellar filament
- bacteria can have 1 or more flagella arranged in clumps or spread all over the cell
- some common flagellar arrangements:
Polar or Monotrichous
Amphitrichous (new)
Lophotrichous
Peritrichous
- flagellar distribution is a genetically-distinct trait that is occasionally used to characterize or distinguish bacteria
Ex. among GNR, pseudomonads have polar flagella that distinguish them from enteric bacteria, which have peritrichous flagella
C. Chemical Structure
- the filament is mostly composed of a contractile protein called flagellin
- flagellin is bound in long chains & wraps around itself in a left handed helix
Picture of flagellin being synthesized off of a ribosome
- the hook is made up of different proteins than the filament
- hook connects to basal body
- the basal body contains rings - structures that impart motility
GN - 4 rings
GP - 2 rings
Picture of GN and GP flagella
D. Motility
- the flagellum is a rigid structure that is rotated like a propeller by a motor apparatus located in the plasma membrane
- rings in the basal body rotate relative to each other causing the flagella to turn
Ex. GN cell
the innermost rings, the M and S rings, located in the plasma membrane, comprise the motor apparatus
the outermost rings, the P and L rings, located in the periplasm & the outer membrane respectively, function as bushings to support the rod where it is joined to the hook of the filament on the cell surface
as the M ring turns, powered by an influx of protons, the rotary motion is transferred to the filament which turns to propel the bacterium
the energy to drive the basal body is obtained from the PMF established on the bacterial membrane, rather than ATP hydrolysis that powers eukaryotic flagella
How fast do bacterial cells move?
Average 50 µm/sec, which is about 0.0001 miles/hr.
Relative Speeds of Organism
Organism Kilometers/hr Body lengths/sec
Cheetah 111 25
Michael Johnson 37.5 5.4
Bacteria 0.00015 10
E. Detection of motility
1. Indirect - looking for the presence of flagella
a. dyes - the binding of the dye adds extra width to the structure & absorbs light, making them visible
b. antibody (Ab) stains - Ab that recognize flagellin. By attaching a fluorescent or colored dye to the Ab & using a special microscope, it's poss. to detect the flagella
c. electron microscopy
2. Direct - looking for movement
a. microscopy - watch living bacteria swim around using the phase scope
b. motility medium - a semisolid medium that will hold non-motile bacteria in place, but motile microbes can swim thru it. The presence of turbidity is a + test for motility
Why are bacteria motile?
- typically microbes that live in aqueous environments will continually move around looking for nutrients
- sometimes this movement is random, but in other cases it is directed toward or away from something
- bacteria are capable of a tactic response to various stimuli
F. Taxis or Directed Motility
Chemotaxis - toward or away from a chemical stimulus
Phototaxis - toward or away from light
Aerotaxis - toward or away from oxygen
Magnetotaxis - orientation in a magnetic field
There are a few, but one ex. is Aquaspirillum magnetotacticum, which has magnetosomes. These structures orient themselves in a magnetic field (The earth's magnetic field under natural conditions). The microbe uses this to determine which way is up and that helps it to find nutrients or adjust its depth in an aquatic environment. Other animals have magnetosomes; birds, dolphins, tuna, green turtles. In these cases they are used for navigation on long migrations.
Chemotaxis
- accomplished by sensing the present environ, comparing it to the one encountered just a few secs agao, & adjusting the rotation of the flagella in response to stimuli
- flagella can rotate clockwise (cw) or counterclockwise (ccw)
- when flagella rotate ccw this creates a force pushing on the bacteria this causes the bacteria to move in a straight line, called a run
Animation of run
- when flagella rotate cw, they all pull on the microbe. W/ all these forces pulling in different directions, it causes the bacteria to tumble or twiddle.
Animation of twiddle (tumble)
- when the twiddling is over, the bacteria will start out a new run in a completely random direction.
Animation of Random
- particular materials (stimuli) in the environ are recog by a collection of proteins - methyl accepting chemotactic proteins (MCPs)
- recog is followed by a change in methylation of the proteins involved in transmission of the signal to flagellar motors
- causes the motors to turn ccw resulting in the run - directed motility
1. Neutral conditions
- in plain medium containing no attractant or repellent, the length of runs is random & the bacteria move about the solution aimlessly
2. Attractant
- when cells are put in an environ containing an attractant (like gluc) they will move toward the source of the attractant
- the microbes are sensing the change in conc of the attractant as they move through solution
- if they are moving up the gradient to higher attractant conc, the length of the run will increase
- if they are moving down the gradient, the length of the run will be much shorter
- in this way, the bacteria eventually moves to the source of the attractant
Animation of Attractant
3. Repellent
- exactly the opposite of an attractant
- when the compound is nasty (like acid) the microbe will shorten runs that go up the gradient & lengthen those that take it away from the repellent
Animation of Repellent
Remember that cells do not detect absolute concentration of chemicals but detect a change in gradient, this is actually a type of chemical memory. They detect the gradient by using a complex molecular mechanism. The attractant or repellent binds to receptors in the membrane and this signal is passed through the cell, eventually regulating the direction of flagellar rotation.
G. Different modes of motion
1. Periplasmic flagella - spirochetes
- flagella are located w/in the periplasm, encased in a membranous sheath
- move by ccw rotation of flagella around the cylinder of the cell
2. Gliding motility
- depends upon contact with a solid surface
- best hypothesis suggests that little circular motors (similar to the basal body of flagella) are spinning underneath the microbe
- the circular force causes the bacteria to glide across a surface