Bacteria- motility and chemotaxis

Question 1

What is motility

Answer 1

1. Motility important to many different aspects of bacterial life.
2. Involved in diverse processes from virulence to biofilm formation.
3. Coupled with sensing of environmental conditions allows bacteria to respond and move away or towards stimuli

Question 2

What are the different Types of motility

Answer 2

1. Swarming - flagella
2. Swimming - flagella
3. twitching - Pilus retraction
4. Gliding - Focal adhesion complexes
5. Sliding - Spreading growth

Question 3

Flagella vs Pilli vs Fimbriae

Answer 3

Fimbriae and pilli
1. Small hair like 
2. Extend away from cell surface
Flagella
1. Extends much further out

Question 4

What are the different flagella locations

Answer 4

1. Polar - One or both ends of cell
2. Lophotrichous - Multiple flagella on one or both poles of the cell
3. Medial - Flagella cited somewhere near centre region of cell
4. Peritrichous - Multiple flagella filaments arising on both sides of the cell

Question 5

How do Flagella act as rotary propellers

Answer 5

1. Unlike flagella found in eukaryotic microbes, bacterial flagella do not beat.
2. Instead the helical flagella filament acts as a propeller and is driven by a rotary motor embedded in the cell envelope.
3. Driven by proton motive force across cytoplasmic membrane
4. Flagella increase or decrease rotational speed in relation to strength of the proton motive force

Question 6

Describe the Modular construction of flagella

Answer 6

1. Several components are easily identifiable
2. The basal body and rings that anchor it to the envelope
3. The hook complex that connects the basal body to the filament
4. The filament (the ‘propeller’)- external part of flagella apparatus
5. The motor for driving the flagella rotation
6. The secretion system that exports the subunits
7. The ATPase that energises the system
8. L ring , P ring and Ms ring - House rod structure to which hook and filament are attached
9. surrounding MS ring- various motor complexes and ATPase secretion complexes- FliG, MotA/B

Question 7

Describe Flagella synthesis

Answer 7

1. Many genes are required for flagella synthesis and motility
2. Organised in operons related to timing of generation of key structures
3. MS ring is made first - ~5 minutes
4. Other proteins and hook are made next
5. Filament grows from tip ~40 minutes

Question 8

Describe Regulation of Flagella biosynthesis

Answer 8

1. Best understood are the E. coli and Salmonella systems
2. Regulation of genes needs to be tight to ensure the right proteins generated at the correct time.
3. Don’t fill up cell with components that can’t be used
4. Genes organised into 3 tiers of regulation:
5. Class 1: Early genes –proteins that control expression of the regulon
6. Class 2: Middle genes required for cell envelop components
7. Class 3: Late genes – filament, motor force generators, and chemosensory machinery
8. FliA expressed as class 2 to facilitate expression of class 3
9. FlgM is produced as class 3 and inhibits class 2

Question 9

What causes flagella to move

Answer 9

1. Multiple flagella filaments form an aligned ‘bundle’ behind cells to propel them in a straight line
2. Polarly flagellated cells often move in a circular or curved pathway

Question 10

Describe Bacterial movement in a “random walk”

Answer 10

1. Switching between run and tumble generates a “random walk”
2. Straight line period takes cell into concentration gradient
3. After a while you have a tumble as there is a reorientation
4. If concentration gradient drops in straight line swimming then rotation happens faster
5. Random walk: bacteria switch randomly from run to tumble (by reversing rotation of flagellar motor CW –> CCW)
6. Taxis- Biased random walk: bacteria move toward attractant by regulating the switch between run and tumble (longer runs as concentration of attractant increases)
7. Attractant diffusing from point of high concentration diffusing out
8. In presence of chemoattractant, the bacteria will now undertake a random biased walk to move towards chemical attractant
9. Start with straight line period of swimming- run- towards nutrient source
10. After a while there is a switch in the flagella rotation to clockwise and there is a tumble
11. Bacteria can then sense nutrient source- if tumble takes it away- lower concentration gradient it switches rotation of flagella filaments much sooner
12. If reorientation allows it to move towards nutrient source you get delay in rotation
13. Results in biased random walk towards nutrient source

Question 11

How is movement of bacteria regulated by chemotaxis

Answer 11

1. When signal is low (or no attractant) MCP (methyl accepting chemotaxis) assisted by CheW stimulates auto~P of CheA
2. CheA transfers ~P to CheY- activating it
3. CheY~P interacts with switch complex in flagella C-ring and causes tumbling
4. Methylation of receptor (by CheR) acts like a temporary record of signal strength.
5. Methylation makes CheA work better = more tumbling.
6. It can help to think of this as methylation making MCP less responsive to stimuli.
7. This helps ensure that cells keep moving accurately ‘up’ a concentration gradient, because methylation makes tumbling more likely unless concentration of attractant keeps getting stronger, and makes ‘runs’ that start to move cells down the gradient shorter.
8. CheB~P works to remove methylation and reset the record.

Question 12

What do MCPs do

Answer 12

1. MCP arrays sense external stimuli and modulate flagella rotation.
2. E. coli has MCPs that sense different signals
3. MCP homodimers span the membrane and sense signals in the periplasm (direct or indirect)
4. Arranged in a large array, most often at or near one pole: allows them to integrate different signals
5. Some bacteria have ~60 MCPs

Question 13

What is Swarming

Answer 13

1. Complex multicellular behaviour
2. A surface associated motility which enables bacteria to move across solid surfaces
3. Many bacterial species capable of this behaviour, although it is particularly pronounced in Proteus Sp. E.g. Proteus mirabilis.
4. P. mirabilis believed to use flagella as a ‘tactile’ sensor.
5. When cells read a solid surface, flagella rotation is inhibited. This stimulus is used to control the switch to swarming motility.

Question 14

Describe Twitching motility

Answer 14

1. Type IV pili
2. Elongation and retraction
3. ATP dependent
4. Exploration of surfaces
5. At least in P. aeruginosa twitching also involved in chemotaxis