Cell envelopes and mobility

Question 1

What are the different types of bacterial motility?

Answer 1

Flagella/flagellum - eg e. coli, salmonella
Axial filaments - helical bacteria
Gliding - myxococcus
Twitching - eg p. aeuruginosa
Swarming - flagellated bacteria
'Mysterious' mobility

Question 2

How do E. coli swim (overview)?

Answer 2

Propelled by a rotating bundle of filaments
Rotation generated by flagellar motor
Changing direction through cell tumbles through switching motor direction

Question 3

How are bundles of filaments formed?

Answer 3

Filaments much longer than cell, stiff, and located randomly along cell body
Has bacterial flagellar hook - FlgE acts as a universal joint
Allows bending so filaments can come together
Filaments can also change structure a little

Question 4

What is the flagellar motor?

Answer 4

Located in cell membrane allowing swimming
Ions flow through motA and motB proteins (stator) enabling rotation of the rotor made of 3 rings of protein
Powered by proton motive force (PMF)
Motor rotates CW and CCW

Question 5

What is proton motive force?

Answer 5

Electrochemical gradient of protons
Form of ion motive force used in E. coli
‘Driving force’ for ion transport given in Volts
PMF = Vm + (kT/e)ln(ci/co)

Question 6

What is Reynolds number?

Answer 6

The ratio between intertial forces and viscous forces
Bacteria swim in low Reynolds number - high viscous forces
Drag dominates things so mass doesn’t matter and there’s no inertia

Question 7

How do bacteria change direction?

Answer 7

Cell tumbles - a few motors change direction and bundle disassembles
Every 1-10 seconds and changes direction roughly randomly
E. coli effectively diffuses if there’s no external stimuli - Brownian motion

Question 8

How do bacteria choose direction?

Answer 8

Reduce tumble frequency - to increase probability of going towards something good
Increase tumble frequency - to decrease probability of going towards something bad

Question 9

How do bacteria decide when to tumble?

Answer 9

Chemoreceptors serve as ‘antennas’ that sense and transmit outside info to the motor
Info transmitted through a series of phosphorylation and de-phosphorylation events that serve as feedback loops and parameters
Motor, with a signalling protein (phosphorylated CheY) bound changes direction of rotation
Once one or more motors change direction, filament is disrupted and cell tumbles
Happens quickly to respond to environment

Question 10

How does gliding motility occur?

Answer 10

S-motility - social motility powered by Type IV pili

A-motility - adventurous motility powered by motor proteins
Membrane-bound cytoplasmic complexes consisting of motor and regulatory proteins directionally transported down axis of cell at constant velocity

In Myxococcus xanthus

Question 11

What do bacterial cell envelopes do?

Answer 11

Protect from environment
Ensures crucial components kept in cell
Reactions can occur on cell membrane

Question 12

What are the two main categories of bacterial envelopes?

Answer 12

Gram +ve - peptidoglycan, periplasmic space, plasma membrane, cytoplasm
Gram -ve - liopopolysachharide outer membrane, periplasmic space, peptidoglycan, plasma membrane, cytoplasm

Question 13

What roles do the outer membrane have?

Answer 13

Lipopolysachharides - endotoxic shock

OMPs - beta-barrels, porins such as OmpF and OmpC allow passive diffusion of small molecules, some diffuse specific molecules, often gated, main role as a barrier

LPs - lipoproteins

Question 14

How does the environment of the periplasm differ from the external environment?

Answer 14

Very thin so difficult to tell but may differ in pH

Can sequester potentially harmful degradative enzymes

Question 15

What is the cell wall made up of?

Answer 15

Repeating units of N-acetyl glucosamine-N-actyl muramic acid
Cross linked by pentapeptide side chains
Gram +ve and -ve similar structure except thickness

Question 16

How is peptidoglycan visualised?

Answer 16

AFM image of gram +ve

Question 17

What is the inner membrane made of?

Answer 17

Phospholipid bilayer - length of lipid tail can vary influencing fluidity
Characteristic membrane functions performed on inner membrane
May have inner membrane reservoirs

Question 18

How do gram +ve and gram -ve peptidoglycan differ?

Answer 18

Gram +ve have thicker cell wall, covalently attached proteins (CAPs), wall teichoic acid (WTA) and lipoteichoic acid (LTA)
LTAs and surface proteins important for attachment to host cells

Question 19

What causes turgor pressure in bacteria?

Answer 19

Inside concentration of ions and molecules higher than outside - water wants to equalise - move into the cell
Pt ~ Ci - Co
Cell wall helps maintain pressure so prevents bursting

Question 20

E. coli pressurised with inside higher than outside what happens if outside conc. increases a lot more than inside?

Answer 20

Cell will shrink then expand - hyperosmotic shock

Question 21

Why does the value for turgor pressure of E. coli vary in literature?

Answer 21

Can be measured in diff conditions
Some can be incorrect
Measured by poking
Can use AFM (atomic force microscope) - testing osmotic contribution but also mechanical properties of cell wall
Use a strain that allows ‘bulging’ - bleb of inner membrane leaking out of cell wall

Question 22

How is new material added to the cell wall?

Answer 22

Insertion helical
MreB polymers - localisation of peptidoglycan synthesis enzymes in the periplasm between IM and OM
E. coli and B. subtilis twist as they elongate
Left-handed helical insertion pattern

Question 23

What bears the pressure in a gram -ve bacteria?

Answer 23

Cell wall and outer membrane
Cell wall stiff
OM gel-like structure