Cell envelopes and mobility Flashcards
(23 cards)
What are the different types of bacterial motility?
Flagella/flagellum - eg e. coli, salmonella Axial filaments - helical bacteria Gliding - myxococcus Twitching - eg p. aeuruginosa Swarming - flagellated bacteria 'Mysterious' mobility
How do E. coli swim (overview)?
Propelled by a rotating bundle of filaments
Rotation generated by flagellar motor
Changing direction through cell tumbles through switching motor direction
How are bundles of filaments formed?
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
What is the flagellar motor?
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
What is proton motive force?
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)
What is Reynolds number?
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
How do bacteria change direction?
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
How do bacteria choose direction?
Reduce tumble frequency - to increase probability of going towards something good
Increase tumble frequency - to decrease probability of going towards something bad
How do bacteria decide when to tumble?
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
How does gliding motility occur?
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
What do bacterial cell envelopes do?
Protect from environment
Ensures crucial components kept in cell
Reactions can occur on cell membrane
What are the two main categories of bacterial envelopes?
Gram +ve - peptidoglycan, periplasmic space, plasma membrane, cytoplasm
Gram -ve - liopopolysachharide outer membrane, periplasmic space, peptidoglycan, plasma membrane, cytoplasm
What roles do the outer membrane have?
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
How does the environment of the periplasm differ from the external environment?
Very thin so difficult to tell but may differ in pH
Can sequester potentially harmful degradative enzymes
What is the cell wall made up of?
Repeating units of N-acetyl glucosamine-N-actyl muramic acid
Cross linked by pentapeptide side chains
Gram +ve and -ve similar structure except thickness
How is peptidoglycan visualised?
AFM image of gram +ve
What is the inner membrane made of?
Phospholipid bilayer - length of lipid tail can vary influencing fluidity
Characteristic membrane functions performed on inner membrane
May have inner membrane reservoirs
How do gram +ve and gram -ve peptidoglycan differ?
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
What causes turgor pressure in bacteria?
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
E. coli pressurised with inside higher than outside what happens if outside conc. increases a lot more than inside?
Cell will shrink then expand - hyperosmotic shock
Why does the value for turgor pressure of E. coli vary in literature?
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
How is new material added to the cell wall?
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
What bears the pressure in a gram -ve bacteria?
Cell wall and outer membrane
Cell wall stiff
OM gel-like structure
