Module 2 - Bacterial structure and function Flashcards
Binary fission: the process
Cell doubles in length, all other organelles are partitioned with an equal split on each side, septum forms, and then the cells fully split
What stimulates cell division?
The FtsZ ring
MreB: where is it present, what is it, and what shapes are the mutants?
Only present in rod-shaped bacteria
An actin homologue which forms a filamentous structure which acts as a scaffold for peptidoglycan synthesis machinery
Coccoid
How does the insertion of new peptidoglycan work in rod-shaped bacteria?
Autolysin breaks the b1-4 linkage between NAM and NAG
Transglycosylase adds a new peptidoglycan unit
How is more peptidoglycan formed in coccoid-shaped bacteria?
Since there is no MreB to synthesize the breakdown and synthesis of peptidoglycan, synthesis can only happen at the septum, when the cell divides
The 4 phases of bacterial growth in culture
Lag phase, Log phase, Stationary phase, and Death phase
Lag and Log phases: what are they and why do they occur?
Lag phase - The phase where there is no increase in the log number of viable organisms due to the bacteria adjusting to the new environment and growing
Log phase - The phase where there is an exponential growth of organisms. This usually forms a straight line.
Stationary and Death phases: what are they and why do they occur?
Stationary phase - The phase where the culture size is stable and there are no more divisions due to lack of nutrients
Death phase - The phase where the culture begins to die out due to intolerable environment (build-up of toxic material, temp etc)
Psychrophiles: what temps are their optimum?
<15°C
Mesophiles: what temps are their optimum?
15-45°C
Thermophiles: what temps are their optimum?
45-80°C
Hyperthermophiles: what temps are their optimum?
> 80°C
Examples of extremophile adaptations (psychrophiles)
1 - Unsaturated fatty acid lipid bilayer. This prevents the membrane from becoming waxy at low temps
2 - Altered proteins. More α-helices give enhanced flexibility
3 - Antifreeze molecules which bind ice crystals have been found in fish and other eukaryotes, but not in prokaryotes
Examples of extremophile adaptations (thermophiles)
1 - Saturated fatty acid lipid bilayer. This remains stable at high temps
2 - Altered proteins. Fewer α-helices give more folding conformations to withstand heat
Examples of extremophile adaptations (hyperthermophiles)
1 - No lipid bilayer. Phytane (C₄₀) hydrocarbon is present which creates a lipid monolayer
How do bacteria cultures die out?
Not instantly - they die at a constant rate over time
What steam sterilisation kills all known pathogens?
Temp - 121°C
Time - 20 mins
Pressure - 138 kPa
Variety of bacterial cell shapes
Range from circular (coccus) to anything (ie can be stellar (star) shaped)
What colours are Gram-positive and Gram-negative cells?
Positive - Purple
Negative - Pink
Difference between the cell walls of Gram-positive and Gram-negative cells
Gram-positive cells have a thick layer of peptidoglycan as an outer membrane
Gram-negative cells have a thin peptidoglycan layer between the inner and outer membrane
Gram stain process
Heat fix to raise cells to the top, crystal violet to dye the cells, fix with iodine to form a larger stained crystal, decolourise with ethanol/acetone, and counterstain
Both will be dyed purple after the crystal violet is used and the Gram-positive cells will remain purple throughout
Gram-negative cells will be dyed purple but then will be decolourised with acetone/ethanol and coloured pink with the counterstain
The use of peptidoglycan in the cell wall
- The rigid structure of the wall gives the cell shape
- Protects from osmotic lysis
- Withstand high internal osmotic pressure
(Gram-positive - 20 atm, Gram-negative - 0.3-3 atm)
Strengths and weaknesses of peptidoglycan
Strength: extremely strong - boiled cells can have their intracellular components destroyed with the peptidoglycan bag remaining intact
Weakness: Antibiotics, lysozyme, and bacteriophage lysins
Why can lacking peptidoglycan be beneficial?
Antibiotics won’t bind, protection value
Peptidoglycan: what is its basic structure?
Altering amino acid chains (N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM)) with glycosidic bonds binding them at the top and bottom of the peptidoglycan. The vertical gap between the top and bottom is filled with amino acids
Glycan is conserved highly, peptides may be variable
The backbone carbohydrate sugar is around 10-65 sugars per chain
How is peptidoglycan bonded in the cell wall of bacteria?
Vertical bonding - peptide bonds which bind the amino acids
Horizontal bonding - glycosidic bonds
Gram-positive cell wall structure
Homogenous architecture vertically filled with large amounts of glycine with peptidoglycan (PG) and teichoic acid (TC, a polymer of glycerol (3C) and ribitol (5C) phosphate)
Gram-negative cell wall structure
Heterogenous architecture with an outer membrane, periplasm on both sides of the peptidoglycan, and an inner membrane
Contains lipopolysaccharides
Lipopolysaccharides: what are they and what do they consist of?
Complex glycolipids that are exclusively present in the outer leaflet of the outer membrane of Gram-negative bacteria which renders outer membrane asymmetric and less fluid
LPS consist of:
* Conserved hydrophobic membrane anchor - lipid A
* Variable oligosaccharide core region
* Highly variable O-antigen polysaccharide forming a repeating oligosaccharide
Endotoxin: what is it and what can it cause to occur?
Stimulates the innate immune system to trigger an inflammatory response which can cause the body to go into endotoxin shock which can lead to multi-organ failure and death
Endotoxin = extracellular Lipid A
Archaeal cell envelope - extremophile
Contains an S-layer of two proteins and membrane anchoring proteins
Tetra-ethyl lipids in the lipid monolayer (also impermeable to protons)
Archaeal cell envelope - mesophile
S-protein layer above pseudomurein which is above a membrane of diether lipids
Archael pseudomurein
Essentially a substitute for peptidoglycan
Some may have it (methanogens) but some may not
Altering amino acid chains (N-acetyl glucosamine (NAG) and N-acetyl talosaminuronic acid (NAT)) with glycosidic β1-3 bonds (which are resistant to lysosome degradation) binding them at the top and bottom of the pseudomurein. The vertical gap between the top and bottom is filled with amino acids (only L-amino acids present, no D-forms)
Surface layers
The outer membrane of archeal cells which is mostly composed of a paracrystalline protein layer
What types of arrangements can flagella have?
Monotrichous - one
Lophotrichous - many on one side
Amphitrichous - one each side
Peretrichous - many on every side
Flagella mediated motility
Flagella are semi-rigid and rotate up to 300 revolutions per second
Bacterial cells swim at 0.00017 km/h but this is 50-60 cell lengths per second
For comparison, Cheetah runs at 110 km/h or 25 body lengths per second
Flagella ultrastructure
Filament - acts as the propellor
Hook - joins filament and basal membrane
Hook’s structure:
L ring, P ring, MotB, MS, MotA ring, and C ring
The flagella’s hook’s structure
- L ring - in line with the outer membrane
- P ring - in line with peptidoglycan
- MotB - in the periplasm below the peptidoglycan
- MS - in line with the inner membrane, top of the machinery that moves the flagella
- MotA - in line with the inner membrane, a channel for protons
- C ring - Inside the inner membrane, the bottom of the machinery that moves the flagella
What facilitates flagella movement
The rotation of the C and MS rings because of MotA, causing the hook and filament to move.
The P and L rings do NOT move at any point (and are not present in Gram-positive bacteria)
Flagella: what are the types of flagella movement and how does the motility work?
Phototaxis and chemotaxis
CCW (counterclockwise - flagella bundle to move in one direction) to move in one direction and CW (spread out flagella) to stop moving
Fimbriae and pili on the bacterial cell surface: what are they, what are their functions, and what on earth is type IV pili and what does it do?
Thin (2-10 nm wide) proteinaceous and filamentous structures that project from the cell surface
Fimbriae are often peritrichous and enable commensals and pathogens to adhere to surfaces and tissues (often via adhesins at the tip to counteract sheer forces), obtain nutrients at specific niches, and form pellicles/biofilms at interfaces/surfaces
Pili have a similar structure (though fewer and more extended than fimbriae) and they allow adherence to host tissues (ie E. coli mannose receptor on pili tip), exchange of genetic material (conjugation pili)
Type IV pili – transformation, twitching motility on a surface, and adherence essential in pathogens such as G-ve’s Vibrio cholerae, Neisseria gonorrhoeae and Streptococcus pyogenes (G+ve), also found in Archaea
E.Coli: the different types of P-type and what do each of them do?
- PapG - adhesin
- PapF - adaptor protein
- PapE - fibrillar protein
- PapK - adaptor protein
- Pap A - structural subunit approx.1000 copies
- PapC - assembly/secretion of subunits
