4 - Microbial Membranes And Cell Walls Flashcards
Structures on outer layer of bacterial cells
- capsule, S layer
- gram + - cell wall
- gram - - outer membrane
- periplasmic space - within peptidoglycan in gram pos and neg bacteria
- cell plasma membrane
Capsule structure info
- sometimes called slime layer or glycoalyx
- polysaccharide components of cell wall
- usually loose network of polymer fibres extending outward from wall
Capsule function
- not required for growth or reproduction
- Carbon store
- protection against desiccation (drying out)
- May be involved in capture of nutrients - acquisition of ions from environment
- confer advantages in vitro such as attachment to surfaces - biofilms, holdfast to eukaryotic cells
- exclude phage, antimicrobials and disinfectants - offer some resistance
- pathogens often capsular can resist phagocytosis, otherwise killed by host when it isn’t
Different capsules in pathogens
- most commonly is a polysaccharide structure
- glycolipid capsule
- protein capsule
- extracellular slime
S-layer info
- Paracrystalline outer wall composed of protein/glycoprotein
- regularly structured layer external to cell wall
- s layer is only cell wall structure in some archaea
- May protect against ion and pH fluctuations, osmotic stress, and predators (e.g. Bdellovibrio)
- May protect against host defences - sometimes a virulence factor (can combat immune systems)
Peptidoglycan structure
- alternating residues of:
NAG (N-acetylglucosamine)
NAM (N-acetylmuramic acids) - arranged in diners which are cross linked by amino acid side chains creating amide bonds
- is a mesh like polymer that retains the gram stain in gram +ve cells
Contains non-protein amino acids:
- D-glutamic acids
- D-alanine
- DAPA
D amino acids used as they protect against degradation by proteases and enzymes
Why D amino acids used in peptidoglycan structure in bacterial cell walls
- connected to form a glycan tetrapeptide
- Protect against degradation by proteases
Peptidoglycan synthesis - cross links
- chains of linked peptidoglycan subunits joined by cross-links between the peptides
- often carboxyl group of terminal D-alanine connected to amino group of DAPA (meso-diamino-pimelic acid)
- sacs are strong enough to retain shape when isolated yet are porous, elastic, and stretchable
Difference between gram +ve and -ve bacterial cell walls
Gram +ve thicker than gram -ve cell walls
- gram +ve contain outer membranes
Which type of bacteria is penicillin more effective against
Gram positive
Function for techioc acid in bacterial cell walls
- only found in gram +ve cell walls
- role is unclear
- May make membrane -vely charged
- May help with binding if Ca2+ and Mg2+
How does penicillin affect bacteria and their cell walls
- briefly describe peptidoglycan synthesis
- penicilin inhibits peptidoglycan synthesis in gram +ve bacterial cells by inhibiting transpeptidation - lyses cells
PG synthesis: - linker peptide initially has two D-alanine peptides
- one is cleaved during linkage with DAPA
- called transpeptidation - this step is penicilin sensitive
- penicilin halts cell wall synthesis
- osmotically sensitive cells lyse
Lysozyme info
- ‘antibacterial’ enzyme
- degrades the beta, 1,4 glycosidic bond in peptidoglycan backbone
- loss of peptidoglycan makes cells sensitive to changes in osmotic pressure
- important host defences against bacteria
- present in saliva, tears, secreted in airways
- lots in egg whites
Where lysozymes found
Saliva
Tears
Secreted in airways
Techioc acid structure
Which bacteria is techioc acid found in
Only gram +ve
Archaea cell walls info
- no peptidoglycan
- some contain pseudomurein
- Contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid in bacteria
- contains B - 1,3 links instead of B-1,4 links
- not degraded by lysozyme, not sensitive to penicilin
- no D-amino acids in linker molecules
- other archaea contain glycoproteins, polysaccharides or S layers (protein or glycoprotein)
Basics of a membrane
- Unit membrane comprised of phospholipids
- Hydrophobic groups inside, hydrophilic groups outwards
- Proteins that traverse the bilayer have hydrophobic regions
- Hydrophilic/charged substances may attach to the hydrophilic surfaces
Sterols/hopanoids info in membrane structure
- hopanoids in bacteria, sterols in eukaryotes
- rigid planar molecules while fatty acids are flexible
- this stabilises membrane structure
- hopanoids not found in archaea
- this role in archaea filled by isopropene structures
What archaea have instead of hopanoids/sterols in membrane
- Isopropene structures
- used to stabilise membrane structure
Gram -ve outer cell membrane info
- Outer cell membrane only in gram -ve bacteria
- asymmetric due to insertion of lipopolysaccharide into external layer of outer membrane
Two ways outer membrane linked to cell in bacteria
- Braun’s lipoprotein
- Adhesion sites - continuum of inner and outer membrane, where two membranes adhere
Braums lipoprotein info
- Most abundant protein in outer membrane
- covalently linked to peptidoglycan and embedded in outer membrane by hydrophobic end
Adhesion sites of outer membrane info
- continuum of inner and outer membrane where two sites adhere
- Around 400 adhesion sites in E. Coli cell
- allow transport of substances to outer membrane and outer cell
- can be visualised using TEM
- plasmolysis makes cell flaccid
- this increases space between membranes, makes adhesion sites more visible
- immunoglobulin staining of a phage
How to make adhesion sites visible in bacteria
- can be visualised using TEM
- plasmolysis makes cell flaccid
- this increases space between membranes, makes adhesion sites more visible
- immunogold staining of a phage
- MS2 lysis protein in adhesion sites
differences between bacterial and archaeal cell walls
bacteria:
- contains peptidoglycan
- usually contain outer membrane (gram +ve)
- B-1,4 glycosidic bonds between sugar derivatives
- contaons some L and some D-isomer amino acids
- peptidoglycan broken down by lysozymes and penicilin
archaea:
- no peptidoglycan
- usually have S layer (protein) instead of polysaccharide containing cell wall to prevent osmotic lysis
- typically lack outer membrane
- some methanogens (methane producing archaea) contain pseudomurein polysaccharides
- B-1,3 glycosidic bonds between sugar derivatives
- psuedomurein contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid in peptidoglycan
- all amino acids are L-stereoisomers in linker molecules
- pseudomurein immune to penicilin and lysozymes
difference and similarities between bacterial and arachaeal cytoplasmic membranes
differences:
bacteria: fatty acid tails are bound glycerol by ester linkages
- forms a lipid bilayer
archaea: contains isopropenoid tails
- hydrophobic isopropenoid tails bound to glycerol by ether bonds (C-O-C)
- forms a lipid monolayer
- formed from phosphoglycerol diethers, with C20 or C40 side chains
similar: both membranes amphipathic in nature (hydrophobic end facing in, hyddrophilic facing out)
main functions of cytoplasmic membrane
- cells permeability barrier - prevent passive leakage of solutes into and out of cell
- anchors many proteins that catalyse many key cell functions - transport proteins, etc.
- role in energy conservation and consumption (AT?)
Cytoplasmic membrane as a permeability barrier info
- barrier to diffusion of most substances, especially polar or charged molecules
- membrane therefore has specific transport proteins to move required solutes into and out of the cell
Cytoplasmic membrane as a site containing key proteins info
- transport proteins usually highly specific, transport only one type of molecule
- other transporters may be more general, and transport certain types of molecules, e.g. amimo acids, or sugars
- if accumulating solutes against conc. gradient, then ATP required for AT
- ensures sufficient amount of nutrients in cell required for chemical reactions
- rate of uptake at max. when transporters are saturated
- some nutrients transported by low- affinity transporter when nutrient present at high external conc.
- nutrients transported by high-affinity transporters when external nutrient conc. is low
Cytoplasmic membrane role in energy conservation and consumption info
- involved in respiration
- involved in generating and dissipation of the proton motive force, in ATP production, in mitochondria and chloroplasts
similarities between bacterial and archaeal cell walls
which stage of PG synthesis does penicilin inhibit
transpepidation
how and why archeae have a monolayer cytoplasmic membrane
- contains double-headed ether lipids
- stabilises membrane at extremes of pH/temp
difference between L and D- amino acids
- L amino acids rotate left (levorotation)
- D amino acids rotate right (dextrorotation)
peptidoglycan cross links structure and info
- Chains of linked peptidoglycan subunits joined by cross-links between the peptides
- Often carboxyl group of terminal D-alanine connected to amino group of diamino pimelic acid (DAPA)
- Sacs are strong enough to retain shape when isolated yet are porous, elastic and stretchable
