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BS127 - Agents Of Infectious Diseases > 4 - Microbial Membranes And Cell Walls > Flashcards

4 - Microbial Membranes And Cell Walls Flashcards

1
Q

Structures on outer layer of bacterial cells

A
  • capsule, S layer
  • gram + - cell wall
  • gram - - outer membrane
  • periplasmic space - within peptidoglycan in gram pos and neg bacteria
  • cell plasma membrane
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2
Q

Capsule structure info

A
  • sometimes called slime layer or glycoalyx
  • polysaccharide components of cell wall
  • usually loose network of polymer fibres extending outward from wall
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3
Q

Capsule function

A
  • 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
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4
Q

Different capsules in pathogens

A
  • most commonly is a polysaccharide structure
  • glycolipid capsule
  • protein capsule
  • extracellular slime
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5
Q

S-layer info

A
  • 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)
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6
Q

Peptidoglycan structure

A
  • 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

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7
Q

Why D amino acids used in peptidoglycan structure in bacterial cell walls

A
  • connected to form a glycan tetrapeptide
  • Protect against degradation by proteases
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8
Q

Peptidoglycan synthesis - cross links

A
  • 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
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9
Q

Difference between gram +ve and -ve bacterial cell walls

A

Gram +ve thicker than gram -ve cell walls
- gram +ve contain outer membranes

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10
Q

Which type of bacteria is penicillin more effective against

A

Gram positive

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11
Q

Function for techioc acid in bacterial cell walls

A
  • only found in gram +ve cell walls
  • role is unclear
  • May make membrane -vely charged
  • May help with binding if Ca2+ and Mg2+
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12
Q

How does penicillin affect bacteria and their cell walls
- briefly describe peptidoglycan synthesis

A
  • 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
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13
Q

Lysozyme info

A
  • ‘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
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14
Q

Where lysozymes found

A

Saliva
Tears
Secreted in airways

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15
Q

Techioc acid structure

A
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16
Q

Which bacteria is techioc acid found in

A

Only gram +ve

17
Q

Archaea cell walls info

A
  • 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)
18
Q

Basics of a membrane

A
  • 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
19
Q

Sterols/hopanoids info in membrane structure

A
  • 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
20
Q

What archaea have instead of hopanoids/sterols in membrane

A
  • Isopropene structures
  • used to stabilise membrane structure
21
Q

Gram -ve outer cell membrane info

A
  • Outer cell membrane only in gram -ve bacteria
  • asymmetric due to insertion of lipopolysaccharide into external layer of outer membrane
22
Q

Two ways outer membrane linked to cell in bacteria

A
  1. Braun’s lipoprotein
  2. Adhesion sites - continuum of inner and outer membrane, where two membranes adhere
23
Q

Braums lipoprotein info

A
  • Most abundant protein in outer membrane
  • covalently linked to peptidoglycan and embedded in outer membrane by hydrophobic end
24
Q

Adhesion sites of outer membrane info

A
  • 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
25
Q

How to make adhesion sites visible in bacteria

A
  • 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
26
Q

differences between bacterial and archaeal cell walls

A

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

27
Q

difference and similarities between bacterial and arachaeal cytoplasmic membranes

A

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)

28
Q

main functions of cytoplasmic membrane

A
  • 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?)
29
Q

Cytoplasmic membrane as a permeability barrier info

A
  • 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
30
Q

Cytoplasmic membrane as a site containing key proteins info

A
  • 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
31
Q

Cytoplasmic membrane role in energy conservation and consumption info

A
  • involved in respiration
  • involved in generating and dissipation of the proton motive force, in ATP production, in mitochondria and chloroplasts
32
Q

similarities between bacterial and archaeal cell walls

A
33
Q

which stage of PG synthesis does penicilin inhibit

A

transpepidation

34
Q

how and why archeae have a monolayer cytoplasmic membrane

A
  • contains double-headed ether lipids
  • stabilises membrane at extremes of pH/temp
35
Q

difference between L and D- amino acids

A
  • L amino acids rotate left (levorotation)
  • D amino acids rotate right (dextrorotation)
36
Q

peptidoglycan cross links structure and info

A
  • 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

BS127 - Agents Of Infectious Diseases (11 decks)

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