Bacterial Cell Envelope

Question 1

What is found in the cell envelope?

Answer 1

Plasma membrane (lipid bilayer), peptidoglycan cell wall, & envelope proteins

Question 2

Explain the Gram positive cell envelopes?

Answer 2

Thick peptidoglycan layer, single membrane, lipoteichoic acid polymer (embedded in membrane), & teichoic acid (anchored to cell wall)

Question 3

Explain the Gram negative cell envelope?

Answer 3

Thin peptidoglycan layer, inner & outer layer membrane, LPS (endotoxin) surface polymer in outside membrane

Question 4

Explain the mycobacterial cell envelope ?

Answer 4

Thin peptidoglycan, single membrane, outer layer of mycolic acid (hydrophobic - prevent influx of antibiotics & toxic m/os - similar function to 2nd membrane) - arabinogalactan links peptidoglycan to mycolic acid

Question 5

Explain the cell envelope of wall-less bacteria?

Answer 5

No peptidoglycan synthesis - obligate as must live in osmotically balanced environment

Question 6

What are the most common forms of cell envelopes?

Answer 6

Gram positive, Gram negative. mycobacterial, & wall-less

Question 7

Explain the Gram stain?

Answer 7

1880s - Hans Christian Joachim Gram - distinguish between Streptococcus pneumoniae (+) & Klebsiella pneumoniae (-) - Crystal violet (+ve) binds to peptidoglycan via -ve polymers of LPS & teichoic acid, iodine complexes with crystal violet - trapper (prevents it being washed away), 95% ethyl alcohol decolourises Gram -ve bacteria via interactions with outer membrane, safranin binds to associated polymers of peptidoglycan (pink)

Question 8

Gram stain visualisation?

Answer 8

Gram +ve = purple
Gram -ve = pink

Question 9

Explain the Ziehl-Nielsen stain?

Answer 9

Mycobacteria - hydrophobic mycolic acid makes Gram staining impossible - carbol fuchsin (pink), phenol, & heat allow penetration of mycolic acid layer & binds with -ve mycolic acid- counterstain is added

Question 10

What is the problem with taking colistin?

Answer 10

Disrupts bacterial & human membranes

Question 11

Explain the cell membrane in bacteria?

Answer 11

Permeability barrier, protein anchor, energy conservation - selective permeability (entry & exit of dissolved material, transport large & charged molecules via carrier proteins), ATP & essential co-factor generation via membrane bound proteins

Question 12

Explain the structure of bacteria & archaea cell membrane?

Answer 12

Hydrophilic region - glycerol, phosphate, ethanolamine
Hydrophobic region - fatty acids
majority are straight chain fatty acids densely packed - branching allows substance to enter (decrease in response to antimicrobials)

Archaea - more branching - lipids sometime bind to each other - more fluid characteristics

Question 13

Explain the function of the cell wall of bacteria?

Answer 13

Strength, contain membrane, capture high conc. of soluble material, contain osmotic pressure up to 208kPa, anchor for surface molecules, target of betalactams

Question 14

Explain the structure of peptidoglycan in the bacterial cell wall?

Answer 14

Mesh-like layer - repeated disaccharides form a polysaccharide backbone (N-acetylmuramic acid (NAM) & N-acetylglucosamine (NAG)) cross-linked of stem peptides on NAM molecules (position 3 to position 4) - gycosidic bonds in x axis & peptide bonds in y axis

GRAM +: indirect linkage L-lysine & d-alanine via pentaglycine bridges

GRAM -: direct linkage between meso-diaminopimelic acid (DAP) to d-alanine

Question 15

What is pseudopeptidoglycan?

Answer 15

Made of G (n-acetylglucosamine) & T (n-acetyltalosaminuronic acid) - L-lys to L-glu - found in archaea

Question 16

What are membrane proteins?

Answer 16

Hydrophobic domains span membrane & hydrophilic domain (Cytoplasmic & ECF) - integral (significantly embedded) & peripheral (slightly attached e.g. lipoprotein)

Question 17

What are the major forms of transport?

Answer 17

1) Osmosis - diffusion of water across membrane
2) Diffusion - movement of particles from area of high conc. to low conc.
3) Active transport - movement of particles against conc. gradient

Question 18

What is teichoic acid?

Answer 18

Polymer of repeated sugar - Gram +, high amount of phosphate (-ve) attracts +ve antimicrobials & immune cell

Question 19

Explain lipopolysaccharide?

Answer 19

Endotoxin - Gram -ve - Lipid A (hydrophobic domain (endotoxin)) ,core oligosaccharide (non-variable core), & O-antigen (repeat hydrophilic distal polysaccharide- varies- dictate host range & how they colonise)

Question 20

What is the s-layer?

Answer 20

self-assembling 2D protein arrays - some bacterial & all archaea - anchored non-covalently to LPS or teichoic acid

Question 21

What are fimbriae & pili?

Answer 21

Hair-like proteinaceous attachment on surface

Question 22

What is the capsule/glycocalyx?

Answer 22

Gelatinous polysaccharide layer, cover cells - form biofilms - hydrophilic (90% water) - protect against phagocytes - diverse (dictate serotype of m/o)

Question 23

Explain active transport?

Answer 23

Simple transport uses H+ ions to facilitate transport - Antiporter (opposite direction to H+ outisde cytplasm) & symporter (Same direction as H+) - lactose transport

Question 24

Explain the ABC Transport System?

Answer 24

3 components - substrate binding protein (periplasm), transmembrane transporters, ATP hydrolysing enzyme (cytoplasm) - maltose transport

Question 25

Explain the phosphotransferase system?

Answer 25

Group translocation, multiple enzymes, substances are chemically altered e.g. dephosphorylation of pyruvate to enter TCA cycle & phosphorylation of glucose required for entry into glycolysis - glucose transport