Lecture 4. Microbial Cell Walls Flashcards
What are the layers of bacterial cells from outer to inner?
Capsule, S-layer
Gram +ve cell wall or Gram -ve outer membrane
Periplasmic space
Cell (plasma) membrane
What are capsules?
Sometimes called slime layers or glycocalyx (‘sugar coat’)
Polysaccharide components outside the cell wall usually loose network of polymer fibres extending outward from wall
What is glycocalyx involved in?
Biofilm formation, aids in establishing complex consortia of bacteria
What is the role of capsules?
Not required for growth or reproduction
Carbon store
Protection against desiccation
May be involved in the capture of nutrients (acquisition of ions from the environment)
Confer advantages in vivo such as attachment to surfaces
Exclude phage, antimicrobials and disinfectants
Pathogens often capsular and resist phagocytosis
What is the most common form of capsules in pathogens?
Polysaccharide structure (Streptococcus pneumoniae, Haemophilus influenzae, Neisseria meningitidis, Bacillus anthracis)
What are examples of bacterial adhesins?
Capsular polysaccharide, extracellular slime, fimbriae, lectins
What is the S-layer?
Paracrystalline outer wall layer composed of protein/glycoprotein
Regularly structured layer external to cell wall
In some archaea the ONLY cell wall structure
May protect against ion and pH fluctuations, osmotic stress, predators such as Bdellovibrio
May protect against host defences
What makes up peptidoglycan?
Alternating residues of NAG (N-acetylglucosamine) and NAM (N-acetylmuramic acid (lactyl ether of NAG))
How is peptidoglycan arranged?
In dimers which are cross linked by amino acid side chains creating amide bonds
Is a mesh-like polymer that retain the Gram stain in Gram +ve cells
How are chains of linked peptidoglycan subunits joined together?
Cross-links between 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
What is a lysozyme?
‘Antibacterial’ enzyme
Degrades the beta 1,4-glycosidic bond in PG backbone
Loss of PG makes cells sensitive to changes in osmotic
pressure
Important host defence against bacteria
What inhibits peptidoglycan synthesis?
Penicillin
What is found in Gram +ve cell wall but not in Gram -ve cell wall?
Teichoic acid, role unclear but might make the membrane negatively charged
What are the differences between archaeal cell walls and bacterial cells walls?
Archaeal cell walls do not contain peptidoglycan
Some methanogens contain pseudomurein
Contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid
What is the structure of the 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
