Lecture 5 (Linda Stewart) - Microbial Cell Structure 1 Flashcards
Describe the fluid mosaic model of cell structure and identify the types of lipids typically found in bacterial membranes
- Phospholipid bilayer with floating proteins.
- A phospholipid consists of glycerol with ester links to two fatty acids and a phosphoryl head group. May also contain a side chain.
- Amphipathic lipids - polar ends (hydrophilic) and non-polar tails (hydrophilic).
- Embedded proteins are called integral membrane proteins.
- Peripheral membrane proteins are loosely attached to the membrane and can be easily removed.
- The membrane is stabilised by hydrogen bonds and hydrophobic interactions.
- Bacterial membranes contain sterol-like molecules called hopanoids
- Hopanoids help stabilise the membrane.
Describe peptidoglycan (murein) structure
- Strands in peptidoglycan are helical.
- The backbone of each strand is composed of two alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), linked to form long strands.
- Contains alternating D- and L- amino acids.
- The presence of D-amino acids protects against degradation by most peptidases.
- Strands in peptidoglycan are cross-linked by covalent bonds between peptides. The more extensive the cross-linking the greater the rigidity.
- Interbridges and peptidoglycan sacs (interconnected networks) may form to increase strength.
Compare and contrast the cell walls of typical gram-positive and gram-negative bacteria
Gram-positive:
- Composed of one layer.
- Thick cell wall (20-80nm).
- Does not have an outer membrane.
- Small periplasmic space or not present at all.
- Chemically composed of peptidoglycan, teichoic acid, and lipoteichoic acid.
- Thick layer of peptidoglycan.
Gram-negative:
- Composed of two layers.
- Thin cell wall.
- Outer membrane is present.
- Large periplasmic space.
- Chemically composed of lipopolysaccharides, lipoprotein, and peptidoglycan.
- Thin layer of peptidoglycan.
Relate bacterial cell wall structure to the gram-staining reaction
Gram-positive bacteria:
- Thick layer of peptidoglycan is dehydrated by the alcohol during the decolourisation step.
- This causes pores in the walls to close and prevent the insoluble crystal violet-iodine complex from escaping.
- Therefore the cells stain purple.
Gram-negative bacteria:
- Thin peptidoglycan layer and large pores does not prevent the loss of crystal violet.
- The alcohol penetrates the lipid-rich outer membrane and extracts the crystal violet-iodine complex from the cell.
- This decolourises the cells.
- When the cells are counterstained with safranin they then appear pink.
Compile a list of all the structures found in all the layers of bacterial cell envelopes, noting the functions and the major component molecules of each
Outer membrane (gram-negative bacteria): - More permeable than plasma membrane due to presence of porin proteins and transporter proteins. - Porin proteins from channels to let small molecules pass. - Contains Braun's lipoproteins which connect the outer membrane to the peptidoglycan. - Contains lipopolysaccharides (LPS) - Consists of three parts (lipid A, core polysaccharide, and O antigen). 👉 Contributes to negative charge on cell surface. 👉 Helps stabilise the outer membrane structure. 👉 May contribute to surface attachment and biofilm formation. 👉 The O antigen helps protect against host defences. 👉Lipid A can act as an endotoxin.
Periplasmic space (gram-negative and some gram-positive):
- Lies between plasma membrane and cell wall.
- Larger in gram-negative bacteria.
- Periplasm of gram-positive bacteria secretes enzymes called exoenzymes which aid in degradation of large nutrients.
- Hydrolytic enzymes and transport proteins are present in theperiplasm of gram-negative bacteria.
Plasma membrane (both):
- Encompasses the cytoplasm and acts as a permeability barrier.
- Fluid Mosaic structure.
- Composed of a phospholipid bilayer containing integral proteins and peripheral proteins.
- The proteins serve a number of functions:
👉 Structural support.
👉 Detection of environmental signals.
👉 Secretion of virulence factors and communication signals.
👉 Ion transport and energy storage.
- Contains sterol-like molecules called hopanoids which stabilise the membrane.
Cell wall (peptidoglycan) (both):
- Confers shape and rigidity.
- Can withstand osmotic pressures and prevent lysis.
- Protects the cell from toxic substances.
- Can contribute to pathogenicity in pathogens.
- Site of action of many antibiotics.
Glycocalyx:
- Contains capsules composed of polysaccharides which are well organised and not easily removed, as well as having protective advantages (resistant to phagocytosis protects from desiccation).
- Contains slime layers which are similar to capsules but are unorganised and easily removed. May aid in motility.
External structures:
- Pili and frimbae assist in surface attachment and DNA uptake.
- Flagellum is responsible for motility and swarming behaviour, attachment to surfaces, and may be a source of virulence factors.
- Endospore is a dormant structure found in some bacteria. It is resistant to heat, radiation, chemicals, and desiccation (aids in survival of cell under adverse conditions).
S Layers:
- Regularly structured layers of protein or glycoprotein that self-assemble.
- Protect from ion and pH fluctuations, osmotic stress, enzymes, and predation.
- Maintains shape and rigidity.
- Promotes adhesion to surfaces.
- Protects from host defences.
What two major groups are prokaryotes divided into
Bacteria and Archaea
Outline the differences between archael membranes and bacterial membranes
- Phospholipids in archaea contain ether linkages instead of ester linkages.
- Archael lipids lack fatty acid and instead have multiple units of isoprene.
- The membrane in Archaea can exist as a lipid monolayer, bilayer, or mixture.
Name a type of bacteria that never make cell walls
Mycoplasmas
Outline the differences between archael cell walls and bacterial cell walls
- Composed of peudomurein, a polysaccharide similar to peptidoglycan.
- Backbone is composed of alternating repeats of N-acetylglucosamine and N-acetyltalosaminuronic acid.
- Sugar derivatives linked by beta-1,3-glycosidic bonds.
How does lysozyme destroy peptidoglycan
Breaks the beta-1,4-glycosidic bonds between NAG and NAM.
How does penicillin inhibit cell wall formation
Inhibits the transpeptidase that cross-links the peptides in peptidoglycan.
How does vancomycin inhibit cell wall formation
Prevents cross-bridge formation in peptidoglycan by binding to the terminal D-Ala-D-Ala dipeptide.
