What are the two major differences between Gram+ and Gram- cells?
- Are Acid fast bacteria more similar to Gram+ or Gram- bacteria?
- How are mycoplasma different?
- Gram- bacteria have an outer membrane
- Gram+ bacteria have a thicker peptidoglycan layer than Gram- bacteria
- Acid-fast are more similar to Gram+ b/c no outer membrane (remember enclosing membrane is an artificial slide creation)
- Mycoplasmas are smaller, lack a peptidoglycan cell wall, and can use exogenous sterols in their membrane (Remember: cause pneumonia and contaminate, ideally sterile env’ts).
What are the three components of Peptidoglycan cells walls of bacteria? Briefly describe each.
- Glycan Backbones - Consists of Repeating disaccharide units of N-acetyl-glucosamine & -muramic acid linked by B(1-4) bonds.
- Tetrapeptide side chain - aa chains, consisting of four aas. The third aa must be able to form 3 peptide bonds and the aa in position four is always D-alanine
- Amino Acid Cross Bridge: Links tetrapeptide side chains, via peptide bonds between aas 3 & 4, on DIFFERENT glycan backbones.
What is the 3-D shape of glycan backbones of the peptidoglycan?
They form rings circling the cell in all possible directions and planes. Think a random ball of yarn or rubber band ball.
Which glycan backbone sugar, glucosamine or muramic acid, do the tetrapeptide side chains always connect to?
Muramic acid
Cross bridges are formed between the #3 and 4 aas of the tetrapeptide side chains via peptide bonds, what type of bond is this? Ionic, covalent, etc…?
Peptide bonds are covalent bonds = strong
What is special about D,L-diaminopimelic acid (DAP)?
It is the special amino acid ONLY found in position #3 in bacterial peptidoglycan tetrapeptide side chains.
-It has 2 amino groups (hence, ‘diamino-‘) AND 2 carboxyl groups allowing it to form 3 peptide bonds (normal aas can only form 2).
What is the big difference in composition of Gram+ and Gram- bacterial peptidoglycan?
- Gram+ peptidoglycan is cross-linked at every possible position (every tetrapeptide chain is crosslinked), creating a very rigid structure.
- Gram- bacteria has far fewer cross links.
What’s a descriptive analogy for peptidoglycan that explains its rigidity and porosity?
A chain link fence. Peptidoglycan is a stiff, porous structure, therefore it provides stability w/o necessarily providing much in the way of filtration.
Why is peptidoglycan so important to bacteria?
Peptidoglycan cell walls allow bacteria to survive in variable turgor (osmotic) pressure conditions.
- Won’t lyse in a hypotonic solution (cytolysis)
- Won’t shrivel in a hypertonic solution (plasmolysis)
What are three cell shapes that instill evolutionary advantages to the bacteria?
- optimum motility = medium length rods
- Helical shape = highly viscous environment
- Very large, small or awkwardly shaped = avoid being eaten
What determines bacterial shape?
Peptidoglycan cell wall determines cell shape.
Shape of wall determined by how deposited, this is determined by the intracellular matrix (tubulin and actin like protein fibers control this).
What is the enzyme found in human secretions, including saliva, that specifically attacks bacterial peptidoglycan? Product?
Lysozyme. Cuts Glycan backbone at B(1-4) bonds resulting in a product of two cross-linked disaccharides.
What are amphiphilic bacterial polymers?
They are usually amphiphilic polymers of glucose that are embedded in bacterial membranes. Often have antigenic properties.
What is Lipoteichoic Acid (LTA)? Where is it found?
What are LTA’s two main parts?
LTA is an amphyiphilic bacterial polymer found in Gram+ bacteria.
- It has a glycolipid (hydrophobic, membrane) portion and a Polyglycerophosphate (long, hydrophilic, ECM) portion
- Note: Polyglycerophosphate portion extends through capsule/glycocalyx region.
What is Lipopolysaccharide (LPS)? Where is it found? What are the 3 main parts? What is an alternative name for LPS?
LPS or Endotoxin is Amphiphilic bacterial polymer found in outer membrane of Gram- bacteria.
- 3 parts
1. Lipid A (toxic): Conserved membrane anchor
2. Core Polysaccharide: Variable sugar composition
3. O-antigen: O-polysaccharide, acts as antigen
KDO is a sugar in linker-region b/t lipid A and core regions. What is its significance?
KDO is an easily cleaved sugar that was cleaved to isolate the lipid A portion of LPS.
- Acid Labile Region
- Allowed to isolate and determine that Lipid A is the region of LPS responsible for toxic effects.
What purpose do Porins serve in the outer membrane of Gram- bacteria?
Because Gram- bacteria have two membranes it is difficult for them to get nutrients into the cell. The porins provide aqueous transport through the outer membrane for hydrophilic substrates.
-there is a size restriction, however, sugar polymers larger than pentosaccharides generally can’t pass through (ex glycogen).
Why is the outer layer of gram- bacteria an assymetric bilayer?
Because Lipid A displaces most of the phospholipids on the outer membrane.
What is mycolic acid?
Bacterial surface polymers on Acid-fast bacteria.
- Causes acid-fast bacteria to have a waxy-like coating = the reason acid-fast bacteria can’t absorb normal dyes.
- Inhibits absorption of water and water associated compounds. Leads to slow growth; important for development of disease (TB)
What is a glycocalyx?
What bacteria express glycocalyx?
What evolutionary advantages does it instill?
Glycocalyx or capsule = Variably expressed, Carbohydrate polymers deposited on surface of bacterium (there are some protein types as well).
- Certain species of both Gram+ and Gram- bacteria express capsules. Also dependent upon growth factors in env’t.
- Capsules give bacteria antiphagocytic properties, may block lysozymes, and also may be ingested by bacteria in times of nutrient deficiency.
What are flagella used for? What filament makes up the major structure?
Motility.
Axial Filaments
What is the major function of Pili?
What’s another name for Pili?
Pili or Fimbriae are generally thought to be adherence appendeges. Also coaggregation, and conjugation (Sex pilus)
-Important in oral cavity
Describe flagellum powered movement.
- What are peritrichous flagella?
- Describe the axial filaments of spirochetes.
Single flagella are rotated in one direction to cause straight forward movement or rotated in the opposite direction to cause scattered backwards movement (Related to chemotaxis!)
- Peritrichous flagella are groups of flagella
- The axial filaments of spirochetes attach at both polar ends of the spirochets and assist in the corkscrew type movement of spirochetes.
What is E. Coli O157:H7?
It is a strain of E. coli that was implicated in contaminating certain meats and vegetables.
-It has the O-antigen 157 and the H-antigen 7
We saw that LPS has the O-antigen. What bacterial structures house the H and K antigens?
- H antigen is located on Pili (and flagella?) of Gram+ and Gram- bacteria
- K antigen is located in the capsule of Gram- bacteria
What is arabinogalactan?
It is a surface polymer in acid-fast bacteria that inhibits phagolysosomal comonents (similar to mycolic acid).
