LecTop4: Bacterial Cell: Struct & Funct

Question 1

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?

Answer 1

1. Gram- bacteria have an outer membrane
2. 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).

Question 2

What are the three components of Peptidoglycan cells walls of bacteria? Briefly describe each.

Answer 2

1. Glycan Backbones - Consists of Repeating disaccharide units of N-acetyl-glucosamine & -muramic acid linked by B(1-4) bonds.
2. 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
3. Amino Acid Cross Bridge: Links tetrapeptide side chains, via peptide bonds between aas 3 & 4, on DIFFERENT glycan backbones.

Question 3

What is the 3-D shape of glycan backbones of the peptidoglycan?

Answer 3

They form rings circling the cell in all possible directions and planes. Think a random ball of yarn or rubber band ball.

Question 4

Which glycan backbone sugar, glucosamine or muramic acid, do the tetrapeptide side chains always connect to?

Answer 4

Muramic acid

Question 5

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…?

Answer 5

Peptide bonds are covalent bonds = strong

Question 6

What is special about D,L-diaminopimelic acid (DAP)?

Answer 6

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).

Question 7

What is the big difference in composition of Gram+ and Gram- bacterial peptidoglycan?

Answer 7

• 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.

Question 8

What’s a descriptive analogy for peptidoglycan that explains its rigidity and porosity?

Answer 8

A chain link fence. Peptidoglycan is a stiff, porous structure, therefore it provides stability w/o necessarily providing much in the way of filtration.

Question 9

Why is peptidoglycan so important to bacteria?

Answer 9

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)

Question 10

What are three cell shapes that instill evolutionary advantages to the bacteria?

Answer 10

• optimum motility = medium length rods
• Helical shape = highly viscous environment
• Very large, small or awkwardly shaped = avoid being eaten

Question 11

What determines bacterial shape?

Answer 11

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).

Question 12

What is the enzyme found in human secretions, including saliva, that specifically attacks bacterial peptidoglycan? Product?

Answer 12

Lysozyme. Cuts Glycan backbone at B(1-4) bonds resulting in a product of two cross-linked disaccharides.

Question 13

What are amphiphilic bacterial polymers?

Answer 13

They are usually amphiphilic polymers of glucose that are embedded in bacterial membranes. Often have antigenic properties.

Question 14

What is Lipoteichoic Acid (LTA)? Where is it found?

What are LTA’s two main parts?

Answer 14

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.

Question 15

What is Lipopolysaccharide (LPS)? Where is it found? What are the 3 main parts? What is an alternative name for LPS?

Answer 15

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

Question 16

KDO is a sugar in linker-region b/t lipid A and core regions. What is its significance?

Answer 16

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.

Question 17

What purpose do Porins serve in the outer membrane of Gram- bacteria?

Answer 17

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).

Question 18

Why is the outer layer of gram- bacteria an assymetric bilayer?

Answer 18

Because Lipid A displaces most of the phospholipids on the outer membrane.

Question 19

What is mycolic acid?

Answer 19

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)

Question 20

What is a glycocalyx?
What bacteria express glycocalyx?
What evolutionary advantages does it instill?

Answer 20

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.

Question 21

What are flagella used for? What filament makes up the major structure?

Answer 21

Motility.

Axial Filaments

Question 22

What is the major function of Pili?

What’s another name for Pili?

Answer 22

Pili or Fimbriae are generally thought to be adherence appendeges. Also coaggregation, and conjugation (Sex pilus)
-Important in oral cavity

Question 23

Describe flagellum powered movement.

• What are peritrichous flagella?
• Describe the axial filaments of spirochetes.

Answer 23

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.

Question 24

What is E. Coli O157:H7?

Answer 24

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

Question 25

We saw that LPS has the O-antigen. What bacterial structures house the H and K antigens?

Answer 25

• H antigen is located on Pili (and flagella?) of Gram+ and Gram- bacteria
• K antigen is located in the capsule of Gram- bacteria

Question 26

What is arabinogalactan?

Answer 26

It is a surface polymer in acid-fast bacteria that inhibits phagolysosomal comonents (similar to mycolic acid).