Bacterial Structure and Function I&II-Felton

Question 1

What are the major characteristics of eukaryotic cells?

Answer 1

1. The cells of animals, plants, algae, fungi, and protozoa are eukaryotic.
2. usually much bigger than prokaryotic cells.
3. have a nuclear membrane and nucleoli.
4. usually have more than one chromosome.
5. have a cytoskeleton composed of tiny rods called microfilaments and intermediate filaments, and cylinders called microtubules.
6. Flagella (cilia), when present have a complex structure and are membrane-bounded. (a lot bigger than bacteria)
7. have 80S ribosomes in their cytoplasm, composed of a 60S subunit and a 40S subunit. These are composed of rRNA and protein.
8. Animal cells don’t have walls. Plants (cellulose) and fungi (chitin) do.
9. Cell membranes contain sterols.
10. Some protozoa produce resting cells called cysts which are resistant to environmental conditions such as heat and drying.
11. Fungi may produce spores which are a means of reproduction. Fungal spores may be somewhat resistant to environmental conditions.
12. Cell-surface proteins and extracellular secreted proteins are glycosylated (called glycoproteins).

Question 2

What are the major characteristics of prokaryotic cells?

Answer 2

1. The cells of bacteria and cyanobacteria are prokaryotic.
2. No nuclear membrane is present.
3. They have no membrane-bounded organelles such as mitochondria, endoplasmic reticulum, Golgi apparatus, or lysosomes.
   - Mesosome: might be a membrane-bounded organelle. Function unknown.
4. Bacteria do have some cytoskeletal elements, but they are much less extensive than the highly developed cytoskeleton of eukaryotes.
5. Cell division is by binary fission. (instead of mitosis)
6. They have 70S ribosomes formed of 30S and 50S subunits.
7. Almost all bacteria of medical importance have a chemically complex cell wall containing peptidoglycan (murein). (substance only found in prokaryotes)
8. When they have flagella, they are almost always unsheathed.
9. They may have other projections from the cell, called pili (fimbrie).
10. They have a very high surface-to-volume ratio, which allows a high metabolic rate. (due to their small size, contributing to rapid growth)
11. Proteins are not normally glycosylated.
12. Does not contain sterols
    - Exception: Mycoplasmas: don’t have a cell wall but do have sterols which they must obtain from their host or environment.
13. Have a nucleoid where the genome resides. (usually on one chromosome)
    - Exception: some bacteria might have 2 chromosomes.

Question 3

What is the basic size of bacteria?

Answer 3

0.5-1.0 μm in width, and up to several μm in length

Question 4

What are the basic shapes of bacteria (4)?

Answer 4

1. Coccus (spherical)
2. Rod or bacillus (cylindrical)
3. Spiral: many turns or only one or two. (spirochetes)
4. Pleomorphic: variable in size and shape.

Question 5

What is the importance of the cell wall?

Answer 5

Protects cells against osmotic lysis, imparts rigidity, and confers their characteristic shape to bacteria. Also determines the Gram staining characteristics.

Question 6

What are the characteristics of Gram (+) cell walls?

Answer 6

1. Stained purple.
2. Has one cell wall with a thick layer of peptidoglycan on the outside and then a cell membrane inside.
   - Has teichoic acid and lipoteichoic acid attached to the peptidoglycan layer.
3. Some Gram-positive bacteria have polysaccharides (Streptococcus), and even in some cases proteins in the outer part of their cell walls. These may also contribute to the antigenic specificity of the cell. Members of the genus Mycobacterium have waxy lipids in their cell walls which makes them more resistant to environmental insults.

Question 7

What are the characteristics of Gram (-) cell walls?

Answer 7

1. Stained pink
2. Has an outer membrane and inner membrane which surround a small peptidoglycan cell wall layer.
   Outer membrane:
   -Inner leaflet: phospholipid layer
   -Outer leaflet: has lipopolysaccharide (LPS)
   -Only found in gram (-).
   Periplasmic space: between 2 membrane layers where the peptidoglycan layer is located.
3. Have porins in the outer membrane which are nonspecific channels that permit the passage of small hydrophilic molecules up to MW of 700.
4. Envelope: collectively the cell wall and the cell membrane, commonly used with gram-negative organisms.

Question 8

What is the basic structure of peptidoglycan?

Answer 8

1. The polysaccharide glycan chains of peptidoglycan are made up of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).
   -NAM is only present in bacteria. NAG is present in both eukaryotes and prokaryotes.
2. There are short peptide chains attached to the N-acetylmuramic acid residues.
   -There is a linker attached to NAM thru which side chains can be attached. (pentapeptide)
   •L-alanine
   •D-Glutamic acid
   •L-Diaminopimelic acid (DAP)
   •D-Alanine (1 or 2)
3. Adjacent glycan chains are held together by cross-linkages between peptides.
   -DAP participates in the cross-link thru its amino group. (lysine can too)
   •Attaches to the carboxyl group of D-Ala on another cross-link.
   •Energy used to make the cross-link is generated by the leaving of the 2nd D-alanine.

Question 9

What is the basic structure of Teichoic acid (in Gram +)?

Answer 9

1. Teichoic acids may be composed mainly of ribitol or mainly of glycerol connected by phosphate bonds.
2. They are polymers which form long filaments.
3. There are two forms of teichoic acids: wall teichoic acids and lipoteichoic acids.
   -The wall teichoic acids are covalently attached to the peptidoglycan layer.
   •Teichoic acids provide much of the Gram-positive cell wall’s antigenic specificity.
   •Have a role in adherence and biofilm formation.
   -The lipoteichoic acids are linked to the cytoplasmic membrane and span the peptidoglycan layer.
   •Lipoteichoic acids mediate adherence to mucosal cells.
   •Lipoteichoic acid possesses an endotoxin-like activity which activates toll-like receptors and results in the release of cytokines.
4. They have a net negative charge. They bind cations and regulate their movement into and out of the cell. There is overlapping function between wall teichoic acids and lipoteichoic acids.

Question 10

What is the basic structure of Lipopolysaccharide (LPS)?

Answer 10

It makes up most of the outer leaflet of the outer membrane lipid bilayer. This is the classical endotoxin. It consists of three parts.

1. Lipid A is embedded in the membrane. This compound is quite toxic to animals, and is a major virulence factor of most Gram-negative bacteria.
2. The Core oligosaccharide connects the O antigen to the lipid A.
3. The repeating polysaccharide which extends into the environment is called the O antigen. The tri-saccharide subunits of the O antigen polysaccharide vary between types of bacteria. The O antigen is a very important antigenic determinant of Gram-negative bacteria. If O antigen is missing then called lipo-oligosaccharide or LOS.

Question 11

What are the major functions of the outer membrane (3)?

Answer 11

1. Strong negative charge important in evading phagocytosis and complement.
2. Barrier to certain antibiotics, some hydrolytic enzyme and other environment chemicals. The hydrophilic properties of LPS compared to the hydrophobic properties of phospholipids makes Gram- bacteria more resistant to hydrophobic antibiotics and certain organic solvents.
3. Prevents loss of metabolite-binding proteins and hydrolytic enzymes found in periplasmic space.

Question 12

What is the periplasmic space(3)?

Answer 12

1. Contains degradative enzymes, detoxifying enzymes and binding proteins involved in membrane transport.
2. Contain oligosaccharides which are osmoregulatory.
3. Lipoprotein connects the peptidoglycan w/ the outer membrane of the Gram- cell.

Question 13

Describe the synthesis of peptidoglycan.

Answer 13

Autolysins create sites for insertion of newly synthesized peptidoglycan precursors.
•Glycosidases break the glycoside backbone.
•Amidase releases the tetrapeptide from NAM.
•Endopeptidases attack the various peptide bonds of the side chains and cross links.

Question 14

What is the mechanism of synthesis of peptidoglycan?

Answer 14

1. Soluble monomeric precursors are made in the cytoplasm as UDP-NAM and UDP-NAG.
2. Particular amino acids are sequentially added to UDP-NAM to build up a UDP-NAM-pentapeptide.
3. The NAM-pentapeptide is then transferred to a membrane carrier lipid called undecaprenol.
4. NAG is then added to form NAG-NAM-pentapeptide (also called disaccharide pentapeptide), still attached to the membrane carrier lipid. At this point the membrane carrier lipid, with its attached disaccharide pentapeptide, flips from the inner surface of the cytoplasmic membrane to the outer (periplasmic) surface.
5. The disaccharide pentapeptide is transferred from the carrier lipid to the cell wall glycan backbone and connected to it via glycosidic bonds.
6. Then peptide cross links are formed via transpeptidation to complete the cell wall structure. The antibiotic penicillin acts by interfering with transpeptidation, thus preventing formation of a functional fully cross-linked cell wall.