Bacterial Structure and Function III-Felton

Question 1

What is the basic structure of the bacterial membrane?

Answer 1

Similar to eukaryotes except w/o sterols.

Question 2

What are the functions of the bacterial membrane?

Answer 2

1. Contains enzyme systems for energy conversion via ATP synthesis (electron transport/oxidative phosphorylation).
2. Contains enzymes for synthesis of cell wall components.
3. Participates in secretion of extracellular proteins.
4. Participates in cell division and as an anchoring site for DNA.
5. Controls transport in/out. Selectively permeable.
   •Porins located in outer membrane only.
   •Aquaporins are the only porins located in the inner membrane.

Question 3

What is the basic structure of bacterial DNA?

Answer 3

1. All essential genes usually on 1 continuous circular piece of double-stranded DNA (nucleoid or bacterial chromosome).
2. Chromosome varies from 1000-6000 genes.
3. Proteins in membrane are responsible for replication of bacterial DNA and segregation of new chromosomes into daughter cells during cell division.

Question 4

What is a bacterial plasmid? What are the characteristics of plasmids?

Answer 4

Plasmid=small circular double-stranded DNA molecules

• Replicate independently of chromosomal DNA. (autonomous)
• Associated w/ cytoplasmic membrane protein.
• Contain 5-100 genes that are NOT crucial for survival under normal situations. (accessory genes)
• –Genes may be for antibiotic resistance, tolerance to toxic metals, productions of toxins or synthesis of enzymes.
• Plasmids can be transferred from one bacterium to another.

Question 5

What is the basic structure of bacterial ribosomes?

Answer 5

70S ribosomes (50S and 30S subunits)
•Subunits = rRNA + numerous protein molecules.
Occur as polysomes (polyribosomes) during protein synthesis.

Question 6

What is the basic structure of flagella?

Answer 6

-Composed of globular protein, flagellin, arranged in several chains that coil together to form a helix around a hollow core.
-The filament is anchored in the cell envelope via a 2-part structure consisting of a hook and basal body.
-Can be seen only by special stains or electron microscopy.
-Arrangement of flagella classifies the organism.
•Monotrichous, peritritous, polar (lophotrichous), amphitrichous and axial filament.
-Spirochetes: flagella are present in the periplasmic space as axial filaments, which are also involved in motility. (corkscrew motility)

Question 7

What is the function of flagella? What is chemotaxis? How is flagella synthesized?

Answer 7

Function = motility
Each flagellum is a semi-rigid helical propeller that moves the cell by rotating from the basal body, which functions like a motor.
Chemotaxis: swim toward or away from a stimulus (glucose)
Synthesis: Addition of subunits to growing tip of flagellar filament; Subunits reach the tip by moving out thru the hollow 5nm channel that lies at the interior of the flagellar basal body, hook, and filament.

Question 8

What antigen is specific to flagella?

Answer 8

H-antigen

Question 9

Describe the Type III secretion system (injection system).

Answer 9

The Type III secretion apparatus shows structural similarities to the basal body of the bacterial flagellum, but instead of the flagellar filament it has a short hollow needle through which certain effector proteins (toxins, etc.) can be injected into target cells of the infected host. There they can produce cytologic effects that favor the survival and growth of the infecting bacteria, and also produce symptoms of disease. So Type III secretion systems can deliver effector proteins into the extracellular culture medium (leakage), or into the cytoplasm of the target cell (their true purpose).

Question 10

Describe the Type IV secretion system (conjugation system).

Answer 10

The Type IV secretion apparatus shows structural similarities to conjugation systems encoded by certain plasmids. These conjugation systems deliver DNA, attached to a pilot protein, into the recipient cells. But many Type IV secretion systems produced by pathogenic bacteria cause the transfer of effector proteins alone (without DNA) into the recipient (i.e. target) cells. They are like Type III secretion systems in that way.

Question 11

What is basic structure of Pili (fimbriae)? What is its function?

Answer 11

-Thin filamentous appendages distributed often in large numbers over the surface of some bacteria. Mostly in Gram- bacteria.
-Too small (3-8nm) to be seen in light microscope.
-Much shorter than flagella.
-Pilum with fimbrum at the tip.
-Assembled and lengthened by addition of new subunits to the base of the pilus, where it is anchored to the cytoplasmic membrane.
Functions:
•Adhesion (in Gram-)
•Antiphagocytic
•Changeable to avoid the immune system (change pilus type).
•Sex: F-pilus which permits transfer of DNA from one bacterium to another by conjugation. Analog of sex pilus seen in Type IV secretion system.

Question 12

What are the other polymers external to the cell wall (exopolysaccharides)?

Answer 12

1. Slime layer: amorphous, unorganized and loosely attached to the bacterial cell.
2. Capsules

Question 13

What is the basic structure of bacterial capsules?

Answer 13

-Organized and firmly attached to cell wall.
-NOT essential for viability.
-Associated w/ virulence in bacteria like Strep. pneumoniae.
•They prevent or resist phagocytosis.
•They may promote attachment to surfaces.
-May protect against dehydration.
-Can be lost via mutation.
-Usually are antigenic.

Question 14

What are endotoxins?

Answer 14

• Toxic proteins.
• Are antigenic and usually heat-labile.
• Usually responsible for the particular symptoms of the diseases caused by the organisms.
• The DNA that codes for these exotoxins is often located on a plasmid or some unusual genetic site.

Question 15

What are endospores? What are they found in?

Answer 15

• Highly resistant, metabolically dormant structures.

- Found primarily in certain Gram+ rods, members of the genera Bacillus and Clostridim.

Question 16

What is the basic structure of endospores?

Answer 16

-Contain a core (consisting of a complete nucleoid, ribosomes, and energy-generating components) enclosed w/n a cytoplasmic membrane, cortex and coat. The cortex is composed of peptidoglycan, while the coat is made of protein, and is highly cross-linked with disulfide linkages.

Question 17

When/why are endospores formed?

Answer 17

-Spores are very dehydrated.
-Spore formation (sporulation) usually occurs when nutrients are exhausted or when water is unavailable. Vegetative cells (normal growing cells) are converted into spores via a developmental process.
-Spore formation is a protective device and not a means of reproduction. 1 bacterial cell can only form one spore.
-Spores are important clinically and in the food industry because they may resist the usual means of infection control or sterilization.
-Spores are resistant to the usual bacterial stains but can be stained with special stains.
-Germination of spores (reverse sporulation)
•Activation occurs in presence of certain nutrients or environmental conditions.
•Begins as a degradative process. Endospore enzymes break down the extra layers surrounding the endospore, water enters, metabolism resumes, and the spore is converted back into a vegetative cell.

Question 18

What are storage granules?

Answer 18

Some bacteria under certain circumstances produce intracellular granules. These inclusion granules are usually metabolic reserves used for storage of nutrients (or “energy”) in a polymerized form, e.g., glucose as glycogen, beta-hydroxybutyric acid as poly-beta-hydroxybutyrate, or phosphate as inorganic polyphosphate (volutin). Polymerization minimizes the osmotic effect of the stored material.