Infectious Agents

Question 1

Describe the generalised structure of a bacterium

Answer 1

Single-called organisms
Prokaryotes (cells are non-compartmentalised, no membrane-bound nucleus)
Circular DNA plasmid found in cytoplasms
Flagellum (tail for locomotion but also as sensory organelle)
Inclusion bodies (represent sites of viral multiplication and usually consist of viral capsid proteins)
Ribosomes
Pilus/fimbria (hair-like appendage0)
Capsule/slime layer
Cell wall
Cell membrane
Cytoplasm

Question 2

Draw a diagram of a bacterial cell and label its constituent parts

Answer 2

See image

Question 3

State the two ways in which bacterial cells can reproduce

Answer 3

Binary fission and spore formation

Question 4

Name the five groups of microorganisms

Answer 4

Bacteria, fungi, viruses, parasites, prions

Question 5

What is the shortest length of time in which bacteria can divide?

Answer 5

E.coli can divide every 20 minutes

Question 6

What do bacteria need in order to multiply?

Answer 6

Energy source, eg. Glucose 
Water
Nitrogen, sulphur, iron etc.
Some need complex organic materials
Atmospheric conditions - some are strict aerobes, some facultative (can use oxygen but also have anaerobic methods of energy production), some strict anaerobes 
Temperature
pH

Question 7

How do bacteria achieve genetic variation, given that they reproduce asexually?

Outline the different forms

Answer 7

Horizontal gene transfer - transfer of genetic information between bacteria of the same generation.

3 forms:

• transformation (naked DNA is released from one cell and up taken into another. The released DNA is transformed into the bacterial DNA of the recipient cell)
• transduction (DNA is transferred by a type of virus called a bacteriophage)
• conjugation (a plasmid is passed via a cytoplasmic bridge between 2 cells)

Question 8

Can viruses survive outside of a host cell?

Answer 8

No

Question 9

In a virus, the nucleic acid (either DNA or RNA) is enclosed in a protein coat called a…

Answer 9

Nucleocapsid

Question 10

Size of bacteria vs size of viruses

Answer 10

TBC

Question 11

Outline the stages of bacterial growth and division - binary fission

Answer 11

1. DNA replication - one section of the bacterial is the origin of replication, and copying of DNA by replication enzymes begins here. The origin of replication opens and DNA replication begins.
2. Genome segregation - the cell elongates and the origins move towards opposite ends of the cell as the DNA is copied. The rest of the chromosome is pulled along with the origins. Replication continues until the entire chromosome is copied and the replication enzymes meet on the far side.
3. Septum formation - the septum, a new dividing wall, forms down the middle of the cell.
4. Division - the cell pinches in two.

Question 12

How many planes do the Gram positive Staphylococci, Streptococci, Enterococci and rods divide into?

Answer 12

Staphylococci - 3 planes
Streptococci and enterococci - 1 plane
Rods - 1 plane

Question 13

Outline spore formation

Answer 13

The bacteria breaks up into a number of pieces. These eventually form adult cells.
TBC

Question 14

TBC insert image of bacterial growth curve

Describe each stage of the bacterial growth curve

Answer 14

1. Lag phase - bacteria mature, synthesise RNA/enzymes/other molecules, and adapt themselves to their conditions. No growth in terms of number of bacteria.
2. Log/exponential phase - replication occurs so the population doubles at a constant rate, until nutrients are depleted and enriched with wastes.
3. Stationary phase - due to growth-limiting factor eg. depletion of essential nutrient, or bacteria produce some kind of inhibitor that kills off cells. Line is flat as rate of growth and death are equal.
4. Death phase - bacteria die due to lack of nutrients or toxin build up or adverse conditions.

Question 15

Outline the structure of Gram negative and Gram positive cell walls

Answer 15

Peptidoglycan (murein) forms a mesh-like layer and makes bacterial cell walls rigid. Target for many antibacterial drugs.

Gram positive - thick layer of peptidoglycan (90% of cell wall).

Gram negative - thin layer of peptidoglycan (10% of cell wall) and high lipid content

Question 16

Outline Gram stain procedure. How would Gram positive and Gram negative bacteria differ with staining?

Answer 16

1. Fixation with heat (or methanol) on slide
2. Add crystal violet, then wash slide.
3. Add iodine treatment, then wash slide.
4. Decolorise with acetone
5. Flood slide with counter stain (safarin).

Gram + holds on to pigment - blue/black
Gram - does not hold on to stain - red/pink

Question 17

What is the difference between commensalism, pathogenic and opportunistic organisms?

Answer 17

Commensals: organisms that are a normal inhabitant of the human body. Do not cause disease under normal circumstances.

Pathogens: organisms that are generally absent from the human microbiota in healthy individuals but cause disease when exposed to them.

Opportunistic pathogens: organisms widely distributed in healthy adults that can become invasive when the host has a weakened immune system

Question 18

Define microbiome

Answer 18

TBC slides or IRP

Question 19

Define microbiota

Answer 19

TBC - slides or IRP

Question 20

Why is peptidoglycan a useful antibiotic target?

Answer 20

It is only found in bacteria, so a drug that breaks it down will not harm human cells.

Question 21

Describe the features of staphylococcus aureus

Answer 21

Catalase positive (catalase breaks down hydrogen peroxide)
Coagulase positive (coagulase converts fibrin to fibrinogen)
Aureus means golden -> when grown on blood agar plates, forms golden yellow colonies

Question 22

What are the virulence factors of S. aureus?

Answer 22

TBC