Infectious agents

Question 1

List some differences between prokaryotic and eukaryotic organisms.

Answer 1

1. EUKARYOTIC: - contains a chromosome - has a nucleus.
2. PROKARYOTIC: - no nucleus - no mitochondria - mainly circular DNA (plasmids).

Question 2

Describe the structure of a virus.

Answer 2

• Most are 20nm to 30nm in diameter.
• They can have an RNA or DNA genome.
• The genome can be linear, segmented, single or double stranded witha size of around 9Kb - 300Kb.
• It has a protein nucleocapsid (individual capsomeres in a helical, cubic or complex arrangement).
• No cytoplasm - it may be enveloped or non-enveloped (may have components derived from the host cell).
• It is an obligate intracellular organism (meaning it can only replicate inside host cells).
• They have receptors on the nucleocapsid that allow it to bind to cells.

Capsid meaning - The capsid is the protein shell surrounding the genome.

Question 3

Describe the structure of bacteria.

Answer 3

1. They have a diameter of 0.5um to 3um.
2. It is prokaryotic.
3. Contains haploid DNA as a circular genome.
4. It has no nucleus.
5. It usually has a rigid cell wall outside the cytoplasmic membrane (which retains the osmotic potential).
6. It has no mitochondria.
7. It divides by binary fission.

Question 4

How does the Gram stain help us identify different bacteria?

Answer 4

The Gram stain divides bacteria into :-

1. GRAM-POSITIVE - blue/purple.
2. GRAM-NEGATIVE - red.

These bacteria have differing cell wall structures which accounts for the different staining properties.

Question 5

Describe becterial growth and list some of its requirments.

Answer 5

• Bacterial growth occurs through binary fission.
• Their doubling time is approximately 20-30 minutes.
• Their growth requires :-
  
  • carbon, water and nitrogen.
  • nutrients and minerals
  • the correct pH
  • the correct osmotic pressure
  • the correct temperature
  • the correct atmosphere

Question 6

Describe the structure of fungi (mycoses).

Answer 6

• They have a diamter of over 2um.
• They are eukaryotic.
• They have a haploid or diploid DNA and have single or multiple nuclei.
• They have a rigid CHITINOUS cell wall outside. the cytoplasm.
• They can be single or multicellular.
• They come in two forms:
  
  • YEAST: spheroid single cells budding and dividing.
  • FILAMENTOUS FORM/ MOULDS: multinucleate branching hyphae forming mycelium.

Mycelium meaning - Mycelium is the vegetative part of a fungus or fungus-like bacterial colony, consisting of a mass of branching, thread-like hyphae. (The white fibre bit that appears on expired bread)

Question 7

What is the simplest way to classify fungi?

Answer 7

• The classification of fungi is complex, and is based on morphology (growth form), mode of reproduction (sexual or asexual) and the disease caused.
• The easiest way to look at different fungi is to consider the type of infection they cause.
  
  • SUPERFICIAL mycoses: affect the skin, hair, nails and mucous membranes.
  • CUTANEOUS mycoses: affect the deep skin.
  • SYSTEMIC mycoses: affect the body’s systems.

Question 8

Describe the structure of parasites.

Answer 8

• They have a diameter of around 5 - 300um.
• They are single cells.
• They have single or multiple nuclei.
• They have haploid DNA.
• Their morphology varies throughout the life cycle.
• They may have flagella.
• Trophozoites, cysts and other morphotypes are parts of their complex life cycles.

Parasite meaning - A microbe that lives in or on another animal host, causing it harm.

Question 9

Describe helminths (a type of parasite).

Answer 9

• Helminths are multicellular parasitic worms.
• They have a different morphology and sizes at different life cycle stages.
• They can either be microscopic or visible to the eye.
• They are categorised into 3 groups :-
  
  • CESTODES are tapeworms.
  • TREMATODES are flatworms or flukes.
  • NEMATODES are roundworms.

Question 10

List some examples of viruses.

Answer 10

1. Herpes Virus
2. HIV
3. Ebola Virus
4. Flu Virus

Question 11

List some examples of bacteria.

Answer 11

1. Campylobacter jejuni - causes food poisoning.
2. Clostridium perfringens - Gas Gangrene.

Question 12

List some examples of fungi.

Answer 12

1. Tinea pedis - athlete’s foot

Question 13

List some examples of parasites.

Answer 13

1. Plasmodium falciparum - malaria causing
2. Entamoeba histolytica
3. Trichomonas vaginalis - common STI

Question 14

How are microbes visualised?

Answer 14

• Bacteria, fungi, protozoa and certain lifecycle stages of helminths can be visualised using light microscopy.
• Viruses at too small to be seen using light microscopy but can be visualised with an electron microscope after special staining.
• This allows detailed morphologic examination of the viruses.