Infectious Agents

Question 1

What are the distinctive biological characteristics?

Answer 1

All organisms are cells apart from viruses
Membranes 
Cellular and genome organisation 
Single cells
Multiple stages- life cycles
Multicellular- parasites
Prokaryotic: 
No nucleus
No mitochondria
Mainly circular DNA e.g. bacteria 
Eukaryotic:
Chromosome 
Nucleus

Question 2

What are viruses like?

Answer 2

Most are 20nm to 300nm
DNA or RNA genome
Linear, segmented, single or double-stranded genome size 9Kb-300Kb
Protein nucleocapsid- individual capsomeres I cubic or complex arrangements
No cytoplasm
Enveloped or non-enveloped- may have components derived from host cell
Obligate intracellular organism - can only grow inside eukaryotic cells

Question 3

What are some common viral infections in the UK?

Answer 3

Community
Upper respiratory tract infections- rhinovirus, influenza, RSV
Gastroenteritis e.g. Norovirus (SRSV), adenovirus, rotavirus
Rashes e.g. varicella-zoster virus (chickenpox), measles
Meningitis e.g. enterovirus
Hepatitis virus e.g. Hep B
Human Papillomavirus (HPV)- genital warts

Question 4

What are fungi like?

Answer 4

Fungi- Mycoses
Size: over 2um
Structure: Eukaryotic
Haploid or diploid DNA
Single or multiple nuclei
Rigid chitinous cell wall outside the cytoplasm
Single or multicellular
E.g:
Yeast (spheroid single cells budding and division)
Or
Filamentous form/moulds- hyphae (e.g. Aspergillus spp) multinucleate branching hyphae forming mycelium
Classification is complex and is based on morphology (growth form), mode of reproduction (sexual or asexual) and disease caused: 70,000 species ~300 human pathogens
Easier to consider by type of infection
Superficial mycoses:
Skin, nail and mucous membrane (dermatophytes) or thrush (Candida albicans)
Cutaneous:
E.g. Tanea- aka ringworm
Systemic mycoses: e.g. Histoplasmosis

Question 5

What are some opportunistic of AIDS?

Answer 5

Cryptococcus neoformans- Fungal yeast-like form- encapsulated- large capsule
Pneumocystis carinii (now jiroveci)
Previously classified as a protozoan
PCP in AIDS
Single cells
Forms cysts containing spores

Question 6

What are parasites like?

Answer 6

Eukaryotic organisms- Protozoa and Helminths
Term parasite- sometimes applied to any microbe that lives in or on another animal host, causing it harm
Protozoa:
Single cells 5 to 300um
Single or multiple nuclei
Haploid DNA
Morphology varies throughout life cycle
May have flagella
Trophozoites, cysts and other morphotypes

Question 7

What are helminths?

Answer 7

These are multicellular parasitic worms
Different morphology and sizes at different life cycle stages
They can be microscopic or visible to the eye
Multicellular
3 groups:
Cestodes are tapeworms e.g. Taenia saginata, the beef tapeworm
Trematodes are flatworms or flukes and include Schistosoma haematobium
The cause of bilharzia or schistosomiasis filarial worm Onchocerca volvulus- River Blindness
Nematodes which are roundworms e.g. Ascaris lumbricoides