Systems For Detection Of Pathogenesis 1

Question 1

What is a commensal non pathogen?

Answer 1

Present but NOT CAPABLE of causing disease in host

Question 2

What is a zoonotic non pathogen?

Answer 2

Present but only CAPABLE of causing disease in another host

Question 3

What is a commensal opportunist pathogen?

Answer 3

Present and capable of causing disease in the host but only in certain circumstances

Question 4

What is a pathogen?

Answer 4

A microbe capable of causing a specific degree of host damage

Question 5

How can a non contaminated sample be obtained?

Answer 5

Sterile sites must be free from contamination
- skin flora in blood cultures

Non sterile sites require decontamination of normal flora
- faeces, mouth, skin

Samples with higher volume or relatively low infected pathogen load require concentration

Question 6

What are the advantages of using microscopy to identify pathogens?

Answer 6

Easy to perform

Rapid screening

Some parasites have specific morphology

Specific immunofluorescence staining possible

Question 7

What are the disadvantages of using microscopy to identify pathogens?

Answer 7

Not sensation

Screening sputum smears requires at least 10-000 orgs per ml to be visualised

General stains are not specific

Labour intensive

Requires specialist interpretive expertise

Question 8

What are the 3 types of media used in cell culture?

Answer 8

Non selective media

Semi selective media

Selective growth temperatures

Question 9

What is the selective atmosphere for a cell culture?

Answer 9

Aerobic culture
- S.aureus
- catalase positive
- E.coli

Microaerophillic culture
- Respiratory pathogens - grow faster with CO2 present (lung conditions)

Anaerobic culture
- no oxygen present
- gas gangrene
- spore forming

Question 10

How do u test for the pathogen causing food poisoning?

Answer 10

Take a stool sample

• MacConkey agar and deoxycholate afar plate
• direct microscopy for cyst/eggs of parasites
• spread on campy CVA selective media plate

Question 11

What are the ways to identify viruses?

Answer 11

1. Culture and microscopy
   - requires permissive cell lines
   - cytopathic effect
2. Direct antigen detection
   - ELISA

Question 12

What are the advantages of classic cell culture and identification?

Answer 12

Cheap

Simple

Reliable reagents

Sensitive

Validated specificity

Easily achieved

Direct in vivo measurement of effectiveness of therapy

Question 13

What are the disadvantages of classical cell culture and identification?

Answer 13

Some pathogens cannot be grown

Some pathogens cannot be well differentiated by biochemistry alone

Some pathogens grow too slowly to aid rapid diagnosis

Labour intensive (expensive)

Requires specialist interpretive expertise (more expensive)