42. Systems for Detection of Pathogens I Flashcards Preview

Clinical Pathology > 42. Systems for Detection of Pathogens I > Flashcards

Flashcards in 42. Systems for Detection of Pathogens I Deck (8)
Loading flashcards...
1
Q

How can we define a pathogen?

A

A microbe CAPABLE of causing a specific degree of host damage

2
Q

How do you get a good sample?

A
  • Sterile sites must be free from contamination - eg. Skin flora in blood cultures
  • Non sterile sites require decontamination of normal flora - eg Faeces, Mouth, Skin
  • Samples with high volume or relatively low infected pathogen load require concentration (centrifugation, filtering) - eg CSF, Ascites, 24 hr Urine
3
Q

What happens in the preparation phase and in the identification phase when there is a direct sample and a culture?

A

•PREPARATION PHASE

  • Culture = Enrichment Purification Amplification
  • Direct = Concentration Sample treatment
•IDENTIFICATION PHASE
- Culture = Molecular DNA/RNA  Gross morphology (Microscopy)
Chemical composition (HPLC MassSpec)
- Direct = Molecular DNA/RNA  Gross morphology (Microscopy)
Chemical composition (HPLC MassSpec)
4
Q

Advantages and disadvantages of microscopy

A

+ Easy to perform
+ Rapid screening
+ Some parasites have SPECIFIC morphology
+ Specific immunofluorescence staining possible

  • Not sensitive ~ e.g. TB
  • Screening sputum smears requires at least 10,000 orgs per ml to be visualised
  • General stains are not specific
  • Labour intensive (expensive)
  • Requires specialist interpretive expertise (more expensive)
5
Q

Classical culture and identification ~ bacteriology and examples of the different types of media

A
  • Bacteriology - This relies on the ability of the test system to be able to grow the pathogen
  • Environment - O2, CO2, ANO2,
  • Quantification, identification, antibiogram

•colony morphology, colour, haemolysis, colony count, colony identification, systematic identification, colony resistance to antibiotics

  • Non Selective Media - eg. Blood Agar
  • Semi Selective Media - eg. MacConkey Agar, DCA, CLED
  • Selective growth temperatures - eg. Campylobacter species
6
Q

What are classical metabolic testings?

A

•Catalase
E.coli = +ve
Clostridium perfringens –ve

•Can cleave indole from tryptophan (Indole test)
E.coli = +ve
Clostridium perfringens –ve

7
Q

What are classical culture identification - VIROLOGY?

A

1) Culture & microscopy
- Requires permissive cell lines e.g. vero cells (kidney epithelial) for measles (Morbillivirus)
- Cytopathic effect
- Immunofluorescent staining of culture

2) Direct antigen detection
- ELISA e.g. influenza virus

8
Q

Advantages and disadvantages of classical culture and identification.

A

+ cheap simple, reliable reagents
+ sensitive e.g. single organisms can be grown and identified
+ validated specificity e.g. gold standards
+ direct in vivo measurement of effectiveness of therapy e.g. antibiotic sensitivity
+ easily archived e.g. epidemiology

  • some pathogens cannot be grown
  • some p cannot be well differentiated by biochemistry alone
  • slow = culture requires at least overnight incubation ~ viral = 3-10days, mycobacterial = 6-12 weeks
  • some pathogens grow too slowly to aid rapid diagnosis e.g. TB
  • Labour intensive (expensive)
  • Requires specialist interpretive expertise (more expensive)

Decks in Clinical Pathology Class (62):