L2 - Laboratory Diagnosis of Infection Flashcards
What are the principles of detection?
- detection of whole organism
- detection of component of organism
How can you detect a whole organism?
Microscopy
e.g. CFS, meningitis
How can you detect a component of an organism?
Antigen
e.g. lateral flow, 96-well
nucleic acid
e.g. probes - unknown & known
What are the uses of direct detection?
establishes presence at particular site
epidemiological info
appropriate therapy
fast
What are the limitations of direct detection?
no information on:
- antimicrobial susceptibility
- typing
What does isolation of the pathogen via culturing enable?
identification
typing (organism relatedness)
susceptibility testing
what are requirements for culturing?
culture medium
incubation
What are the types of culture media
non-selective media
selective media
indicator media
Examples of non-selective media?
blood agar
chocolate agar
nutrient agar
What is selective media?
antibiotics
chemicals (NaCl, Bile salts)
special conditions
What is indicator media?
Chromogenic agar - more than 1 species
MacConkey - lactose fermenters
What are further tests you can carry out on bacteria after growing on media?
Gram stain
Biochemical tests
detection of specific components
Which haemolysis is stronger, alpha or beta?
Beta
What is a positive result for an oxidase (TMPPD) test?
purple
What actually is the oxidase test?
ability to transpire using the ETC
if last cytoplasms is cytoplasms oxidase = purple chemical colour
What is the biochemical test which assesses multiple wells of samples?
API E strip
What is Maldi-tof?
compares to database of known spectra
produces spectrum of bacterial proteins
fast & cheap
What are uses of culturing bacteria?
establishes presence
Allows use of targeted therapy
epidemiolgical information
typing information
What are limitations of culturing bacteria?
slower than direct detection
What is susceptibility testing?
viable microorganisms
culture in presence of antimicrobial agent
see if conc. is high enough to kill microorganism
How is susceptibility testing carried out?
antibiotic discs
zone of clearing/inhibition
What are methods of susceptibility testing?
measure zones of inhibition
commercial e-tests ( gives MIC )
breakpoint plates
What are uses of susceptibility testing?
inform decisions on targeted antimicrobial therapy
What are limitations of susceptibility testing?
correlation between antimicrobial susceptibility in-vitro and clinical response - not absolute
What are immunological tests?
detection of immune response to infection:
antibody detection
fourfold rise in titre
What are the uses of antibody detection?
confirms exposure
epidemiological info
What is the MAIN limitation of antibody detections?
It’s retrospective
