Lab diagnosis of infection Flashcards
What are the four main approaches to diagnosing bacterial infections?
Microscopy (visualisation)
Culture
Detection of organism-related antigen, toxin or nucleic acid
Serology
What colour(s) do bacteria turn on a gram stain?
Gram-positive: purple
Gram-negative: pink
Which bacteria do not show up on gram-stain?
mycobacteria legionella chlamydia spirochaetes - trep pallidum - borrelia burgdorferi - leptospira
When is a Ziehl-Neelsen (ZN) stain used?
To detect acid and alcohol-fast bacilli (AAFB) such as mycobacteria species
What chemical is used in a Ziehl-Neelsen (ZN) stain?
Auramine phenol
What conditions are usually used to incubate bacterial cultures?
5% CO2
37 degrees celsius
(at least one plate should be incubated anaerobically)
For what period of time are bacterial culture plates usually incubated for? Give one exception
24 - 48 hours
An exception is Mycobacterium tuberculosis which takes several weeks
What test can be used to identify staph aureus?
Coagulase test - staph aureus is coagulase-positive
What test is used to differentiate between different species of streptococcus?
Ability to haemolyse blood agar
What is Lancefield Grouping?
A way of classifying some streptococcus species based on their surface antigens
- strep pyogenes is Group A strep
How can gram-negative bacteria be further classified?
Fermentation of lactose
Give two lab tests used to detect bacterial antigens/toxins, and give examples of bacteria which can be diagnosed by these tests
Enzyme immunoassay (EIA)
- C. dificile
- Legionella antigen in urine
Latex agglutination (LA)
- pneumococcus (from blood or CSF)
- meningococcus (from CSF)
- H. influenza (from CSF)
Which lab test can be used to detect bacterial nucleic acids? Give an example of an organism that can be diagnosed in this way
Polymerase chain reaction (PCR)
- chlamydia trachomatis
What type of bacteria can be identified by serology
Mainly atypical pneumonia - chlamydia psittacci - coxiella burnetti - mycoplasma pneumoniae also spirocheate infections - borrelia burgdorferi - treponema pallidum - leptospirosis
Give three approaches for diagnosing viral infections
Antigen detection
Nucleic acid detection
Serology
Which viruses are diagnosed by antigen detection? Which methods are used?
Hep B (EIA)
Resp viruses e.g. influenza A/B, RSV (immunofluoresence)
Rotavirus in faeces (EIA or LA)
When is nucleic acid detection most useful in viral infections?
To monitor viral load in HIV, CMV and EBV infections
When is serology most useful in viral infections?
for detecting evidence of previous viral infections, and for confirming successful immunisation
How are fungal infections usually diagnosed?
microscopy and culture
How are parasite infections usually diagnosed?
microscopy
How is malaria diagnosed?
Blood film microscopy (both thick and thin blood films are required)
