Diagnosing infection

Question 1

Why are replicated fragments the same size in PCR?

Answer 1

long strands can only be synthesised from the original strands; not from newly-formed strands, so they increase arithmetically, but short strands are synthesised
from newly-formed DNA and increase exponentially

Question 2

What is the principle of latex agglutination?

Answer 2

antigen binds antibody on latex molecule and causes latex to ‘clump’

Question 3

How does MALDI-TOF work?

Answer 3

Detects unique biomarkers or patterns by breaking up bacteria/proteins and analyse through detector by mass

Question 4

What is the principle of the Widal test?

Answer 4

Bacteria bind to antibodies and clump at the bottom = +ve result and a titre can be calculated suggesting exposure to a microorganism with that antigen (demonstrates a serological response)

Question 5

Explain the steps and examples of each to Make a Specific Aetiological Diagnosis of Infection?

Answer 5

1. Demonstrate organism or component
   -Microscopy eg. Phase contrast, darkground, Gram stain, Ziehl-Neelsen stain etc.
   - Detecting specific components:
   Antigen detection (Latex agglutination, Solid phase/ capture assay, Immunohistochemistry)
   Nucleic Acid Detection: Hybridisation, PCR
   Detecting Other Components: MALDI-TOF

need confirmatory tests e.g. DNA/RNA analysis, serotype, antimicrobial susceptibility

2. Isolate the micro-organism
   - Timing of specimen: before treatment starts
   - Types of specimen: Sterile site eg. Blood, knee joint fluid, CSF, Site with normal microbiota - eg. throat, feces, skin
3. Demonstrate a serological response (look for an immune response to see if recent exposure) e.g. Widal test, solid phase assay