Systems for Detection of Pathogens

Question 1

What are commensal non-pathogens, zoonotic non-pathogens and commensal opportunists?

Answer 1

• Commensal non-pathogen - present but not capable of causing disease in host.
• Zoonotic non-pathogen - present but only capable of causing disease in another host - e.g. E. coli subclinical in cattle but pathogenic to humans.
• Commensal opportunist - present and capable of causing disease in the host, but only in certai circumstances.

Question 2

Define a pathogen.

Answer 2

A microbe capable of causing a specific degree of host damage under certain circumstances.

Question 3

Describe the advantages and disadvantages of microscopy in pathogen detection.

Answer 3

Advantages:

• Easy to perform.
• Rapid screening.
• Some parasites have specific morphology.

Disadvantages:

• Not sensitive.
• General stains are not specific.
• Expensive - labour intensive, requires specialist interpretation.

Question 4

What conditions can we control in order to select for a specific microbe?

Answer 4

• Selective media
• Selective temperature
• Selective atmosphere
• Antibiotics - e.g. using ampicillin resistance gene

Question 5

Describe the advantages and disadvantages of classical culture.

Answer 5

Advantages:

• Cheap, simple, reliable reagents.
• Sensitive - single organisms grown and identified.
• Validated specificity.
• Direct in vivo measurement of effectiveness of therapy - e.g. antibiotic sensitivity.

Disadvantages:

• Some pathogens cannot be grown.
• Some pathogens cannot be well differentiated by biochemistry alone.
• Slow - culture requires at least overnight incubation.
• Some pathogens grow slowly to aid rapid diagnosis.
• Expensive - labour intensive and requires specialist interpretation.

Question 6

What are the advantages of using microarrays?

Answer 6

• Covers the whole genome.
• Can be used for RNA and transcriptomics.
• Can look for microRNA.

Question 7

Name 4 genetic techniques for detection of pathogens.

Answer 7

• Gene targeting
• Genome targeting
• Targeting biomarkers of virulence
• Rapid sequencing

Question 8

Describe the advantages and disadvantages of mass spectrometry (MALDI-TOF) in pathogen detection.

Answer 8

Advantages:

Disadvantages:

• Can only identify a known profile.

Question 9

Describe the advantages and disadvantages of using biomarkers of virulence to detect pathogens.

Answer 9

Advantages:

• Good specificity.
• Good sensitivity.
• Easily automated.

Disadvantages:

• Serological response not rapid - not useful in acute infections.
• Single sera results are meaningless due to possible previous exposure.
• Some antibodies are cross-reactive.
• Virulence is only inferred by the presence of a biomarker - not proven.

Question 10

Describe the advantages and disadvantages of using molecular detection methods to identify pathogens.

Answer 10

Advantages:

• Faster than traditional techniques.
• Allows for timely therapy and interventions.
• Increased sensitivity in positive samples.
• Can be automated and has potential for point of care testing.

Disadvantages:

• Expensive - labour intensive, requires expertise.
• Does not screen for unknowns.
• Possibility of contamination.
• Negative samples may still need culture.