Introduction to Medical Microbiology

Question 1

List the categories of microorganism that cause infection.

Answer 1

Bacteria
Viruses
Parasites
Fungi
Prions - unclassified

Question 2

Define a prion.

Answer 2

A prion is a protein of unknown function on the surface of brain cells.
An abnormal form of prion protein that in mammals, induces pathogenic forms that arise spontaneously or through transmission and upon accumulation in the brain cause a prion disease.

Question 3

List the common specimen types collected for culture.

Answer 3

Mid-stream specimen of urine (MSSU) - UTI
Sputum - lower respiratory tract infection.
Throat swab - tonsilitis/pharyngitis
Swabs - wound infection, genital tract infection
Faeces - infectious diarrhoea
Blood culture - septicaemia (sepsis)
Cerebrospinal fluid (CSF) - meningitis.
Aspirate of pus - abscess.
Bone - osteomyelitis

Question 4

Explain the role of microscopy in the diagnosis of bacterial infection.

Answer 4

Allows for interpretation of results:
- Knowledge of common pathogens.
- Knowledge of commensal flora.
(Some bacteria (e.g. S.aureus) are both a commensal and pathogen

• Final report with interpretive comments and results.

Question 5

Name some examples of methods of microscopy.

Answer 5

Light microscopy (x1000 magnification)
Light microscopy unstained - to see WBCs and parasites.
Gram stains - to visualise bacteria and yeasts/fungi
Special stains - Ziehl Neelson stain or auramine stain for mycobacteria.

Question 6

Describe microscopy and culture together in the diagnosis of bacterial infection.

Answer 6

Microscopy is rapid, means that information can be urgently relayed to clinicians.
Microscopy is not to specific but can give a provisional diagnosis and empirical microbial therapy.
This is then followed by culture and susceptibility testing.

Question 7

Explain the difference between sterile and non-sterile sites of the body.

Answer 7

Sterile sites are areas where microorganisms are not typically found (e.g. brain or heart).
These are often sites deeper within the body, that are more protected from outside infection.
Non-sterile sites (e.g. lungs, stomach, oesophagus etc)

Question 8

Outline the methods of detecting viruses.

Answer 8

Viruses can be detected by;
- molecular methods (real time, multiplex PCR)
- Antigen detection
- Serology to detect immunity
- Virtually obsolete methods e.g. electron microscopy, cell or tissue culture (inoculate a cell line).