S1: introduction to microbes & infection

Question 1

What is an infection?

Answer 1

Invasion of a host’s tissues by micro-organisms and disease is caused by either mocrobial multiplication, toxins or host response

Question 2

How do people get infections?

Answer 2

Microbiota = microrganisms carried on skin and mucosal surfaces, transfer to other sites can be harmful
Physical contact
Airborne spread
Vector
Ingestion of contaminated food or water
Inhalation of air contaminated by environmental organisms
Contact with contaminated surfaces

Question 3

What are the modes of horizontal transmission? What is vertical transmission?

Answer 3

Modes of horizontal transmission:
-contact: direct, indirect + vectors
-inhalation: droplets + aerosols 
-ingestion
Vertical transmission = mother to child, before or at birth

Question 4

What are the steps done by micro-organisms to cause disease?

Answer 4

Exposure
Adherence
Invasion
Multiplication
Dissemination

Question 5

What are virulence factors? What are the different types?

Answer 5

Virulence factors = various methods it can employ which depend on the individual microbe to cause the disease symptoms
1) exotoxins: cytolytic, AB toxins, superantigens + enzymes
2) endotoxins
All cause host cell damage either directly or as a result of the host’s immune response

Question 6

Name factors which determine the severity of a disease (disease determinants)

Answer 6

Pathogen: virulence factors, inoculum size + antimicrobial resistance
Patient: site of infection + co-morbidities

Question 7

Name supportive investigations

Answer 7

Full blood count: neutrophils (raised in bacterial infection), lympocytes (raised in viral infections)
CRP
Liver and kidney function tests
Imaging
Histopathology

Question 8

Name specific investigations for a bacterial infection

Answer 8

Swabs, fluids, tissues
Microscopy, culture + antibiotic susceptiblity
Antigen detection
Nucleic acid detection

Question 9

Name specific investigations for a viral infection

Answer 9

Antigen detection
Antibody detection
Detecting viral nucleic acid

Question 10

What four categories are micro-organisms broadly categorised in?

Answer 10

Viruses
Bacteria
Fungi (yeasts = single-celled, molds = multicellular)
Parasites (protozoa = single-celled, helminths = worms, multicellular)

Question 11

Describe the structure of bacteria and the difference between gram-positive and gram-negative bacteria

Answer 11

Bacteria can have three different shapes: coccus, spirillus + bacillus
Gram-positive bacteria appear purple (retain the crystal-violet stain due to thick layer of peptidoglycan)
Gram-negative bacteria appear pink (doesn’t retain the crytal-violet stain due to thin layer of peptidoglycan)

Question 12

Explain how to do a gram stain

Answer 12

1) Treat with a solution of crystal violet and then iodine (bacterial cells will stain purple)
2) Stained cells should then be stained with a solvent (eg. alcohol or acetone); gram-positive bacteria retain the stain whereas gram-negative species lose the stain becoming colourless
3) A counterstain, safranin, should be added which stains the clear, gram-negative bacteria pink