Introduction to Infection

Question 1

Define symbiotic relationship

Answer 1

Close and often long term relationship between 2 species. Can be:

• Mutualistic
• Commensal
• Parasitic

Question 2

Define commensal relationship

Give an example

Answer 2

Commensal = symbiotic relationship between 2 different species in which one benefits and the other is unaffected.

E.g. gut microbiota

Question 3

Define mutualistic relationship

Give an example

Answer 3

Mutualistic = symbiotic relationship between 2 species in which both parties benefit

e.g. gut microbiota

Question 4

Define parasitic relationship

Answer 4

Symbiotic relationship between 2 species in which one benefits and the other is harmed

Question 5

Define colonisation

Answer 5

Where a microbe grows in/on an organism without causing disease.

Question 6

Define infection

Answer 6

Invasion and multiplication of microbes in an area where they are not normally present, usually causing disease (can be symptomatic or asymptomatic)

Question 7

What relationship do gut microbiota have with the host?

Answer 7

Commensal or mutualistic (e.g. provide protection from pathogenic bacteria such as C-Diff

Question 8

What is a common side effect of antibiotics on gut microbiota?

Answer 8

Gut microbiota can be eliminated by antibiotics allowing the invasion of pathogenic bacteria. This can lead to C-Diff infections and C-Diff associated diarrhoea

Question 9

What factors can increase risk of infection

Answer 9

• Extremes of age (immature or weakened immune system)
• Physiological stress
• Starvation
• Compromised barriers to infection:
  
  • Physical: e.g. burns
  • Biochemical: e.g. stomach acid
• Immunocompromised host:
  
  • Primary immunodeficiency (from birth)
  • Secondary immunodeficiency (acquired after birth, e.g. AIDS)
  • Immunosuppression (iatrogenic from immunosuppressive treatments)

Question 10

How do the majority of pathogens enter the body?

Answer 10

Nasopharynx

Respiratory tract

Urinary tract

Genital tract

Question 11

What does bacterial pathogenesis require?

Apply these to the example of a UTI

Answer 11

• Access: usually from colon
• Adhesion: pili and adhesion molecules
• Invasion: haemolysin increases invasion potential
• Multiplication: colonisation may precede infection
• Evasion (of immune system): few immune cells in urinary tract.
• Resistance: Many UTI causing bacteria have multiple drug resistances
• Damage to host cells (direct or indirect): urethritis, cystitis, pyelonephritis, nephritis, septicaemia
• Transmission to another host: easily passed out through urine (limited transmission risk)

Question 12

What are some risk factors for UTI?

Answer 12

Female (shorter urethra, closer to the GI tract)

Malformations (PKD, renal and ureteric malformations, strictures)

Internal obstructions (stones, tumours)

Outflow obstructions (pregnancy, prostate enlargement)

Iatrogenic (urethral catheterisation, operations etc)

Immunocompromise: e.g DM

Question 13

Name some investigations for UTIs

Answer 13

Urinalysis: dipstick for protein, blood, WBC, and nitrite

MSSU for MC&S

Bloods: FBC, CRP, U&E

BC

Imaging: USS, CT, urogram

Question 14

Name some investigations for infection

Answer 14

Inflammatory markers (CRP)

MC&S

Nucleic acid detection (PCR) for viruses and unusual pathogens

Antigen detection: usually for unusual pathogens, e.g. Malaria

Imaging