Infections Lecture 1: Aetiology, symptoms and transmission of infection

Question 1

What is an infection?

Answer 1

Colonisation of a host organism by pathogenic organism

Question 2

What is are the branch of medicines that are focused on fighting diseases called?

Answer 2

Infectious disease mechanism

Question 3

What are the six stages of infectious disease?

Answer 3

1. Acquisition
2. Colonisation
3. Penetration
4. Spread
5. Damage
6. Resolution

Question 4

Describe the acquisition step and how infections are initially contracted?

Answer 4

1. They can be acquired from endogenous micro-organisms in or on the body (bacteria, fungi, viruses etc)
2. Contact with exogenous (transmission) microbes

Question 5

What are the 5 ways to contact exogenous microbes?

Answer 5

1. Direct contact- e.g. STIs
2. Inhalation/droplet infection- e.g. cold
3. Ingestion/faecal route- e.g. gastroenteritis
4. Trauma/inoculation e.g. tetanus, malaria, infected topic dermatitis
5. Trans-placenta e.g. congenital toxoplasmosis

Question 6

Describe the general colonisation step?

Answer 6

1. Organism must survive and multiply in the environment
   - dependent on pH, temperature, competing endogenous organisms
2. Some are born with mechanisms to help colonisation
   - Mucolytic enzymes to penetrate mucous layers
   - Influenza virus adheres to glycoprotein receptors on upper respiratory tract mucosal cells

Question 7

What do bacteria seldom lone cells usually form? And what do they do?

Answer 7

1. Usually form complex colonies called “Biofilms”
2. Acts as a slimy protective outer layer- other organisms attach to the biofilm
3. More difficult to treat than isolated bacteria

Question 8

Describe the purpose of the general penetration step?

Answer 8

To invade the organism must breach the surface barrier such as skin and mucous membrane

Question 9

How does the upper respiratory tract deal with foreign particle exposure?

Answer 9

Muco-cillary clearance and cough reflex protects against particulate exposure

Question 10

Where are the three main areas where cells can penetrate?

Answer 10

1. GI tract
   - organisms can damage the surface by releasing toxins
2. Crossing the placenta
   - Rubella, syphillis
3. Cell penetration
   - Viruses enter some bacteria and enter host cells via penetration

Question 11

What are the three main ways infections can spread?

Answer 11

1. Along tissue planes
2. Extension of colony into surrounding area
3. Via vasculature/ lymph (effective rapid spread)

Question 12

What is the first line of defence against an infection spreading?

Answer 12

Non specific
1. Intact skin and mucous membranes

Body defence

1. Sweat (pH)
2. Mucous traps bugs
3. Anti-bacterial secretions e.g. stomach acid
4. Lysozyme in tears

Question 13

What is the second line of defence against an infection spreading?

Answer 13

Non specific
1. Phagocytosis

Body defence:
1. Phagocytic cells in blood (monocytes, neutrophils)

2. Tissues and lungs (macrophages) engulf and destroy micro-organisms

Question 14

What is the third line of defence against an infection spreading?

Answer 14

Non specific:
1. Inflammation

Body defence:
1. Alongside phagocytic cells they increase at local site and body tries to limit the spread of the organism by forming clot

Question 15

Describe the mechanical damage infections can do?

Answer 15

Mechanical damage from bulk effects- bacteria and biofilm can obstruct a hollow organ- cystic fibrosis

Question 16

Describe the toxin damages infections can do?

Answer 16

• Exotoxins produced inside a bacteria (G+ve and -ve) and diffuse out
• Clostridium tetani neurotoxin- tetanus- very toxic
• Endotoxins are LPS protein based complexes mainly in cell wall G-ve bacteria

Question 17

Describe the damage the altered function of host cell can do?

Answer 17

• Can affect how the cell functions overall in the tissue or organ (common with viruses)
• Can occur as the body tries to combat infection

Question 18

Describe the damage the host response can have?

Answer 18

1. The host damages itself through response to the infection

2. This is usually an inflammatory response which includes swelling, pus, scarring and necrosis

Question 19

What is a pathogen?

Answer 19

Ability of an organism to cause a disease

Question 20

What is virulence

Answer 20

Degree of pathogenicity of an organism (ability to cause disease)

Question 21

What is Attenuation?

Answer 21

Reduction in the virulence (ability to cause disease) of a pathogen

Question 22

What is Exaltation?

Answer 22

Increase in virulence (ability to cause disease)

Question 23

Describe the what resolution is in terms of infections? And give examples with HIV and herpes?

Answer 23

1. Organism is gone and everything is back to normal
2. HIV- you can have a long time with no symptoms, still in the body until it gets worse and immune system cannot take it and it shows
3. Herpes- they can flare up and become blisters again

Question 24

Describe what chemotherapy means?

Answer 24

The use of synthetic chemical to destroy infective agents

Question 25

Describe the three classes of biochemical reaction that are potential targets for chemotherapy?

Answer 25

Class 1: (poor targets)
- Utilises glucose and other carbon sources to produce ATP and other carbon compounds

Class 2: 
- Pathway uses energy and class I compounds to make small molecules (e.g. amino acids and nucleotides)

Class 3:
- Pathways convert small molecules into macromolecules like proteins, nucleic acids and peptidoglycan

Question 26

Give examples of how class 2 reactions are used as targets?

Answer 26

1. Folate synthesis is inhibited by sulfonamides
2. The utilisation of the folate is inhibited by folate antagonists- e.g. trimethoprim (bacteria) and methotrexate (cancer cells)
3. Pyrimidine and purine analogues (e.g. mercaptopurine) for cancer

Question 27

Give examples of how class 3 reactions are used as targets?

Answer 27

1. Beta lactam antibiotics can selectively inhibit bacterial peptidoglycan
2. Antibiotics can selectively be inhibit the bacterial protein synthesis stage
   - Prevention of tRNA binding (tetracyclines)
   - Misreading of mRNA (aminoglycosides)
   - Inhibit transpeptidation (chloramphenicol)
3. Nucleic acid synthesis is impaired by altering base pairing of DNA template (antiviral vidarabine)
   - inhibits the DNA polymerase (aciclovir)
   - inhibiting DNA gyrase