Infections I: Aetiology, Symptoms & Transmission of Infection Flashcards
Define: Infection
The colonisation of a host organism by a pathogenic organism
List the 6 stages of infectious disease
- Acquisition
- Colonisation
- Penetration
- Spread
- Damage
- Resolution
What is the acquisition stage known as if it is an exogenous infection?
Transmission
How can exogenous microbes be transmitted?
Direct contact e.g. STIs
Inhalation/droplet infection e.g. common cold
Ingestion/faecal-oral route e.g. gastroenteritis
Trauma/inoculation e.g. malaria
Trans-placenta e.g. congenital toxoplasmosis
List 3 factors that the success of colonisation depends on
- pH
- Temperature
- Competing endogenous organisms
List 2 mechanisms that some organisms have evolved to help colonisation
- Mucolytic enzymes = penetrate mucous layers
2. Adhesins = help stick to colonisation site
Define: Biofilms
Complex colonies of bacteria
Cells stick together and adhere to a surface (slimy protective outer layer)
More difficult to treat than isolated bacteria
What must an organism do in order to invade/penetrate?
Breach the surface barrier
e.g. skin, mucous membrane
What in the upper respiratory tract protect against particulate exposure?
Muco-ciliary clearance
Cough reflex
What defends the GI tract from penetration
Stomach acid
However organisms can damage the mucosal surface by releasing toxins or getting taken into cells
List the 3 ways an invading organism can spread
Along tissue planes
By extension of colony into surrounding area
Via vasculature/lymph (= effective rapid spread)
What are the 3 lines of defence against an invading organism?
1st = intact skin and mucous membranes 2nd = phagocytosis 3rd = inflammation
How do intact skin and mucous membranes act as defences against invading organisms?
Mucous traps bugs
Sweat (pH)
Secretions e.g. stomach acid, lysosomes in tears
How does phagocytosis act as a defence against invading organisms?
Phagocytic cells in blood (monocytes, neutrophils), tissues and lungs (macrophages) engulf and destroy micro-organism
How does inflammation act as a defence against invading organisms?
As well as phagocytic cell increase as local site, body tries to limit spead by e.g. forming clot
How is damage caused by an invading organism?
Mechanical damage from bulk effects e.g. biofilm obstructing a hollow organ
Toxins
Altered function of host cell
Host response
How do toxins from an invading micro-organism cause damage?
Exotoxins = produced inside bacteria and diffuse out, very toxic Endotoxins = LPS-protein complexes, mainly in cell wall, G-ve bacteria, less toxic
How does the altered function of a host cell cause damage?
Can affect functioning of cell, tissue or organ
Can be as the body tries to combat infection e.g. c.dif causes diarrhoea
How does host response cause damage?
Usually inflammatory then immune response
Can get swelling, pus, scarring, necrosis
Describe the ‘vicious cycle’ of inflammation
- Tissue infected by bacteria and toxins
- Inflammation of tissue
- Immune system chemicals attack tissue to contain infection
- More tissue damage as collateral effect
- More immune system chemicals released
6 More inflammation
etc etc
What is the resolution stage of infection?
Subsiding of pathologic (disease) state = reduction of inflammation
Define: Pathogen
An organism capable of causing disease
Define: Virulence
Degree of pathogenicity of an organism = ability to cause disease
Define: Attenuation
A reduction in the virulence of a pathogen
avirulent if gone
Define: Exaltation
An increase in virulence
What is microbial virulence controlled by?
Controlled genetically
e.g. genes encoding toxins
In what ways do the expression of virulence factors respond to the environment?
Switching virulence on/off allows bacteria to avoid mechanical, non-specific and immune defences
Some species can also form spores
Define: Chemotherapy
The use of synthetic chemicals to destroy infective agents
Define: Chemotherapeutic agents
Chemicals intended to be toxic for the pathogenic organisms but safe for the host
Define: Microbe
General term used to describe bacteria, viruses and fungi
Define: Parasite
General term used to describe protozoa and helminths
What does the selective toxicity of chemotherapy for bacteria depend on?
It depends on exploiting the biochemical differences between pathogen and host
What are the targets of chemotherapy for bacteria?
3 general classes of biochemical reaction (I, II, III)
Describe the 3 classes of biochemical reaction which are the targets of chemotherapy for bacteria
Class I = reactions which utilise glucose & other carbon sources to produce ATP and simple carbon compounds Class II = pathways utilising energy and class I compounds to make small molecules (e.g. amino acids, nucleotides) Class III = pathways that convert small molecules into macromolecules (e.g. proteins, nucleic acids, peptidoglycan)
Why are Class II reactions better targets for chemotherapy then Class I reactions?
Class II:
Folate synthesis in bacteria is inhibited by sulphonamides
Folate utilisation is inhibited by folate antagonists e.g. trimethoprim
Why are Class III reactions important targets for chemotherapy?
Bacterial protein synthesis selectively inhibited by antibiotics that:
Prevent the binding of tRNA = tetracyclines
Promote misreading of mRNA = aminoglycosides
Inhibit transpeptidation = chloramphenicol
Inhibit translocation of tRNA = erythromycin
