1.1 - Intro to Infection Flashcards
What is an infection
the invasion of a host’s body tissues by disease-causing agents, followed by multiplication of these agents and the subsequent reaction of these agents with the host’s defences
Disease caused by
- Microbial multiplication
- Toxins (in some cases)
- Host response
What are the 5 generic stages for how a microorganism could cause an infection within a human
exposure
- How the microbe comes into contact with the host
- This can be vertical or horizontal transmission (on sep card)
adherence
- The binding of a pathogen to a host cell
- This is done using an adhesion protein (ligand) on bacterial cell wall or viral capsule surface
- This then binds to a receptor on the host cell
invasion
- The mechanism of this is unique to the pathogen
multiplication
- In bacteria, this is done by binary fission (replication of main strand → replication of plasmids → move to opposite poles of the cell → division)
- In viruses, host cell’s multiplication machinery are taken over → produce further viruses → released from the cell to infect further cells
dissemination
- Involves spreading of the bacteria / viral load from one part of the body to another
- Ie through blood and lymphatics
Vertical and horizontal transmission
vertical
- Transmission of infection from mother → fetus/baby during pregnancy or childbirth
- Transmission may occur during pregnancy across the placenta
- May occur if baby comes into contact during delivery in the birth canal
- Could also occur postnatally via mother’s breast milk
horizontal
any form of transmission that is not from parent to child. Subdivided into three main categories:
☞ CONTACT: involving direct contact between source and host. Can be direct (physical contact between infected + susceptible persons. Most STIs are this way) and indirect (susceptible patient comes into contact with a vector or contaminated surface)
☞ INHALATION: involves entry of microorgansims through resp tract. Aerosols + droplet spread (more detail on separate card)
☞ INGESTION: involves entry of microorganisms through digestive system. Eg faeco-oral ingestion where food/water is contaminated by faeces
What is the difference between direct + indirect contact (transmission)
direct is where there is direct contact between source and host. Most STIs are spread this way
indirect is where susceptible patient comes into contact with a vector or contaminated surface/object. Must be an act of making physical contact with an object that an infected person has contaminated.
Difference between droplet and aerosol spread
droplets are larger particles. Coughed out etc. Short range (so don’t stay in air very long)
aerosols are much smaller particles. Remain suspended in air (and can drift) for a much longer period of time.
What are commensals
- Types of microbes that reside on surface of the body or the mucosal surfaces
- Normally harmless or even beneficial
- Transfer to other sites can be harmful and cause disease
- Ie bowel perforation can lead to periotonitis (commensals that populate large bowel can escape into the usually sterile peritoneal cavity)
What is virulence definition
The ability of a micro-organism to infect a host (at all steps of the process)
What are virulence factors
- chemicals that enhance the survival of the microorganisms and their reproduction inside the host
- These include toxins and haemolysins (proteins + lipids that cause lysis of erythrocytes by destroying their cell membrane) etc
- Toxins are subdivided into endotoxins + exotoxins (sep card)
- The virulence facts cause damage or harm to the patient, and cause the signs + symptoms of disease
Exotoxins + endotoxins
dont need to know??
endotoxins
- consist of the lipopolysaccharide part of the cell wall of gram negative bacteria
- Cause an inflammatory response
- If the response is severe and unregulated the patient may become septic/ develop other systemic symptoms
- Not immunogenic, so don’t produce an aquired immune response
exotoxins
- toxins that are secreted by bacteria
- can destroy body cells + affect normal cellular metabolism
what factors determines the severity of disease
pathogen
- virulence factors
- size
- antimicrobial resistance (doesn’t make pathogen more virulent, but infective process will carry on until there is an effective antibiotic used)
patient
- site of infection (ie the heart is more serious than skin infection)
- co-morbidities (is patient immunosuppressed etc?)
Infection history and clinical examination
history
- potential exposures including travel, contact with infected person, contact with animals
- relative time from exposure to presentation (suggests incubation time)
- vaccination history
- symptoms of the presenting complaint (ie severity, duration, rash, muscular problems, vomiting etc)
examination
- organ dysfunctions
- rashes
investigations for determining cause of infection
- full blood count
- C-reactive protein (acute phase protein that is a sign of systemic inflammation, produced by liver)
- LFTs (liver function tests)
- Urea + electrolytes (can suggest kidney issues)
- Imaging (x-ray, ultrasound and MRI)
- Blood cultures (to grow bacteria if it is present)
- Antigen detection (of micro-organism)
- Antibody detection (understand the patient’s response)
- Amplification of nucleic acids by PCR (to detect specific viruses or bacteria)
Specific vs supportive investigations
specific identifies causative organism ie bacteriology and virology
supportive helping to confirm diagnosis ie imaging and blood tests
Virology and bacteriology
virology
- Antigen detection (the virus)
- Antibody detection (the patient’s response)
- Detecting viral nucleic acid (RNA or DNA)
- Elisa testing can be used (on another slide)
bacteriology
- Growing the causative organism using the specimen
- Culturing using agar (nutrient source for bacteria)
- Using microscopy to try to identify
Basic principles of ELISA testing
- Enzyme-lined immunosorbent assay
- Bottom of tube coated with antibodies against the virus
- Tissue sample added – binds to antibodies
- Another antibody added that is complimentary, with an enzyme attached
- Substrate added that changes colour in the presence of the enzyme
- Washed between each stage to ensure no false positives
