Pathogens And Hosts Flashcards
What is the difference between a pathogen and a commensal?
A pathogen is an organism which can cause disease whereas a commensal is an organism which is part of the normal flora.
What is Koch’s postulates on a pathogen causing the disease?
The organism must be found in all cases of the disease.
It must be able to be cultured outside the body for several generations.
It should reproduce the disease on inoculation.
What is pathogenicity and what are two requirements for it?
The capacity of a microorganism to cause an infection.
Infectivity is the ability to become established in the host.
Virulence is the ability to cause harmful effects once established.
Name four virulence factors
Genetically determined microbial components
Invasiveness
Toxin production
Evasion of immune system
What are the three types of toxins and what do they do?
Exotoxins are released extracellularly by the organism.
Enterotoxins are exotoxins which act on the GI tract.
Endotoxins are structurally a part of the gram negative wall.
Give an example of an exotoxin
Clostridium tetani (gram positive) infects dirty wounds and produces toxins that bind to nerve synapses and inhibit the release of inhibitory neurotransmitters so death is caused by respiratory paralysis. Treated by debridement, antitoxins and antibiotics.
Give an example of an enterotoxin
Vibrio cholerae colonises the small intestine and the toxins increase cAMP which inhibits the uptake of sodium and chlorine ions and secretes more chlorine and carbonate ions so there is a massive outflow of water. Death is caused by dehydration.
What are superantigens and what do they do?
They are certain exotoxins of strep pyogenes and staph aureus. They’re able to stimulate division of T cells in the absence of the specific antigen. The overwhelming cytokine production causes toxic shock.
What is the structure of endotoxins and what do they do?
They’re made up of a polysaccharide antigen, oligosaccharide core and a Lipid A which is what causes the damage.
They induce a severe uncontrolled host response.
What are three virus pathogenic mechanisms?
Cell destruction after virus infection such as HIV killing T4 helper cells.
Virus induced changes to cellular gene expression such as cellular transformation by rumour viruses.
Immunopathogenic disease such as influenza A virus.
Name four types of viruses in terms of time periods.
Acute infections start and finish early such as influenza.
Latent infections are acute early then lie dormant before getting a secondary infection such as herpes.
Chronic infections are acute early then are always present at low levels such as Hepatitis B.
Tumour virus infections have a small response early but will have a large latent response.
What is viraemia and how does it differ from acute infection?
It means the virus is in the bloodstream whereas acute infection is localised to a specific site of the body.
What are antigenic shift and drift?
Shift is abrupt major changes in virus antigenic structures. Non human hosts can help generate new virus types through antigenic shift.
Drift is minor changes in the genes of flu viruses that occur over time to generate antigenic variants.
Name some enteroviruses and what they cause
Poliovirus causes poliomyelitis.
Coxsackie B viruses causes myocarditis and pancreatitis.
Many enteroviruses cause aseptic meningitis and respiratory infections.
How does the latent infection of herpes work?
There is a primary infection of the epithelium then the virus migrates to the ganglia where it lies latent in the nucleus and doesn’t replicate. A stimuli reactivates the virus and it migrates back to epithelial cells to replicate.
