Pathogens and Host Flashcards
What is a pathogen
An organism which can cause disease
What is a commensal
An organism which is part of normal flora (e.g. E. Coli in gut, Staph aureus in nose)
What are the signs and symptoms of a clinical infection
Inflammation Pain Pyrexia Tachycardia Rigors Increased white cell count Increased C reactive protein (CRP)
What is pathogenicity
The capacity of a micro-organism to cause an infection
What is pathogenicity dependent on
Infectivity
Virulence
What is infectivity
The ability to become established on or within a host
What is Virulence
The ability to cause harmful effects (disease) once established
How can infectivity occur
Attachment (e.g. E. coli to P-fimbriae using a receptor on uroepithelial cells) Acid resistance (e.g. Helicobacter pylori using urease makes ammonia from urea)
What is virulence determined by
Virulence factors which are genetically determined microbial components such as:
Invasiveness
Toxin production
Evasion of immune system
Virulence factors are specific to the strain not the species
How are exotoxins released
Extracellularly by the microorganism
What are enterotoxins
Exotoxins which act on the GI tract
What structural role do endotoxins have
Part of the Gram negative cell wall
Give an example of an exotoxin and its mechanism of action
Tetanus
Clostridium tetani causes an infection in a dirty wound and produces toxins
The toxins bind to nerve synapses and inhibit the release of inhibitory neurotransmitters
It can cause death by respiratory paralysis
Treatment is by debridement, antibiotics and antitoxin
Give an example of an enterotoxin and its mechanism of action
Cholera
Vibrio cholerae colonises the small intestine and produces enterotoxins
Toxins:
Increase cAMP levels which inhibit the uptake of Na+ and Cl- ions and stimulates the secretion of Cl- and HCO3- ions
Cause a passive outflow of H2O.
Death is caused by dehydration
Treatment is rehydration
What is the structure of a endotoxin
Lipid A
Oligosaccharide core
Specific polysaccharide chain
What do endotoxins induce
A severe uncontrolled host response which causes cytokine production Fever Rigors Hypotension Tachycardia Collapse
What are the sites of viral entry
Conjunctiva Arthropod Capillaries Skin Respiratory tract Alimentary tract Urinogenital tract
Name two acute viral infections
Influenza A virus causing a respiratory infection
Enterovirus causing enteric and neurological infections
What is antigenic drift
Minor changes (natural mutations) in the genes of viruses.
It occurs gradually over time to produce antigenic variants
What is antigenic shift
Abrupt major changes in the virus antigenic structure
Name some examples of enteroviruses
Poliovirus
Aseptic meningitis
Myocarditis
Pancreatitis
Describe the features of an enterovirus infection
Infection enters the gut
Causes viraemia in non-neuronal tissues and neuronal tissues
Leads to paralysis
Virus is excreted in faeces
What do monocytes in the blood mature into
Tissue macrophages
Name some polymorphs
Neutrophils
Eosinophils
Basophils
Are lymphocytes phagocytic
No
Where do T cells mature
Thymus
What type of organisms are resistant to phagocytosis
Capsulate organisms (e.g. Strep pneumoniae)
Name an organism resistant to intra-cellular killing
Mycobacterium tuberculosis
What are polymorphs
Granulocytes
What type of macrophages are there
Fixed
Free
Where are fixed macrophages found
Liver
Spleen
Lymph nodes
of mononuclear phagocytic system
Where are free macrophages found
Tissues
What happens in innate immunity
Phagocytosis of foreign bodies Organism held in phagosome Fusion with lysosome Forms phagolysosome Eventually causes intra-cellular killing
What does the mononuclear phagocytic system do
Causes the:
Spleen to clear blood
Liver to clear entero-hepatic circulation
Regional lymph nodes to drain peripheral sites
What does IgM do
Produce primary response
What does IgG do
Produce secondary response
What does IgA do
Form mucosal immunity
What does IgE do
Assist in allergy and helminth infection
What is Ig
Immunoglobulins which are proteins with antibody activity.
There are 5 classes
What is a monoclonal antibody
An antibody from one clone of plasma cells which has specificity for a single epitope
What type of specificity do polyclonal antibodies have
Multiple specificity
State two features of B-lymphocytes
Can differentiate into plasma cells when they recognise a specific epitope
Require T-cell help
What is the complement system
A complex cascade of about 20 proteins which is triggered from the combination of an antibody (IgG or IgM) to its specific antigen.
What types of T cells are there
- CD4 helper cells
Th1 used in cell mediated immunity
Th2 which control B cell antibody response - CD8 suppressor ad cytotoxic cells.
Why is cell mediated immunity important in infection
It combats intracellular infection
What type of infections does cell mediated immunity attack
Most viral infections
Fungal infections
Intracellular infections
In what type of infections does humoral immunity mainly occur
Bacterial
Where do humoral infections occur
Extracelluar
Describe the process of cell mediated immunity
Causes macrophages to present antigens and stimulate T-cells
This causes cytokine production which control the response
Th1 cells activate macrophages to ingest and kill or contain the pathogen while the cytotoxic T-cells (CD8) kill infected host/foreign cells
What type of tumour can papillomavirus cause
Cervical carincoma
What type of tumours can retroviruses cause
Lymphomas
Leukaemias
How can a retrovirus induce tumours
Retrovirus attaches to the cell
Fusion with the membrane
Uncoating so viral DNA interacts with reverse transcriptase
RNA/DNA hybrid produced
Formation of viral DNA integrated into cellular genome
Change in cellular gene expression
This leads to uncontrolled cell multiplication and tumour formation
Where are acute viral infections localised
Specific site of the body
Viraemia will be developed with the widespread infection of tissues
What is a feature of viral pathogenesis
Antibodies neutralise viruses in their viraemic stage to prevent adherence of microorganisms and opsonise capsulate organisms
Describe acquired immunity
Produces a specific response to the antigen concerned
Creates immunological memory.
Has humoral (antibody) and cell (T cells) mediated response with each organism being a complex mixture of antigens and each antigen normally being a mixture of epitopes
What is colonisation
The development of a bacterial infection in an individual seen in a positive culture. The individual may have no signs or symptoms but have the potential to infect others
Define latent
An asymptomatic infection capable of manifesting symptoms under particular circumstances or if activated
Define infection
invasion and multiplication of an infectious agents in body tissues of the host and may lead to clinical symptoms or local cellular injury because of competition in metabolism, production of toxins, intracellular replication, or antigen antibody response
Define asymptomatic infection
An infection without symptoms
