A relationship between two organisms
An organism that lives in or on a host that causes disease
a relationship between two species where one species obtains food or other benefits from the other without causing harm or benefit to the host.
What are the characteristics of a typical prokaryotic cell?
Cell division by binary fission
Name 3 differences between prokaroytic and eukaryotic cells
Prokaryotes: Small cells, unicellular,no membrane bound organelles, circular DNA, divide by binary fission
Eukaryotes: Large cells, membrane bound organelles, cell division by mitosis or meiosis,
What is the difference between gram negative and gram positive bacteria?
Gram negative bacteria have a thin layer of peptidoglycan and an LPS layer
Gram positive bacteria have a thick layer of peptidoglycan
Describe the mechanisms of lateral gene transfer in bacteria
Lateral gene transfer occurs whenever genes are transferred from one cell to another independently of the normal inheritance process. Three methods are: transduction, transformation, conjugation
Transduction: DNA is transfered from cell to cell by bacteriophage
Transformation: free DNA is incorporated into competent recipient cells
Conjugation: transfer of plasmid DNA from a donor cell which contains an F pilus, to a recipient cell which does not. Transfer of DNA is via rolling circle replication.
Describe the mechanism of bacterial conjugation
Conjugation requires direct contact between donor and recipient cells.
The donor cell contains a conjugative plasmid (has an F pilus) which makes contact with the recipient cell and the cells are drawn together.
Conjugation is triggered by cell-cell contact and DNA is transfered to the recipient cell via rolling circle replication of the plasmid DNA.
At the end of the process both donor and receipient have the plasmid
the establishment of an organism within a host
any organism that has the capacity to cause disease
quantitative measure of pathogenicity or the likelihood of causing disease
What are virulence factors?
Pathogen produced extracellular proteins that enable a microorgansm to establish itself and replicate on or within a specific host species, and enhance the microbe's potential to cause disease
What are opportunistic pathogens?
AN organism that causes the disease in the absence of normal host resistance i.e. when there is deficiency in the immune defense
Describe the role of virulence factors in bacterial infections
Virulence factors enhance colonisation and growth
1. Aid colonisaton: express adhesion molecules which specifically adhere to cells in a region of the body. Interactions between the bacteria and host epithelium trigger entry (invasion)
2. Aid survival: Produce factors which allow the bacteria to evade the immune system e.g. capsule which prevents phagocytosis, molecular mimicry, intracellular invasion
3. Damage the host: produce enzymes and toxins which target host cells and tissues, resulting in cell lysis and tissue damage
4. Obtain nutrients: Enzymes and toxins degrade host tissue and cause cell damage, products (proteins, nucleic acids) taken up by the bacteria. Produce proteins that chelate iron and Ca2+
What are the obligatory steps for infection
Exposure to pathogen
Attachment and entry into the body
Adherence to the skin or mucosa by evading the natural protective mechanisms of the host
Invasion through the epithelium
Colonisation and growth (produces virulence factors).
Evasion of host immune defences . Allows further growth and local or general spread
Growth and colonisation can result in toxicity (effects can be local or systemic), invasiveness (further growth at original site and distant sites) both of which cause further tissue damage.
What are the different types of haemolysis seen on a blood agar plate?
alpha haemolysis: green/brown zone on blood agar. Not true haemolysis, discolouration caused by loss of K+ from RBCs
beta haemolysis: clear zone of haemolysis around the colony. Caused by release of toxins e.g. lecithinases, phospholipases, streptolysin
gamma haemolysis: bacteria do not induce haemolysis
Describe the action of alpha toxin of S.aureus
Membrane damaging toxin
Monomers bind to the surface of susceptible cells and oligomerise into a pore which lyses the cell.
What determines the expression of virulence factors?
Response to change in enviornmental stimuli (e.g. temperature, pH)
Response to cell density (quorum sensing)
Name four properites of a virus
Small, infectious, obligate intracellular parasite
Non-living (cannot reproduce by themselves)
Make mRNA that is translated by host ribosomes
Describe the replication cycle of a virus
Attachment of the virion to a susceptible host cell
Penetration of the virus or its nucleic acid into the cell wall
Synthesis of nucleic acid and protein by cell metabolism early in the infection
Late in infection, structural proteins (viral capsid) are synthesized.
Assembly of capsomers and packaging of nucleic acid into new virions
Release of mature virions from the cell.
Name two outcomes of virus infection for the cell
Transformation into tumor cells
How does the immune system respone to viral infections?
Cell mediated immunity (T-cells, NK cells)
Antibodies (neutralisation, opsonisation)
What is the diifference between lytic and latent infection?
Lytic infection: Virus replicates and distorts cell function, causing cell lysis and virus release (productive infection)
Latent infection: VIrus lies dormant within the cell, minimal or no replication
What is reactivation
When an organism leaves latency and establishes productive infection.
State two differences between yeasts and moulds
Reproduce by budding
Identify using biochemical tests
tubular structures that form hyphae
grow by branching and longitudinal extension
Name two antifungal treatments
Fluconazole (yeasts particularly): Inhibits specific fungal enzymes
AmphotericinB: broad spectrum antifungal, acts on the cell wall (associated with renal toxicity
Name three diseases caused by protozoa and state the treatment
Malaria: Caused by plasmodium sp. Treat with quinine, chloroquine
Giardiasis:giardia sp. causes diarrhoea. Treat with metronidazole
Leishmaniasis: Causes cutaneous and systemic infections. Treat with amphotericin B