Virulence Factors Flashcards
Koch’s postulates
- The bacterium must be found in all people with the disease in the correct location
- The bacterium should be isolated from the infected site and maintained in pure culture
- The pure culture should be capable of causing the disease
- The same bacterium should be isolated from the intentionally infected host.
Virulence
Ability of a bacterium to cause disease
Virulence Factor
A bacterial product or structure or strategy that contributes to virulence
Molecular Koch’s Postulates
- Gene present in virulent strain should be absent or inactive in avirulent strains
- Disrupting the gene in a virulent strain should diminish virulence or introducing the egen to an avirulent strain should make it virulent
- Gnese should be expressed during infection
- Antibodies to gene product should be protective
Example of afimbrial adhesion
Strep pyogenes has an adhesion protein F which binds to fibronectin - casting throat and wound infections
Explain evading complement
- Attachment of sialic acid to the LPS O antigen alters LPS which is the main target for complement
- bacteria produce proteins that bind to antibodies (e.g. Protein A by S.aureus) which bind to Fc portion and cause neutralisation
- Loss of capsule - prevents opsonisation
Exotoxin
- produced inside the cell (mostly gram +) and are secreted following lysis into surrounding medium
- can be cytotoxic or specific
- e.g. Chlorea toxin
Endotoxin
- part of the outer portion of the cell wall of gram - bacteria (LPS)
- liberated when cell dies or cell wall breaks apart
- all gram - have LPS thus all have endotoxin in outer leaflet of outer membrane
Toxic component of LPS
Lipid A
Function of LPS
- LPS binds to LPS -binding protein which triggers cytokine release which can lead to a cascade of event characterised as shock
- shock can lead to MOSF
*
What are the categories of exotoxins?
- A-B toxin (combo of A and B subunits; can be complex or simple)
- Membrane disrupting (pore-forming or membrane damaging)
- Superantigen (responsible for toxic shock)
Explain simple A-B toxins
- produced as a single protein from a single gene
- cleaved by host protease into A and B subunits
- linked together by S-S bridge
- A: active subunit
- B: binding subunit - binds to target cell and delivers A subunit
Explain complex A-B toxins
- 2 genes
- A subunit proteolytically cleaved into A1 and A2 - held together by disulphide bridge
- A1 subunit contains toxic activity
- 1 A subunit: 5 B subunits
- A2 subunit interacts with B subunits - delivers A1 to target cells
Types of membrane disrupting toxins
- Pore-forming - binds to receptor, inserts into membrane forming a pore, cell content leak out and cell dies (e.g. BT toxin)
- Phospholipases - cleaves phospholipid heads which destablises the membrane (e.g. Phospholipase A2)
Bioassay used to identify membrane-disrupting toxins
Hemolysins - lyse RBCs
Superantigens
- Binds the MHC and TCR in the absence of an antigen
- Tricks the T cell into thinking it has recognised a foreign antigen
- Results in the increased activation of T-cells known as a cytokine storm
Hydrolytic enzymes
- enhance infection progression but not toxins
- DNAse - reduces viscosity
- Hyluronidase - causes tissue damage
Ways of measuring virulence
- LD50 - no. of cells to cause death in 50% animals
- ID50 - no. of cells to cause infection in 50% animals
- no. in organ
- symptomology
- luciferase - detects in real time
- cultured cells
*a pathogen can have a low ID50 but high LD50 (or no LD50)
*ID50 will always be ≦ LD50
→ the lower the number the more virulent the organism
Advantages and Disadvantages of using cultured cells to measure virulence
Advantages
- human origin
- cost efficient
- defined
- readily available
- few ethical concerns
- can measure adherence, cytotoxicity, invasion of pathogens
Disadvantages
- incorrect gene expression
- lack of polarisation
- loss of traits overtime
- often transformed as they are cancer-derived
- cannot reproduce symptoms, organ specific traits, immune system response
Virulence Strategies
- production of toxins - tetanus, Botox
- overcome body defences - vibrio
- invade tissues and organs - diphtheria, salmonella
