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Microbiology > 2. Virulence and Virulence Factors > Flashcards

2. Virulence and Virulence Factors Flashcards

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1
Q

How do we know a bacterium is a pathogen?

A

Koch’s Postulates

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2
Q

Briefly summarise Koch’s Postulates

A
  • Bacterium should be found in all people with disease, & in correct location
  • Bacterium should be isolated from infected site & maintained in pure culture
  • Pure culture should be capable of causing disease
  • The same bacterium should be isolated from the intentionally infected host
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3
Q

What are some problems associated with Koch’s postulates?

A
  • Doesn’t always account for host factors
  • Not always possible to isolate organisms in pure culture
  • Some diseases involve polymicrobial infections
  • Cultivation of bacteria can lead to loss of virulence factors
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4
Q

What is virulence?

A

The ability of a bacterium to cause disease

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5
Q

What is a virulence factor?

A

A bacterial product or structure or strategy which contributes to virulence

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6
Q

Define infection

A

An infection occurs when a bacterium (including those capable of causing disease) becomes established in the body

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7
Q

True or False: Not all bacteria that infect cause disease

A

True

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8
Q

Define disease

A

An infection producing symptoms that are detrimental to the host

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9
Q

How do we know that a specific attribute of a pathogen is a virulence factor?

A

Molecular Koch’s Postulates - attempts to define a virulence factor (gene product) rather than a pathogen

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10
Q

What is the most important thing to know re Molecular Koch’s Postulates?

A

Disrupting the gene in a virulent strain should diminish virulence, or, introducing the gene to an avirulent strain should make it virulent

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11
Q

What are examples of virulence factors that mediate colonisation & survival in infection?

A

Adherence, motility, invasion & intracellular residence, evasion of host defences

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12
Q

What virulence factors damage the host (determine disease)?

A

Toxins: endotoxins (lipid A), exotoxins
Enzymes: hyaluronidase, DNAse

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13
Q

What are some factors that have been implicated in order for bacteria to survive antibacterial features of the mucosal surface?

A
  • Motility & chemotaxis
  • sIgA proteases (sIgA binds Ag & mucin)
  • Iron acquisition (produce siderophores to compete against transferrin)-
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14
Q

What genes are pili encoded by?

A

Pap genes

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15
Q

What genes are fimbriae encoded by?

A

Fim genes

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16
Q

What do fimbriae tightly bind to?

A

Mannose residues (found on most human cells)

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17
Q

What are pili?

A

Protein hair-like structures found on the outside of many bacterial cells - long hair-like structures that have a protein at their tip that binds to host sugars

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18
Q

What is an example of an infection that best represents the function of both pili and fimbriae?

A
  • UTI caused by E.coli
  • Fimbriae stick to bladder cells, pili stick to kidney cells
  • Fimbriae and pili help the bacterial cell to attach to host cells (don’t get washed out in urination)
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19
Q

Specificity of binding to host cell carbohydrate depends on…

A

the pilus tip

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20
Q

What do afimbrial adhesins do?

A

Mediate tight binding between bacterium & host cell

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21
Q

What is an example of a bacterium that has an adhesin?

A

Streptococcus pyogenes has an adhesin, protein F, which binds to fibronectin → causes throat & wound infections

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22
Q

What are invasins?

A

A collective term for proteins that are important for bacterial invasion

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23
Q

How do bacteria like Shigella and Listeria enter host cells which are not naturally phagocytic?

A

Cause rearrangements in the host cytoskeleton (actin) - do this using bacterial surface proteins called invasins. Can also rearrange actin within the cell to promote movement between cells

24
Q

What is an example of a bacteria that targets M cells for entry?

A

Salmonella

25
Q

What are capsules and what do they do?

A

Capsules are loose unstructured polysaccharide polymers that can prevent uptake by macrophages

26
Q

Some capsular material is non-immuogenic. Give examples

A
  • Streptococcus pyogenes - produces a capsule called hyaluronic acid
  • Neisseria meningitidis - sialic acid
27
Q

How can complement be evaded?

A

Can evade complement by altering the major target, LPS, by attaching sialic acid to the LPS O antigen. Also produces enzymes which degrade complement proteins

28
Q

What happens when sialic acid is attached to the LPS O antigen?

A

The LPS essentially becomes invisible to complement

29
Q

What bacterial protein binds Fc portion of Ab and neutralises the Ab response to that bacteria?

A

Protein A of Staphylococcus aureus

30
Q

What are endotoxins?

A

LPS (intergral part of Gram negative outer membrane)

31
Q

Where are exotoxins mostly produced?

A

Inside Gram positive bacteria as part of their growth & metabolism. They are then secreted/released following lysis into the surrounding medium.

32
Q

How is LPS chimeric?

A

LPS has a lipid component (Lipid A) and a sugar glycol component (O antigen)

33
Q

What is the toxic component of LPS?

A

It is released by G-ve bacteria as they die & acts as a toxin in eukaryotic cells - an indication to the host that G-ve bacteria are in the vicinity & the host prepares the immune system

34
Q

What happens if LPS is found in the wrong place at the wrong time?

A

Death by septic shock

35
Q

What do LPS binding proteins bind to?

A

Lipid A part of LPS → delivers LPS to circulating immune cells e.g. macrophages

36
Q

TLR4 surface protein on macrophages binds to?

A

CD14, which interacts with LPS binding protein → signalling cascade in macrophage → cytokine production

37
Q

What is shock?

A

Shock describes a set of events which lead to the collapse of the circulatory system, primarily, and can progress to multiple organ system failure and death (80%)

38
Q

What are the 3 main categories of exotoxin?

A
  1. A-B toxin (simple or complex)
  2. Membrane disrupting (pore-forming or membrane-damaging)
  3. Superantigen (responsible for toxic shock)
39
Q

How are simple A-B toxins formed?

A

They are produced as a single protein from a single gene, cleaved by proteins - remain connected by a disulphide bridge (joins A & B fragments together)

40
Q

What does the B subunit do?

A

B subunit binds to target cell and delivers A subunit to that cell

41
Q

What are complex A-B toxins made up of?

A

One A protein (may contain 2 subunits linked by a disulphide bridge), multiple B proteins (usually 5). A & B produced independently from separate genes. A subunit proteolytically cleaved into A1 & A2. A2 part responsible for interacting with B subunits - together deliver A1 subunit to target cells. A1 subunit has the toxic activity.

42
Q

What happens when a toxin binds to its receptor?

A

Triggers an endocytic response → toxin is taken up into endosome → active subunit is delivered across endosomal membrane into cytoplasm

43
Q

What is an example of a membrane disrupting toxin?

A

Phospholipase A2 - cleaves phospholipid heads, destabilises the membrane, cell dies

44
Q

True or False: Membrane-disrupting toxins are often indiscriminate and attack many cell types

A

True

45
Q

What do superantigens do?

A

Superantigens bring a MHC and a TCR together in a non-specific way, without the presence of an antigen → much bigger immune response → toxic shock syndrome (inappropriate massive immune response)

46
Q

How do superantigens trick T cells?

A

They trick T cells into thinking it has recognised a foreign antigen

47
Q

What happens when a T cell interacts with a superantigen?

A

Cytokine storm → shock → death

48
Q

What are hydrolytic enzymes?

A

Enzymes which can contribute to the progression of an infection, but are not normally categorised as toxins i.e. proteases, hyaluronidase (cause tissue damage), DNAse (reduce viscosity)

49
Q

How is virulence measured?

A

2 strains of the same species (e.g. Salmonella typhi vs Salmonella enterica)
or
A parent and mutant (lacking a potential virulence factor)

50
Q

Which strain of Salmonella only infect humans?

A

S. typhi

S. typhimurium causes similar effects in mice

51
Q

What is a very important measure of virulence?

A

LD50 (number of cells to cause death in 50% of animals) - can be used to compare virulence of different strains or mutants
(the lower the number, the more virulent the organism)

52
Q

True or False: ID50 will always be lower or equal to LD50

A

True

53
Q

What other features can be used to measure virulence?

A

Numbers in a particular organ, symptomatology, luciferase (light)

54
Q

Advantages and disadvantages of cultured cells

A

Advantages: human origin, cost efficient, defined, readily available, few ethical concerns
Disadvantages: incorrect gene expression, lack of polarisation, loss of traits over time, often transformed (cancer-derived, different metabolism)

55
Q

What can you use cultured cells to measure?

A

Specific attributes of pathogens such as adherence, cytotoxicity and invasion (but obv can’t reproduce symptoms, organ specific traits, systemic spread, response to immune system etc.)

Microbiology (13 decks)

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