Pathogenecity of Staph Aureus

Question 1

What are the nonspecific characteristics of s. aureus making it a successful apthogen

Answer 1

Ability to colonise skin esp. miost - opportunistic infectinos
Resistanct to dessication
Very adaptable - cal develop anitbiotic resistance
Cyclinc of predominant types - would be useful in evading the immune response

Question 2

Describe the s. auerus genome structure

Answer 2

2/3 core genome –> this is clonal with point mutatiosn accounting for the majority of variation and recombination rare

1/3 acessory genome - high peropeortion of mobile elemtnsl genomic/pathogenicity islands, bacteriophages, plasmids and transposons. Reassortment occurs due to hosizontal exchange

Question 3

What virulence factors are encoded in the core genome

Answer 3

Coagulase
Protein A
Alpha haemolysin
Fibrinogen binding protein

Question 4

What virulence factors are encoded in the accessory genome

Answer 4

Enterotoxins
Toxic shock toxins
Antibiotic resistance genes
Leucocidins
Exfoliative toxins
Collagen adhesin proteins

Question 5

What are the cell envelope virulence factors

Answer 5

1) capsule
2) protein A
3) Clumping factors ( a cell bound coagulae +/- fibringoen binding protein)
4) other cell wall associated adhesions

Question 6

What is the s. aureus capsule

Answer 6

A thin lauer of uronic acid containing polysaccharides
Present in most clnical isoltes
Impedes phagocytosis by masking cell wall components e.g. teichoic acid

Question 7

How many seroligcally distinguished types of S. Aureus are there

Answer 7

13

5 or 8 account for >70% of blood isolates though

Question 8

What is protein A

Answer 8

It is a cell wall associated protein projecting from S. Aureus
Protein A binds the Fc domain of IgG and incapacitates it!
Also secretes Sbi protein binds the Fc domain of IgG

Question 9

What are MSCRAMMS

Answer 9

Microbial Surface Components Recognising Adhesive Matrix Molecules

Question 10

Name some MSCRAMMS

Answer 10

1) Protein A
2) clumping Factors (fibrinogen binding portein
3) Fibronectin binding protein
4) Collagen binding protein

Question 11

What do the MSCRAMMS do

Answer 11

They each have a domain for binding target proteins
upon binding a conformational shape change occurs to increase their affinity for the moecule and sometimes to other molecuels

Question 12

What does the sortase enzyme do

Answer 12

The sortase enzyme catalyses transpeptidisation ractions between the PXTG motif of adhsind and pentaglysine cross bridges of peptidoglycan to anchor the surface proteins to the cell wall

deficient in sortase = less virulent

Question 13

Describe the enzymes as virulence factors

Answer 13

Coagulase - initiates fibrin polymerization by binding prothrombin
Staphylokinase
Hyalourindase - breaks down tissues of dermis in particularly
Lipase

Question 14

Describe toxins as virulence factors

Answer 14

Haemolysins
Leucocidins
enterotxins
Exfoliative toxins
Staohpylococcal toxic shock syndrome

Question 15

What do alpha and gamma haemolysisn and leucocidins form

Answer 15

Form multimeric beta barrel pores in host cell membranes composed of 7 identical subunis

Question 16

What are the gamma toxins and leucocidins including PVL comprosed of

Answer 16

Comprised of the F & S polypeptides that assemble in the cell membrane to form a 7-mer structure –> resemble a beta barrel pore

Question 17

What is PLV associated with

Answer 17

Severe necrotising skin infections and penumonias

Question 18

What are PSMs

Answer 18

They are amphipathic and alpha helical peptides

Question 19

What are the groups of PSMs

Answer 19

4 x PSMs alpha - 21-22a.a coded in a gene cluster
2 x psm beta - 44 aa peptides in a second cluster
PSM Delta - a delta haemolysin - 26 a.a. encoded in a 2nd gene cluster
PSM - mc - a 20 a.a encoded on some SCC Mec elements but less common

Question 20

How d PSMs cause virulence

Answer 20

Stimualtes neutrophils to increase chemotaxis, increase cell lysis and increase IL-8

Question 21

What is the sign you get in SSSS

Answer 21

Nikolskys sign - extensive blistering and desquamation on gentle rubbing

Question 22

What are the 2 disaese mediated by exfoliative toxins

Answer 22

1) Bullous impetigo

2) SSSS

Question 23

What are the 2 s. aureus exfoliative toxins

Answer 23

1) ETA - heat satble, gene on chromosmal temperate phage
2) ETB - heat labile and on plasmid

Only 40% sequence homology but serologically distincyt but functionally similar

Question 24

How do the exfoliative toxins work

Answer 24

They bind to keratohylin structures in the stratum granulosum
The toxins have an active site with sequence homology to a trypsin like serine protease –> cleaves desmoglein - 1 which is a cell-cell adhesion molecule!

Question 25

How many s. aureus enterotoxins are there

Answer 25

5 SEA –> SEE

THey are proteinase and heat resistant

Question 26

How do the enterotxins have emetic activity

Answer 26

They stimulate the vomiting centre by stimulating the vagus nerve

Question 27

What do some of the enterotoxins also possess

Answer 27

Superantigenic activity - can stimulate excessive cytokine production

Question 28

What are the definining clinical fatures of Toxic Shock Syndrome

Answer 28

Fever >38.9
Hypotension
Involvement of 3 or more organ systems
Diffuse macular rash
Desquamation 1-2 weeks after onset if you survive

Question 29

Differences in toxins between menstural and non=menstrual women who have TSS

Answer 29

Menstural: 95% TSS alone 5% TSST and enterotoxin

Non-Menstural: 50% had TSST Alone, 5% had TSST and enterotoxin rest = enterotoxin only!!!

Question 30

What are superantigens

Answer 30

TSST and SEB/C1 –> these activate a high propertion of T cells causing massive cytokine production

Question 31

What is the name of the main system regulating toxin production in S. Aureus

Answer 31

The Accessory Gene Regulator Syste (Agr)

Question 32

describe the make up on the Agr system

Answer 32

Several genes arranged in 2 operans
Regulates the expression of RNA III –> the RNAIII mRNA upregulates the expession of s. aureux exoproteins at the expesne of surface proteins in the early stationary phase

Mutants of agr = less virulent

Question 33

How does the agr gene system functions

Answer 33

agrB encodes a cell membrane bound protein
agrD encodes a signal peptide –> it is processed by AgrB to a 7-9aa fragments –> these fragments are sensed by AgrC (a sensor histidine kinase) which then phosporlates AgrA which then goes to promote P2 nad P3 hence upregulating both agr and RNA III transcription!

Question 34

Name the other s. aureus regulatory sstems

Answer 34

1) Staphylococcal accessory regulator
2) S. aureus exoprotein expression regulator
3) RNA III INhibitory Peptide

Question 35

How does the Staphylococcal accessory regulator work

Answer 35

Binds at the P2 and P3 to upregulate RNA III

ALso directly binds to the promoters of some genes e.g. alpha-haemolysin

Question 36

How does the staphyloccal exoprotein expression regulator work

Answer 36

2 component system

Activates exoportein prodction at the level of transcription

Question 37

Is S. Aureus an intracellular pathogen

Answer 37

Not though to be But is internalied by a variety of cells
Internalisation is promoted by fibrinogen binding protein
Cells defective in Agr are internalised in greater numbers and fail to induce endosomal or cell lyss!