0505 - Staph Aureus and Virulence Factors

Question 1

What does it mean to be coagulase positive? (clinically and microbiologically)

Answer 1

The ability to catalyse the conversion of fibrinogen to fibrin in the body. Microbiologically, form clumps on slide or in tube to test.
Coagulase is an important invasin and impedin, allows it to form a clot, evade phagocytosis, and then use staphylokinase to break the clot down and continue invading.

Question 2

What is the role of DNase? (Clinically and microbiologically)

Answer 2

Degrades DNA in tissue. In lab, DNase clears an area around colony. S. Aureus only one that’s both coagulase and DNase positive.

Question 3

How does staph aureus differ from coagulase negative staph species?

Answer 3

Number of toxins =>30 vs None
Number of adhesins = >20 vs Less than 10
Staph Aureus only one that produces both coagulase and DNase.

Question 4

What is an Adhesin

Answer 4

Coordinates binding of the organism to host tissue

Question 5

What is an Invasin

Answer 5

Enable organism to invade host cell

Question 6

What is an Impedin

Answer 6

Enable organism to evade some host defence mechanisms

Question 7

What is an Aggresin

Answer 7

Causes direct damage to the host

Question 8

What is a Modulin

Answer 8

Induces indirect damage to the host (e.g. antigen-antibody complexes).

Question 9

How are Staph Aureus virulence factors categorised

Answer 9

Genetic component and product.
Regulatory systems - Accessory Gene regulator (agr complex) - influences production of downstream virulence factors
Cell surface - adhesins (lots of binding proteins)
Secreted enzymes and proteins
Toxins and haemolysins
Small colony variants - tailored for small colonies, may allow survival in colonisation.

Question 10

What are some cell surface determinants of Staph Aureus virulence?

Answer 10

LOTS - mostly adhesins
Capsule - impedin to resist phagocytosis
Surface adhesins - Clumping factors A and B (coagulase), fibronectin/colllagen/elastin binding proteins
Teichoic and Lipoteichoic acid
Peptidoglycan
Ability to form biofilms - (e.g. on prosthetics) - KEY

Question 11

What are some enzymes/proteins secreted by staph aureus

Answer 11

All are responsible for tissue damage and invasion.
Coagulase (also impedin)
Phospholipase C - damage RBCs
Staphylokinase (also impedin)

Question 12

What are the toxins and haemolysins of Staph aureus?

Answer 12

These are Aggresins - most important virulence factors
Haemolysins
Leucocidins (lyse WBCs)
Exfoliative toxins - damage skin and subcut tissue
Superantigen toxins - very important - aggressins and impedins, can cause auto-immune diseases. TSST-1 and Enterotoxins.
Phenol soluble modulins.

Question 13

What are the Staph Aureus Haemolysins?

Answer 13

Pore-forming toxins that lyse RBCs and other cells. Alpha haemolysin is most commonly secreted, causes skin and soft tissue infections (SSTI), and pneumonia.

Question 14

What are the Leucocidins of Staph aureus?

Answer 14

Bi-component toxins - come together on cell surface to form a pore.
Panton-Valentine Leucocidin (PVL) is implicated in many infections, but only present in 5% of isolates. Responsible to severe soft tissue infections and pneumonias. 85% of community associated MRSA has this toxin.

Question 15

What Staph Aureus toxins are associated with Staphylococcal scalded skin syndrome?

Answer 15

Exfoliative toxins A and B. They directly break down the stratum granulosum.

Question 16

What are the two types of superantigen possessed by Staph Aureus?
(KEY EXAM CONCEPT)

Answer 16

TSST-1 - Toxic Shock Syndrome Producing Toxin - associated with menstrual TSS
Staph Enterotoxins - cause food poisoning.

Question 17

How to Staph superantigen toxins work?

KEY EXAM CONCEPT

Answer 17

Don’t follow normal path of other antigens - activate T-cells non-specifically (20% T-cells activated, normally 0.0001% activated by infeciton).
Leads to cytokine storm and overwhelming inflammatory response. This results in endothelial leakage, haemodynamic shock, multi-organ failure and possibly death.

Question 18

How does Staph Aureus evade the immune system?

Answer 18

Inhibit leukocyte migration and phagocytosis
Prevent Opsonin coating via capsule - Ig binding and opsonisation blocked
Protein A and Staphylokinase are peptides that destroy Ig’s
Complement is evaded by capsular polysaccharides, staphylokinase and protein surfaces.
Antimicrobial peptides - staphylokinase, aureolysin, to provide positive environment.

Question 19

What role does the agr gene regulator play in Staph Aureus infection?

Answer 19

agr plays a role in quorum sensing, and expression of key virulence factors.
When agr activated, adhesins decrease, toxins increase, and vice versa. So colonising strains (with more adhesins) are less likely to produce virulence factors and invade.
agr=infection.

Question 20

How can Staph Disease present?

Answer 20

Pyogenic/Invasive and toxin mediated
Invasive - Actual infections and most inflammation - particularly skin and soft tissue and bacteraemia and associated internal infection. Often linked with healthcare facilities.
Toxin mediated - toxic shock syndrome, staph scalded skin syndrome, food poisoning.

Question 21

What are the Skin and soft tissue infections caused by staph?

Answer 21

Epidermis - Impetigo
Superficial dermis - Folliculitis
Deep dermis - furuncle or carbuncle
Subcutaneous infection - cellulitis, fasciitis
Staphylococcal scalded skin syndrome (actually toxin-mediated)

Question 22

Briefly outline Staph Bloodstream infections

Answer 22

Mostly healthcare associated
20-30% mortality rate
1/3 develop metastatic complications, particularly infective endocarditis. Unlike strep, staph can actually damage the valve and form vegetations on an otherwise undamaged valve.

Question 23

Briefly outline Staph bone and joint infections

Answer 23

Osteomyelitis - 50-70% staph aureus.
Often diabetic foot infections or foot trauma in kids.
Prosthetic joint infections - can be acute or delayed (12 week post-surgery divid). CoNS most common, then staph aureus.
Small colony variants implicated as cause of persistant bone infections and refractory to treatment.

Question 24

Briefly outline Staph toxin-mediated diseases

Answer 24

Staph food poisoning - ingestion of pre-formed toxins (not live staph) in food/drink. Symptoms 2-6 hours after ingestion. 10% require hospitalisation.
Toxic shock syndrome - potentially leading to death.
Staphylococcal scalded skin syndrome.

Question 25

What are the characteristics of healthcare associated MRSA?

Answer 25

Resistant to >3 classes AND beta lactams

Highly clonal

Question 26

What are the characteristics of community-associated MRSA?

Answer 26

Resistant to less than 3 classes and beta lactams

Polyclonal generally

Question 27

How is staph able to obtain antibiotic resistance?

Answer 27

Predominantly based on altered penicillin binding protein 2a (PBP2a) - mecA gene.
If damaged, penicillin can’t bind and can’t bind. MecA carried on Staphylococcal chromosome cassette (SCC).