Staphylococcus

Question 1

is S. Aureus coagulase + or -?

Answer 1

S. Aureus is coagulase +

This test differentiates S. Aureus from other staphylococci

Question 2

Where is Staphylococcus aureus found?

Answer 2

in moist skin folds, mucosal surfaces or the nasopharynx

• increased in diabetes mellitus, IV drug users, and where a foreign body is present

Question 3

Describe the pathogenesis of S. Aureus.

Answer 3

1. gets in - portal of entry
2. attaches to the cells
3. defeats/evades the immune system
4. cuases damage to host cells
5. gets out and spreads further

Question 4

What is the most common portal of entry of S. Aureus

Answer 4

1. Ingestion
2. Penetration of the skin

Question 5

How does S. Aureus attach to the cells?

Answer 5

1. surface proteins - attach to epithelial and endothelial proteins (laminin and fibronectin of the extracellular matrix)
2. Capsule - inhibits chemotaxis, phagocytosis and facilitates adherence ot foreign bodies
3. fibrin/fibrinogen binding protein - attachment to blood clots and traumatized tissue
4. matrix binding proteins - fibronectin, fibrinogen and collagen binding where adhesion promotes collagement attachment found in strains that cuase septic arthritis

Question 6

how does S. Aureus defeat/evade the immune system?

Answer 6

inhibition of phagocytosis with capsule

and

production of extracellular substances that promote invasion

• invasins
• enzymes

Question 7

what is the effect of coagulase in S. Aureus?

Answer 7

coagulase is an extracellular protein which binds to prothrombn

*remember that S. aureus is coagulase +*

clots then protect the bacteria from phagocytosis and other host defences

Question 8

what is the effect of S. Aureus’ Protein A ?

Answer 8

it is a cell wall surface protein that binds IgG molecules in the inverted orientation by their Fc Region and therefore prevents phagocytosis

Question 9

What is the role of Alpha toxin in S. Aureus?

Answer 9

it binds to platelets and monocytes cuasing small pores - osmotic lysis

Question 10

What is the effect of leukocidin in S. Aureus?

Answer 10

acts on polymorphonuclear leukocytes2% of all S. Aureus isolates express it

nearly 90% of isolates from severe dermonecrotic lesions are +

Question 11

what is the effect of staphylokinase in S. Aureus?

Answer 11

plasminogen activator that lysses fibrin and dissolves fibrin clots and helps bacterial spread

Question 12

what is the effect of hyaluronidase in S. Aureus?

Answer 12

helps the spread by breaking down hyaluronic acid in connetive tissues

Question 13

how does S. Aureus damage the host cells?

Answer 13

• direct damage (peptidoglycan wall - provides osmotic stabiity and stimulates the release of cytokines)
• enzymes
• toxins
  
  • Super antigens -
  • exfoliative toxins
  • cytotoxins

Question 14

how do you classify staphyloccocal infections?

Answer 14

1. skin and soft tissue ex) styes, folliculitis, mastitis, boils
2. systemic - invasive ex) bacterial stream infection, endocarditis, bone/joint infections
3. systemic - toxic mediated ex) food poisoning/gastroenteritis, scalded skin syndrome, toxic shock syndrome

Question 15

What is Toxic Shock Syndrome?

Answer 15

Historically associated with high absorbency tampons - leads to pyrexia hypotension due to dilitation due to cytokines , rash with subsequent desquamation, other organ involvement (renal failure, CNS etc)

Due to

• TSS toxin 1 acts as a super antigen cytokine release

Question 16

What is MRSA?

Answer 16

methicillin resistant S. Aureus

• results from production of an altered penicillin bidning protein - confers resistance to most beta-lactam antibiotics

a number of treatment options remain

Question 17

Wha tis scalded Skin Syndrome?

Answer 17

S. Aureus has toxins that are exfoliative - meaning they split the intracellular bridges in the skin layer

classically described in young children - very contagious

huge risk of secondary skin infection

Question 18

How should we manage patients with MRSA?

Answer 18

1. history/clinical exam
2. investigationas depend on the infection - ex) abscess need pus swab, BSI need blood cultures, Food poisoning need piece of food
3. Antibiotic treatment - no treatment for mild infections, but 7-10 days for skin/soft tissue infections and RTI
   
   1. Fluclozacillin if susceptible
   2. Vancomycin if MRSA suspected

Question 19

where are coagulase negative staphylocci found?

Answer 19

they are natural inhabitants of human skin and mucosa - harmless while on skin

much less virulent than S. aureus

Question 20

what are S. Epidermidis infections most often associated with?

Answer 20

with prosthetic device infections ex) artificial joint infections

Question 21

what does S. Saprophyticus cause?

Answer 21

most commonly UTIs

Question 22

What is the treatment for a UTI?

Answer 22

generally infection = S. Saprophyticus = 3 days treatment for lower UTI with Trimethoprim

Question 23

generally we treat coagulase + S.cocci with what?

How about coagulase - ?

Answer 23

coagulase + = flucloxacillin or vancomycin b/c risk of MRSA

coagulase - = Trimethoprim for general UTIs