Staphylococcus aureus and MRSA

Question 1

Describe the gram stain appearance of Staphylococcus aureus.

Answer 1

gram-positive (purple) coccus in clusters

Question 2

Is S. aureus a normal part of the human flora?

Answer 2

yes, normal commensal of skin and mucous membranes

Question 3

How is S. aureus transmitted/cause disease?

Answer 3

1. direct contact
2. contamination of fomites/environment
3. contamination of food
4. mov. of skin/mucous membrane commensal to other site of body

Question 4

Which disease is mainly caused by S. aureus?

Answer 4

cellulitis (90%)

Question 5

Which diseases can be caused by S. aureus infection?

Answer 5

1. Localised skin infections, eg cellulitis, impetigo, superficial abscesses
2. Deep localised infections, eg most common cause of septic joint (in children), bone marrow infection
3. acute endocarditis
4. septicaemia
5. pneumonia
6. toxic shock syndrome
7. staphylococcal gastroenteritis

Question 6

Which virulence factors does S. aureus produce to protect itself against the immune response?

Answer 6

1. enzymes, e.g. coagulase: localised blood clotting, restricting access by PMNs
2. haemolysins: mediate cellular osmotic lysis, esp. of RBCs
3. Panton-Valentine leukocidin: pore-forming toxin that lyses neutrophils

Question 7

Why can S. aureus cause abscess formation?

Answer 7

Panton-Valentine leukocidin: pore-forming toxin that lyses neutrophils… inflammation and tissue damage… tissue necrosis and abscess formation (pus).

Question 8

Why can S. aureus cause toxic shock syndrome?

Answer 8

Produces superantigen exotoxins, eg

1. toxic shock syndrome toxin (TSST-1): stimulates enhanced T cell response
2. exfoliatin: cleaves component of desmosomes - loss of superficial skin layer

Question 9

How is S. aureus infection diagnosed?

Answer 9

relies of microscopic and colony morphology - gram stain, and catalase positivity

Question 10

How is S. aureus differentiated from other staphylococci?

Answer 10

coagulase test: is coagulase +ve (causes blood clotting) whilst many other staph are coagulase -ve

Question 11

Which antibiotics are used to treat MSSA and MRSA?

Answer 11

MSSA: flucloxacillin

MRSA: vancomycin

Question 12

How are MRSA cases reduced in hospitals?

Answer 12

Hand washing as part of infection prevention measures important in reducing MRSA cases (rather than P isolation).

Question 13

What is generally required for S. aureus infection?

Answer 13

significant host compromise, e.g. break in skin or insertion of a foreign body (e.g. wounds, surgical infections or central venous catheters)

Question 14

Why does MRSA have increased antibiotic resistance?

Answer 14

chromosomal acquisition of gene for a distinct penicillin-binding protein, i.e. new peptidoglycan transpeptidase with low affinity for all currently available beta lactam antibiotics

some MRSA strains are sensitive only to glycopeptides such as vancomycin