Staph Aureus

Question 1

Name the different Staphylococcus species

Answer 1

S.aureus-S. epidermidis-S. saprophytic

Question 2

Which are coagulase positive/negative?

Answer 2

Negative: saprophyticus, epidermidis-
Positive: S.aureus

Question 3

How does a coagulase test work?

Answer 3

The way to distinguish between the different staphylococcus strains is by looking for coagulase. -Coagulase is an enzyme and is only secreted by S. aureus protein. Coagulase leads to localised blood clotting, this is done by coagulase interacting with human prothrombin and cleaves it to form thrombin and thrombin will convert fibrinogen to fibrin and many of them will form cross links leading to blood clots.-Coagulase is only secreted by S. aureus-To test it, a staphylococcus strain is mixed with rabbit plasma it will form clots.

Question 4

Are they gram +ve or -ve?

Answer 4

+ve

Question 5

How does a catalase test work?

Answer 5

To differentiate staphylococcus from streptococcus bacteria the catalase test
Staphylococcus is positive for the catalase test.-H2O2 is added to the agar plate and when staphylococcus is present, H2O and O2 would be produced. 6 H2O2→ 6H2O + 3O2

Question 6

Which are Protein Factor A/Clumping factor positive/minus?

Answer 6

Negative: saprophyticus, epidermidis-
Positive: S.aureus

Question 7

Which are Mannitol positive/minus?

Answer 7

Positive: S.aureus and saprophyticus
Negative: epidermidis

Question 8

Which are Novobicin positive/minus?

Answer 8

Positive: saprophyticus
Negative: S.aureus epidermidis

Question 9

Suggest 3 others test to distinguish staphylococcus and steptococcus strains

Answer 9

S. aureus has protein A present on it surface which the other staphylococcus strains do not have. Protein A is able to bind to Fc region of antibodies which inhibits normal functions of antibodies and clumping factors causes blood plasma clots.
Can use an organism that utilise specific sugars (mannitol), which in present can identify S. aureus and S. saprophyticus strains.
Microscopy: S. aureus - grape like clusters vs. Streptococci - chain
Culture on plates: S. aureus -golden pigment

Question 10

Name 3 diseases caused by exotoxin release and describe how they arise?

Answer 10

Gastroenteritis
toxic shock syndrome
Staphylococcus Scalded Skin Syndrome - generalized exfoliation of the skin

Question 11

Describe 7 diseases that result from direct organ invasion

Answer 11

pneumonia
meningitis
osteomyelitis
Endocarditis:  is  a  violent  destruction  of  the  heart  valves  sudden  onset  of  high  fever  and  chills.   
Septic  arthritis
skin infections
blood sepsis

Question 12

Describe the envelope of the peptidoglycan layer?

Answer 12

S. aureus is a gram positive bacteria (violet stain) therefore its envelope consists of one lipoprotein membrane and a large peptidoglycan layer covered by a capsule and intracellular adhesion proteins.

Question 13

What 3 functions does the peptidoglycan layer have?

Answer 13

Peptidoglycan is very important in S. aureus as it is essential for their growth. For shape and structure, protection, which prevent osmotic lysis and platform to anchor surface proteins

Question 14

Describe the peptidoglycan structure?

Answer 14

Peptidoglycan contain a peptide chain (5 in S. aureus) and a glycan part. It is synthesised as a precursor inside of the cell and it is flipped out of the cell. Then the penicillin binding proteins (PBPs) cross-links the peptidoglycans, making a long sugar chain and joining glycans.

Question 15

What is the MOA to treat Staphylococcus aureus?

Answer 15

Penicillin looks for the transpeptidase substrate site on PBPs and instead of cleaving the last amino acid it cleaves on the penicillin and is attached there.
The four-membered β-lactam ring of penicillin is bound to enzyme DDtranspeptidase, an enzyme that when functional, cross-links chains of peptidoglycan that form bacterial cell walls. The binding of β-lactam to DD-transpeptidase inhibits the enzyme’s functionality and it can no longer catalyse the formation of the cross-links

Question 16

Discuss the resistance mechanisms developed by Staphylococcus aureus

Answer 16

-The S. aureus can get resistance by destroying penicillin and now all of the S. aureus are resistant to penicillin. S. aureus acquired a gene for β-lactamase, when secreted, it cleaves the β-lactam ring of penicillin and it can no longer inactivate PBPs.
Penicillinase

Question 17

How does the Methicillin’s mechanism of action differ and not result in resistance?

Answer 17

Methicillin cannot be cleaved by β-lactamase and so can still bind and inactivate penicillin binding proteins-Therefore even if S. aureus has acquired a β-lactamase and is resistant to penicillin it isn’t necessarily resistant to methicillin.

Question 18

How did MRSA arise

Answer 18

Methicillin resistant strains of S. aureus (MRSA)arise due to a mutation that produces PBP2A, which has low affinity for methicillin (and penicillin).
Low affinity causes methicillin to be unable to bind to inactivate the PBPs so peptidoglycan cross links can still form enabling normal formation of S. aureus cell wall.

Question 19

How was the MRSA outbreak tracked?

Answer 19

Cultured and sequenced MRSA

Strain only differed by 1 point mutation