Lecture 4: Waves of Resistance S. Aureus

Question 1

What is the common microbiology or S. Aureus? (toxins and enzymes)

Answer 1

Gram positive cocci with many virulent factors

Toxins: cytotoxins, superantigens, enterotoxins, TSSST-1, exfoliative toxins

Enzymes: protease, lipase, hyaluronidase

Question 2

What is the mode of transmission of S. Aureus?

Answer 2

Contact

Question 3

What are the clinical manifestations of S. Aureus?

Answer 3

Skin and soft issue infections
Pneumonia
Osteomyelitis
Bacteraemia
Endocarditis
Toxin-mediated disease

Question 4

What is the epidemiological niche of S. Aureus?

Answer 4

Nasal carriage
GI tract
Perineal
Thorat

Question 5

What percentage of people carry S. Aureus in their nasal carriage? What is the further breakdown of carriers?

Answer 5

30% of adults

20% persistent carriers
60% transient carriers
20% never carriers

Question 6

What are the risk groups associated with high carriage rates of S. Aureus?

Answer 6

Diabetes Mellitus
Dialysis patients
HIV
Chronic skin diseases
IV Drug Abusers
Health care workers

Question 7

What are the 4 mechanisms of resistance of Staphylococcus aureus?

Answer 7

1. Modifying enzymes
2. Degrading enzymes
3. Target change
4. Efflux pumps

Question 8

What does mecA (acquired resistance gene) encode for?

Answer 8

Encodes for new penicillin binding protein

Question 9

What does the mecA gene product PB2a encode for?

Answer 9

Takes over cell wall synthesis

Question 10

How was penicillin resistance first acquired?

Answer 10

Acquisition of bacteriophage

Carried Beta-lactamase that got incorporated into the genome

Question 11

How is S. aureus blaZ resistance acquired?

Answer 11

Induction of staphylococcal Beta-lactamase synthesis in the presence of the Beta-lactam antibiotic penicillin I

DNA-binding protein BlaI binds to the operator region, repressing RNA transcription from both blaZ and blaR1-blaI

Question 12

What does binding of penicillin to the transmembrane sensor-transducer BlaR1 stimulate?

Answer 12

Stimulates BlaR1 autocatalytic activation

Question 13

What does BlaR1 have action on?

Answer 13

Cleaves BlaI into inactive fragments
Allows transcription of both blaZ and blaR1-blal to commence

Question 14

What is Beta-lactamase encoded by?

Answer 14

Encoded by blaZ

Question 15

What is the mode of action of Beta-lactamase?

Answer 15

Hydrolyses the Beta-lactam ring of penicillin

Rendering it inactive

Question 16

What is mecA?

Answer 16

Part of a large, mobile, genetic element - Staphylococcal cassette chromosome mec (SCCmec)

Question 17

What is PBP2 encoded by and what is its mode of action?

Answer 17

Capable of cell wall synthesis

Encoded by mecA

Has low affinity for all Beta-lactams

Question 18

What has caused variance in MRSA isolates?

Answer 18

Selective pressures over decades

Genetic diversity of early isolates showed not all evolved at the same time

Question 19

What are stringents?

Answer 19

Physical stress to an organism to initiate resistance

Question 20

What are the 3 guidelines of susceptibility for S. Aureus isolates?

Answer 20

MIC of vancomycin is <4 ug/ml are susceptible

8-16 ug/ml are intermediate

> 32 ug/ml are resistant

Question 21

What is a heteroresistant strain of S. Aureus?

Answer 21

Susceptible to vancomycin

Contain subpopulations of organisms for which the MIC of vancomycin is in the intermediate range

Question 22

What are the 2 methods of vancomycin resistant staphylococci?

Answer 22

Re-engineered

Removal of terminal amino acid

Question 23

What does the vanA operon from the enterococcus change?

Answer 23

Changes the cell wall pentapeptide seqyence

Question 24

What odes VanY do?

Answer 24

VanY encoded by the vanA gene cluster modifies finished native peptidoglycan precursor

Question 25

What does vancomycin do to the VanS protein?

Answer 25

Causes the VanS protein to autophosphorylate

Turns to phosphorylate VanR

Question 26

What does phosphorylated VanR protein have an effect on?

Answer 26

Binds to the promoter region of vanHAX

Drives transcription that encode essential structural molecules of gene cluster for peptidoglycan

Question 27

What does VanH protein do?

Answer 27

Converts pyruvate into D-lactate

Combined with D-alanine by VanA ligase to create D-Ala-D-lac

Question 28

What does VanX dipeptidase do?

Answer 28

Hydrolyzes D-Ala-D-Ala reducing the pool of D-Ala-D-Ala available to make the vancomycin-susceptible peptidoglycan precursor

Question 29

What does VanY do?

Answer 29

Accessory structural protein that removes the terminal D-Ala residue from the peptidoglycan precursor

Carboxypeptidase augments glycopeptide resistance by removing residual vancomycin binding sites

Question 30

When was the first VRSA strain reported?

Answer 30

2002

Question 31

When was the first MRSA isolate identified?

Answer 31

1961

Question 32

When was the first VISA isolate identified?

Answer 32

1997

Question 33

How does the spread of MRSA occur?

Answer 33

Spread is mainly clonal
(single isolate spread horizontally across community)

Lapses in infection control causes spread

Question 34

How does the role of antibiotic pressure contribute to the clonal spread of MRSA?

Answer 34

Loss of intrinsic protection by antimicrobials

Makes more susceptible to resistant infection

Question 35

What is Panton Valentine leukocidine?

Answer 35

Pore forming cytotoxin

1. Pvl phage infects MSSA
2. Prophage integrated into S. Aureus chromosome carrying pvl genes
   3.MRSA produces PVL toxin

Question 36

How is MRSA controled?

Answer 36

Hospitals:
Infection control
Antibiotic control

Community:
Hard as it is difficult to isolate

Question 37

What are the treatments of community-acquired MRSA?

Answer 37

Clindamycin
TMP-SMX
Rifampin
Vancomycin

Question 38

What are the treatments of hospital-acquired MRSA?

Answer 38

Vancomycin
Linezolid
Daptomycin