Antimicrobial Susceptibility Testing PART 4

Question 1

Mechanisms of resistance:

-DRSP

Answer 1

1. Altered Target Site
2. PBP-2 Penicillin
   Binding Protein

Question 2

Mechanisms of resistance:

- ESBL

Answer 2

1. Arise from point mutations of common beta-lactamases 2. TEM-1 and SHV-1 genes coding for enzyme

Question 3

Mechanisms of resistance:

- VRSA/GRSA

Answer 3

1. May involve alterations in the cell wall and hyperexpression of PBP
2. vanA gene present

Question 4

Mechanisms of resistance:

- MRSA

Answer 4

1. Alteration of target site (classic and modified)

2. Hyperproduction of beta-lactamases

Question 5

Mechanisms of resistance:

- VRE

Answer 5

1. altered precursor formation in cell wall synthesis 2. Carried by transposons or plasmids

Question 6

Mechanisms of resistance:

- ARHI

Answer 6

1. Plasmid-mediated beta-lactamases (most common)

Question 7

Mechanisms of resistance:

- CRE

Answer 7

1. carbapenemase (breaks down antibiotic) 2. cephalosporinase combined with porin loss

Question 8

Lab detection of:

- MRSA

Answer 8

Cefoxitin Disk Diffusion: Induces mecA gene (best drug to detect MRSA)
Media: CAMHB with 2%NaCl; Incubation: 24 Hours, 35’C, Ambient Air;
MRSA Agar Screen: Meuller-Hinton Agar + 4%NaCl + 6 mcg/mL Oxacillin;

Question 9

Lab detection of:

- DRSP

Answer 9

Disk Diffusion using Oxacillin; Media: Mueller-Hinton Agar, 5% SBA; Incubation: 20-24 Hrs, 35’C in CO2;
Broth Dilution: Media CAMHB with 2-5% LHB; Incubation: 20-24 Hrs at 35’C, ambient air;
E-Test Media: Mueller-Hinton agar, 5% SBA; Incubation: 20-24 Hours 35’C in CO2

Question 10

Lab detection of:

- ARHI

Answer 10

Disk Diffusion: Media: HTM agar; Incubation: 16-18 Hrs, 35’C in CO2; Broth Dilution: Media HTM Broth; Incubation: 20-24 Hours, 35’C, Ambient Air; Nitrocefin assay: Chromogenic cephalosporin, If pos, isolate is resistant to penicillin, ampicillin, and amoxicillin/If neg, isolate can be resistant by other mechanisms

Question 11

Lab detection of:

- VRE

Answer 11

Disk Diffusion: Media: Mueller-Hinton Agar; Incubation: 24 Hrs, 35’C, Ambient Air; Broth Dilution: Media: CAMHB; Incubation: 24 Hrs, 35’C, ambient air; Agar Screen Test: Media: BHI with 6ug/mL vacomycin; Incubation: 24 Hrs, 35’C, Ambient air, any growth = resistance; Confirmation of VRE with gram stain

Question 12

Lab detection of:

- VRSA/GRSA

Answer 12

Disk Diffusion and Broth Dilution tests are held for 24 Hrs; Agar Screen: Media: BHI with 6ug/mL Vancomycin, Incubation: 24 Hrs, 35’C, ambient air; D-Test Disk Induction: Erythromycin and Clindamycin disks are placed 15mm apart and flattening is noted, Pos=Resistant to clind, Neg=suscep

Question 13

Lab detection of:

- ESBL

Answer 13

Confirmatory Test: Use cefotaxime and ceftazidime disks and each disk combined with clavulanic acid; >5mm increase with clav=pos; Can also use E-Test Strips, Vitek-2, and Microscan

Question 14

Lab detection of:

- CRE

Answer 14

Modified Hodge Test: 1:10 Dilution of 0.5 Mcfarland (E.coli lawn)(because larger zone size with decreased inoculum), streaked with known + and - and Patient. Shouldering occurs if enzyme present to break down meropenam = pos result/resistance