AA susceptibility and antibiotic resistance

Question 1

Minimum inhibitory concentrations (MIC)

Breakpoint

Answer 1

Lowest concentration of an agent that inhibits the growth of the test organisms
Determined within a set time period that can be reproducible

Value at which an organism is defined as S, I or R

Question 2

Disc diffusion

Answer 2

Diffusion of antibiotic from disc which results in a concentration gradient
Disc are impregnated with antibiotic are placed onto inoculated agar
* Disc absorbs water from agar
o Antibiotic diffuses out through the agar

Question 3

E test

Answer 3

Gradient of antimicrobial strips embedded on the underside
An ellipse of inhibited growth is produced around the strip which intersects at the MC

Question 4

Agar dilution

Answer 4

Antibiotic incorporated into molten agar then poured

Question 5

Broth dilution

Answer 5

A series of dilutions of antibiotic prepared in broth

Question 6

Detect beta lactamases, ESBL and carbapenems

Answer 6

Beta lactamases detected using EDTA or boronic acid

ESBL
* Screen using agar dilution plates
* Confirm with modified casal’s test

Carbapenems
* Phenotypic screen with meropenem plate
* PCR testing most reliable

Question 7

Detect staph and vancomycin resistance

Answer 7

Staph
Methicillin resistance detected by cefoxitin disc
* Inhibition zone <21mm
* Cefoxitin has replaced oxacillin
Penicillin resistance can also be detected using
* Nitrocefin disc
* Disc diffusion

Vancomycin
Detected by MIC by broth dilution/ automation or Etest

Question 8

What are modes of autoprotection

Answer 8

modification of antibiotic and efflux pump
mutation of target
modification of target

Question 9

What are transient changes and explain the mechanism

Answer 9

Expression of genes that express proteins to protect against the action of the antibiotics
Reduced entry of antibiotics through
* Decreasing outer membrane permeability
o Less expression of outer membrane porins
* Increased expression of efflux pumps

Question 10

Inducible resistance for macrolides

Answer 10

MsrA is a ATP binding cassette transporter to pump out the antibiotics
Mef encodes a major facilitator family pump

When expressed pumps confer resistance to 14 and 15 membered rings

Prevents binding of macrolides to the 23S rRNa = inhibition of translation

Question 11

Inducible resistance for vancomycin

Answer 11

VanHAX gene/ Vancomycin resistance operon
* Synthesis of the modified precursor peptides and removal of D-ala-D-ala precursor

o VanH - reduces pyruvate to D-Lac
o VanA - forms ester bond between D-ala and D-Lac
o VanX - cleaves D-ala-D-ala bond