Antibiotic susceptibility testing

Question 1

Forklar Susceptibility

Answer 1

The lowest concentration (µg/ml or mg/L) that inhibits
completely growth of the test strain (MIC)

(The MIC is not the concentration that kill the strain)

Question 2

Forklar forskellen imellem MIC og MBC

Answer 2

MIC er den mængde som skal til for at hæmme vækst

MBC er det der skal til for at dræbe vækst

Question 3

Hvilke metoder bruges til at måle MIC

Answer 3

Dilutions metodes:

Broth dilution:
(flere glas på række. Skyet betyder at der er vækst. Er den ikke skyet, så er det fordi væksten er hæmmet)

Agar diffusion metode:

• Disk (tablet) method (opklaring omkring små hvide pletter. Hvis der er opklaring er den følsom/susceptibel. Er der ingen omklaring er den resistent)
• E-test (Den dråbeformede opklaring omkring en strips)

Question 4

Forklar 1)Susceptible, 2)Intermediate og 3)Resistant

Answer 4

1)Inhibited by the concentrations
achieved with normal dosage

2)Clinical efficacy is possible at body
sites where the drug concentrates
or when dosage can be increased

3)Not inhibited by the concentrations
achieved with normal dosage

Question 5

What is a breakpoint ?

Answer 5

A breakpoint is a drug-specific value to interpret the
results of susceptibility testing and determine if an
antibacterial is potentially useful in the treatment of a
bacterial infection

Breakpoint
Definition MIC (µg/ml) Zone (mm)
Susceptible ≤ 4 ≥20
Intermediate 8-16 15-19
Resistant ≥ 32 ≤ 14

Question 6

Hvad er forskellen på Mikrobiologiske og kliniske breakpoints

Answer 6

• Microbiological breakpoints or epidemiological cut-off values are based on
MIC distribution ONLY

• IN ADDITION Clinical breakpoints are based on:
• PD/PK modeling from in vitro and in vivo studies
• Clinical and bacteriological outcome data from clinical trials
• PD is the study of the drug effects on the bacteria over time
• PK is the study of the changes of drug concentrations in the body over time

Question 7

Når man har fundet et break-point/MIC hvad er da det vigtigste ?

Answer 7

Regardless of the method
it is essential to follow a
standard and perform
quality control!

How is quality control done?
• Reference strains of different species are routinely included in
the testing
• The MIC (or inhibition zone diameter) of the reference strain
has to fall within a given rage to convalidate the test
• If not, the test is not validated and should be repeated

Question 8

Hvad bruges beta-lactam amox/clav til …?

Answer 8

Important for detection of ESBLs, which are susceptible to clavulanic acid.

Question 9

Hvad bruges beta-lactam Cefoxitin til …?

Answer 9

It is used for detection of MRSA and MRSP as well as ESBL detection (ESBLproducing E. coli are susceptible unless they contain an AmpC β-lactamase).
Staphylococci resistant to cefoxitin should be regarded as resistant to all β-
lactams (MRSA/MRSP).

Question 10

Hvad bruges beta-lactam Cefpodoxime til ?

Answer 10

Cefpodoxime, ceftazidime and/or cefotaxime are indicated for ESBL
detection, and are indicators of resistance to cefovecin.

Question 11

Hvad bruges beta-lactam Oxacillin til ?

Answer 11

It has diagnostic value for staphylococci only. The results are used for
identification of MRSA and MRSP. It is the most effective antibiotic for MRSP
detection. Staphylococci resistant to oxacillin should be regarded as resistant
to all β-lactams (MRSA/MRSP).