3 Antimicrobial susceptibility testing

Question 1

What do we measure?

Answer 1

• laveste konc. (µg/ml or mg/L) som inhiberer vækst af teststammen (MIC)
• It is associated with an error of +/- one twofold
dilution
• MIC is not an accurate measurement as it
measures a range of MICs
• The MIC is not the concentration that kill the strain

Question 2

How do we measure it?

Methods

Answer 2

Dilution methods
• Liquid media
• Solid media
Agar diffusion methods
• Disk (tablet) method
• E-test

Question 3

• PD is ?

Answer 3

the study of the drug effects on the bacteria over time

Question 4

• PK is ?

Answer 4

the study of the changes of drug concentrationsin the body over time

Question 5

Factors affecting measurement

Answer 5

• Medium type (Mueller-Hinton, Iso-sensitest, Sensitest medium)
• Medium manufacturer
• Lot-to-lot variation for both medium and disks
• Effect of additives (e.g. blood)
• Inoculum size and concentration
• Incubation conditions (temperature and duration)
• Human factors (e.g. preparation of dilutions)

Question 6

Detektion af mecA :

Answer 6

• mecA is poorly expressed in vitro
• The 2 antibiotics used for detection are OXACILLIN or CEFOXITIN (good inducers of mecA)
• The gold standard for MRSA identification is PCR of mecA
• Strains resistant to oxacillin should be regarded as resistant to all β- lactams irrespective of their susceptibility to penicillins and cephalosporins.

Question 7

Detektion af ESBL :

Answer 7

Følsom for CLAVULANIC ACID
Følsom for CEFOXITIN
Resistant overfor CEFEPIME
PCR and sequencing are required for ESBL identification

If an isolate is not confirmed as an ESBL producer by PCR, the interpretation of penicillins and cephalosporins should not be changed.

Question 8

Betalactamer
- Ampicillin
Siger noget om:

Answer 8

Siger noget om følsomheden overfor andre amineopenicilliner såsom amoxicillin

Question 9

Betalactamer
-Amox/clav
Siger noget om:

Answer 9

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

Question 10

Betalactamer
- Cefazolin
Siger noget om:

Answer 10

It predicts susceptibility to other firstgeneration cephalosporins such as cephalexin, cefadroxil and cefalothin but there is no 100% cross-resistance
between these compounds.

Question 11

Betalactamer
- Cefoxitin
Siger noget om:

Answer 11

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

Question 12

Betalactamer
- Cefpodoxime
Siger noget om:

Answer 12

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

Question 13

Betalactamer
- Oxacillin
Siger noget om:

Answer 13

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).

Question 14

Betalactamer
- Imipenem
Siger noget om:

Answer 14

Last choice antibiotic for treatment of Gram-negative infections in humans. It’s use cannot be justified unless certain criteria are fulfilled!