Antimicrobial suceptibility testing

Question 1

Methods for AST (2)

Answer 1

Dilution methods

Agar difussion methods

Question 2

Dilution methods (2)

Answer 2

Broth dilution tests (QUANTITATIVE)

Agar dilution tests (QUANTITATIVE)

Question 3

Agar difussion methods

Answer 3

Disk/tablet difussion test (SEMIQUIANTITATIVE)

Gradient difussion E-test (QUANTITATIVE)

Question 4

How does the dilution methods give quantification of antimicrobial susceptibility

Answer 4

By MIC (minimum inhibitory concentration), and MBC (minimal bacteriocidal concentration)

Question 5

What is MIC

Answer 5

LOWEST concentration that INHIBITS completely growth of the test train

Question 6

What is MBC

Answer 6

LOWEST concentration that KILLS the test train

Question 7

Broth dilution

Answer 7

Enables automated or semilatunated reading

Question 8

Agar dilution method

Answer 8

Primarly used for research purposes, for example when strains have to be testes with one drug

Question 9

Gradient difussion method (E-test) gives you

Answer 9

Inhibition zone
Bacterial growth
Antibiotic strip
MIC

Question 10

AST standard commitee (3)

Answer 10

Clinical and laboratory standards instituite

EUCAST (European committee on antimicrobial susceptibility testing)

Question 11

How is quality control done

Answer 11

Reference strains of different species are routinely included in the testing
MIC of reference strains has to fall within a given range to convalidate the test
If not test is not validated and should be repeated

Question 12

AST procedure (4)

Answer 12

Adjust inopculum of bacteria
Add adjusted inoculum of plate
Incubate at 35 degrees C for 16-20 hrs
Read plate

Question 13

WHat is a breakpoint

Answer 13

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

Question 14

Suceptible

Answer 14

Strain is inhibted at blood concentrations achieved by standard dosage of the drug

Question 15

Intermediare ((buffer zone)

Answer 15

Clinical efficacy is possible if the strain infects body sites where the drug concentrates or if the dosage can be increased compared to starndad dosage

Question 16

Resistant

Answer 16

Strain is NOT inhibited at blood concentration achieve by standard dosage

Question 17

Mixed cultures are frequent with

Answer 17

Otitis externa, wound infections and UTIs

Question 18

What is used in the lack of approved clinical breakpoints to predict susceptibiliy to other drugs from same class

Answer 18

Surrogate antimicrobial

Question 19

What surrogate antimicrobials are used (4)

Answer 19

Ampicillin for amoxicillin
Sulfamethoxazole for sulfadiazine
Lincomycin for clindamycin
Tetracycline for doxytetracycline

Question 20

What other drugs are used for MRSA and MRSP identification and why

Answer 20

Oxacillin and Cefoxitin

Because the meticillin resistance gene mecA is poorly expressed in lab medias

Question 21

MRSA expert rule

Answer 21

Strains resistant to oxacillin/cefoxitin should be regarded as resistant to all beta lactams irrespective of their susceptibility to penicillin and cephalosporin in the report

Question 22

ESBL confer resistance to

Answer 22

3rd generation cephalosporins susch as cefpodoxine, cefotaxime, ceftazidime

Question 23

ESBL producing strains are suceptible to (3)

Answer 23

Amoxicillin, clavulanic acid and cefoxitin unless they contain an additional beta lactamase of AmpC type