Antimicrobial suceptibility testing Flashcards
Methods for AST (2)
Dilution methods
Agar difussion methods
Dilution methods (2)
Broth dilution tests (QUANTITATIVE)
Agar dilution tests (QUANTITATIVE)
Agar difussion methods
Disk/tablet difussion test (SEMIQUIANTITATIVE)
Gradient difussion E-test (QUANTITATIVE)
How does the dilution methods give quantification of antimicrobial susceptibility
By MIC (minimum inhibitory concentration), and MBC (minimal bacteriocidal concentration)
What is MIC
LOWEST concentration that INHIBITS completely growth of the test train
What is MBC
LOWEST concentration that KILLS the test train
Broth dilution
Enables automated or semilatunated reading
Agar dilution method
Primarly used for research purposes, for example when strains have to be testes with one drug
Gradient difussion method (E-test) gives you
Inhibition zone
Bacterial growth
Antibiotic strip
MIC
AST standard commitee (3)
Clinical and laboratory standards instituite
EUCAST (European committee on antimicrobial susceptibility testing)
How is quality control done
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
AST procedure (4)
Adjust inopculum of bacteria
Add adjusted inoculum of plate
Incubate at 35 degrees C for 16-20 hrs
Read plate
WHat is a breakpoint
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
Suceptible
Strain is inhibted at blood concentrations achieved by standard dosage of the drug
Intermediare ((buffer zone)
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
Resistant
Strain is NOT inhibited at blood concentration achieve by standard dosage
Mixed cultures are frequent with
Otitis externa, wound infections and UTIs
What is used in the lack of approved clinical breakpoints to predict susceptibiliy to other drugs from same class
Surrogate antimicrobial
What surrogate antimicrobials are used (4)
Ampicillin for amoxicillin
Sulfamethoxazole for sulfadiazine
Lincomycin for clindamycin
Tetracycline for doxytetracycline
What other drugs are used for MRSA and MRSP identification and why
Oxacillin and Cefoxitin
Because the meticillin resistance gene mecA is poorly expressed in lab medias
MRSA expert rule
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
ESBL confer resistance to
3rd generation cephalosporins susch as cefpodoxine, cefotaxime, ceftazidime
ESBL producing strains are suceptible to (3)
Amoxicillin, clavulanic acid and cefoxitin unless they contain an additional beta lactamase of AmpC type
