microbiologic lab teests in ID and rapid diagnostic tests Flashcards
MIC/MBC testing - broth dilution
micro v macro dilution - difference in volume of broth used
two-fold (2,4,8,16, etc) dilutions of drug concentration in liquid media - only difference between tubes is drug conc
standard bacterial inoculum (10^5-6 CFU/ml) added to each tude; incubated at 35 C for 18-24 hours
MIC - lowest conc that prevents VISIBLE growth (unaided eye)
MBC - lowest conc resulting in over 99.9% decrease in initial inoculum
MIC - agar dilution
“drug in plate”
two fold dilutions of AB incorporated into molten agar, poured into petri dishes and allowed to solidify
bacterial inoculum applied to surface of agar; incubate at 35 C for 18-24 hours, inspect for growth
MIC - agar plate with lowest conc of drug and no growth of organisms
dots = different organisms
MIC - epsilometer test (Etest)
antibiotic impregnated plastic strips containing a continuous conc gradient of drug
mircoorganism streaked onto surface of agar plate; etest strip applied to surface of plate, incubate for 24 hours, elliptical zone of inhibition formed
MIC - conc on strip where inhibition ellipse intersects the scale of the strip
more “precise” MIC values compared to conventional two-fold dilutions
Automated AST systems
Vitek 2 - uses reagent cards
microscan - algorithm-dervied MIC
BD phoenix - algorithm-derived MIC
susceptible
isolates are inhibited by usually achievable conc when normal dosing regimens are used
susceptible-dose dependent
S-DD
susceptibility is dependent on the dosing regimen used (higher dosing)
intermediate
may be useful when max doses are used or if drug is conc at the site of infection
resistance
never use
how are breakpoints esablished?
clinical pharmacology of the drug
clinical and bacteriologic response from human studies
distribution of MICs within a bacterial population
phenotypic detection of bacterial isolated with certain resistance mechanisms
inoculum effect
increase in MIC when a higher than standard inoculum of bacteria is used in the susceptibility test (i.e. inoculum is over 10^5 CFU/ml)
seen more frequently with B-lactams
important because in the lab our conc is always 10^5 but what is it in a patient???
tolerance
MBC greater than or equal to 32 x MIC
clinical significance not well known - has been associated w poor outcome
rarely identified clinically because MBCs are not routinely determined
MIC stats
in a population of bacteria (same strain, different samples) in which the MIC for each isolate has been determined
MIC50: antibiotic conc that inhibits 50% of the bacteria tested
MIC 90: AB conc that inhibits 90% of bacteria tested
geometric mean MIC - antilog of the mean of the log MICs
modal MIC - most frequently occurring MIC
disc diffusion
fixed amount of AB in commerically supplied disc
placed onto agar plate streaked w organism and incubated
zone of inhibition created
big zone doesnt necessarily mean it’s our best drug clinically - i.e. vanc won’t move that quickly
growth within a zone is considered resistance
synergy
the activity of an antimicrobial combination is greater than that expected from the additive activity of the individual agents
1+1=5
antagonism
the activity of an antibicrobial combination is less than that expected from the additive activity of the individual agents
2+2=3