Exam 3: Rose Testing Mechs Flashcards
Serology
- blood testing
Direct detection
- atigen, staining, assays
Culture
- biochem reactions
- antisera
- molecular methods
- susceptibility
Antimicrobial Susceptibility Testing.. what info do you want to find from this?
Minimum inhibitory concentration and minimum bactericidal concentration
Minimum inhibitory concentrations… are they set concentrations?
- no they differ in each person
Susceptibility breakpoints
- concentration that separates the populations of micro organisms
- classifications are:
- susceptible
- intermediate
- resistant
- nonsusceptibile
Intermediate susceptibility
- infection may be treated with higher doses or drug conectrations istes
Nonsusceptible
- newer antimicrobials with few resistant strains
What is point of susceptibility breakpoint?
- provide info to clinicians to select optimal antibiotic therapy
- may be set for drug classes but unique to each organism and an antibiotic
MIC… low numbers next to antibiotics more or less potent?
- more potent
MIC50
- concentration at which 50% of organism population is halted by the selected antibiotic
MIC 90
- often reported in surveillance studies
What other factors to consider when antibiotic with lowest mc50/mc90 is not the best
- protein binding (only free drug available for activity)
- tissue penetration
- suceptibility breakpoints
Disk Diffusion Test (Kirby-Bauer) (qual vs quantitative, method, how long it takes, pros/cons)
- QUALitative
- organism streaked across surface of agar followed by antibiotic disks
- 18-24 hour incubation, inhibition zones are measured
- results as resistant, sensitive, or intermediate
- advantages: speed, low cost, minimum labor
- cons: no MBC, misinterpretation of new and rare resistance
Broth Dilution Test
- QUANtitative
- macrotube vs microtube
- doesn’t tell if bacteria is killed but does show inhibition of growth if not turbid
Macrotube Broth dilution test
- serial two-fold dilutions of antibiotic made into growth medium.. add bacteria at standard dose
- tubes examined for turbidity 18029 horus
- MIC defined as tube containing highest dilution of antimicrobial inhibiting visual growth
Microtube Broth dilution Test
- smaller volumes in plastic microtiter plates
- adaptability to automation
- pros: can determine MBCs and automated
- cons: labor intensive and time consuming, difficult on large scale
Automated susceptibility testing
- reports susceptibility of up to 25 agents after 4-24 hours of incubation
- pros:
- reduced labor time/costs
- reproducibility
- data management
- rapid results: most <12 hours
- Cons
- increased equipment costs
- predetermined panels
- inability to test all clinically relevant pathogens
- inaccurate detection of resistant phenos
- can have inducable resistance
Epsilometer Test (ETEST) (what it does and how to read it)
- QUANtitative
- similar to disk diffusion
- bacteria streaked on agar, place strip with gradient of antibiotic onto plate
- incubate 18-24 hours
- tear shaped zone of inhibition
- MIC read as lowest point of intersection on E-strip
Pros and Cons of Etest
- pros:
- quantify MIC
- easy to perform, high reproducibility
- multiple antibiotics can be tested per plate
- cons:
- expensive
- MBC cannot be determined
Minimum Bactericidal Concentration (how to get there from Broth test)
- get the MICs
- dilutions of the antimicrobial MICs (ones that didn’t grow) are subcultured onto antibiotic-free agar
- plates incubated for 18-24 hours and then examined for growth
- lowest conc of antibiotic able to kill >99.9 of original inoculum is MBC
MBC fun facts
- not routinely performed
- not standardized between laboratories
- may be determined for serious infections (meningitis, endocarditis)
Tolerance in terms of MIC and MBC
MIC/MBC >/= 16 is tolerance
MIC and clinical utility
- used to determine selection of definitive antibiotic therapy
- pathogen susceptible to antibiotic will be reported as
- susceptible: below achievable blood [ ]
- intermediate: approaching MIC blood [ ]
- resistant: MIC above achievable antibiotic blood [ ]
