Glossary of antimicrobial susceptibility testing
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- 3 types of antimicrobial therapy
- empiric amtimicrobial therapy
- based on diagnosis and probability of susceptibility; 70% of prescriptions are empiric, correct most of te time
- directed antimicrobial therapy
- following susceptibility testing; makes use of the report. only 12% so what are we working for? confirms the efficacy of empiric usually
- prophylactic antimicrobial therapy
- why do a susceptibility test?
- -epidemiologic studies, to build up antibiograms - database of organisms in each hospital
- when antimicrobial susceptibiity testing is not necessary:
- -when organism is routinely suscept. or resistant to certain antibioitcs.
eg., GNB against PENICILLIN.. duh
-when orgn is NORMAL FLORA!
- factors that affect performance of antimicrobial test
- -inoculum size
-incubation time,temp, atm
- how does inoculm size affect antimic testing
- must be standardized with mcfarland standard
- how would incubating the organism for too short of time affect the test results?
- it would look susceptible because didnt have time to grow
- what type of media to use
- mueller hinton agar; neutral pH, ca/mg concentration, specific depth for disk diffusion
- how would an expired antibiotic affect test results?
- organism would look more resistant because antibiotic wasn't working as well.
- how would a delay between prep of standard and actual testing affect results?
- organism continues to grow and so looks more resistant than it is
- advantages of kirby-bauer
- simple, qquick, flexible in which antibiotic to use
- qualitative, not quantitative.
- disk diffusion method is aka
- principle of agar dilution method
- make serial dilutions of the antibiotic; add bacteria; find the MIC
- the lowest concentration of antibiotic necessary to inhibit the bacteria so that there is no visible growth
- advantages of agar dilution method
- -QUALITATIVE AND QUANTITATIVE
-can test many organisms at same time
- principle of schlicter test
- tests the serum of patient to see MIC and MBC of the actual serum, not just standardized concentrations
- what do antimicrobial synergy and antagonism test?
- the additive or interfering effects of combinations of antibiotics.
- why a patient would need more than one antibioitc
- mixed infections
use lower doses of 2 antib so that higher, toxic doses arent necessary
- 3 types of automated susceptibility testing
- what does a b-lactamase test test for?
- the presence of b-lactamase; that's an enzyme that breaks down the b-lactam ring in penicillin or cephalosporins
- 3 b-lactamase testing methods
-chromogenic cephalosporin disk
- how does iodometric method work
- add iodine to incubated inoculation.
if acid, iodine will break down.
indicator is starch, and if breakdown, it is white. if dark brown, iodine is intact.
- how does acidometric method work?
- a pH indicator shows that acid is produced if the b-lactam ring of penicillin or cephalosporins is broken down
- how does the chromogenic ceph disk test work?
- if b-lactam ring of nitrocefin is broken down, there's a color change.
- diff btwn antimicrobial and antibiotic
- antimicrobial is designed to interfere with microbe growth in a host; can be synthetic or even ofor fungi (never virsues tho)
antibiotic is a drug made from antoher organism, designed to kill the bacteria.
- quorum sensing
- signalling by bacteria setting up an infection; mimicked in antibiotics to interfere and clear bugs via immune system
- aminoglycosides - specific toxicity
- oto (mid ear) and renal
- how hepatic function alters microbial action
- inability to metabolize antibiotic leads to toxicity
- factor to consider in admin antibacterials to diabetes mellitus patients
- absorb antibiotics poorly at intramuscular sites
- how renal function alters microbial use
- renal distress, can't handle antibiotics excreted by kidney; it builds up in tubules.
- bacterium tests susceptible; patient doesn't respond:
- 1. buildup of pus - acid; erythromycin and aminoglycos can't handle.
2. Hi Ca+ in bones inhibits antibiotics 3. multiple orgs at one site; confer
4. CNS infectn/abcess/osteomyelitis - drug can't penetrate.
5. vasc. occlusion - cant penetrate.
6. undetected B-lactamase produced
7. Need bactericidal, gave static.
8. Organism set up intracell. houskeeping; can't penetrate.
9. # cells at inoculum site too much for inoculum size to handle.
- why patient responds when shouldnt
- -acid ph activates tetracycline
-kidney concentrates antibiotic, kills UTI bacteria
-host defense removes bacteria w/out antibiotic
- advantages/limitations of kirby bauer
- adv: quick, simjple, flexible in which antibiotic is used- economical
limit: qualitative, not quantitative; only tells if susc. or resist, not HOW susc or resist; not as reproducible; agar depth easy to mess up
- name of instrument that delivers bug to dilution plates in agar dilution
- steer's replicator
- minimal concentration that results in a 99.9# reduction in the colony-forming-units per ml of drug.
minimum conc. of drug necessary to kill the bacteria, not just inhibit it.
- advantages/limitations of e-test
- adv: QUantitative, esp good for fastidious organism tests cuz can grow on enriched media;
limit: depth is crucial; dropping strip wrong onto plate alters diffusion of drug into media.
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