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

-organism i.d.
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
-antibiotic stability
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
-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


-flow cytometry
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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