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Antibiotics Susceptibility
Test
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Antibiotics Susceptibility Test
Two tests can be used: dilution series test andagar diffusion
test
Dilution series testantibiotic agents areprepared in the
nutrient medium, inoculatedwith organism, incubated, then the
lowest
growth-inhibiting concentration level aredetermined
Very time-consuming
Kayser, Medical Microbiology 2005 Thieme
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Antibiotics Susceptibility Test
Agar diffusion testinoculation of nutrient
agar plate with test strain, then disks of filterpaper
containing antibiotic are placed uponagar plate,
incubatedinhibition zone
around the disks (whether or not theydevelop and their size)
provide information
on the resistance of the microorganismstested
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Minimal Inhibitory Concentration
MIC: the lowest concentration of drug thatinhibits the growth of
the organism
The MIC is determined by inoculating the
organism isolated from the patient into aseries of tubes or cups
containing 2-folddilutions of the drugdilution methods
After incubation at 35C for 18 hours, thelowest concentration of
drug that preventsvisible growth of the organism is the MIC
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Disk diffusion methoddisks impregnated withvarious antibiotics
are placed on the surface ofan agar plate that has been inoculated
with the
organism isolated from the patientAfter incubation at 35C for 18
hours, during
which time the antibiotic diffuses outward fromthe disk, the
diameter of the zone of inhibition is
determined. The size of the zone of inhibition iscompared with
standards to determine thesensitivity of the organism to the
drug.
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Minimum Bactericidal Concentration
The minimum bactericidal concentration(MBC): the smallest
concentration of asubstance required to kill 99.9% of the
cells in an inoculum.
Determined using quantitative subculturesfrom the
macroscopically unclouded tubes
or (microplate) wells of an MIC dilutionseries.
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Determination of minimal inhibitory concentration (MIC) and
minimal bactericidalconcentration (MBC). In the top part of the
figure, the patient's organism is added to tubescontaining
decreasing amounts of the antibiotic. After incubation at 37C
overnight, growth
of the bacteria is observed visually.The lowest concentration of
drug that inhibits growth,ie, 3.1 pg/mL, is the MIC. However, at
this point, it is not known whether the bacteria havebeen killed or
whether the drug has only inhibited their growth.To determine
whether thatconcentration of drug is bactericidal, ie, to determine
its MBC, an aliquot (0.1 mL) from thetubes is plated on an agar
plate that does not contain any drug.The concentration of drugthat
inhibits at least 99.9% of the bacterial colonies, ie, 6.2 pglmL,
is the MBC.
Levinson W. Medical Microbiology and Immunology 8th Edition.
Appleton & Lange.
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Antibiotic sensitivity testing.A zone of inhibition surrounds
severalantibiotic-containing disks. A zone of certain diameter or
greater indicatesthat the organism is sensitive.Some resistant
organisms will grow all theway up to the disk eg, disk N.
Levinson W. Medical Microbiology and Immunology 8th Edition.
Appleton & Lange.1998.
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Antibiotics Susceptibility Test
This method, also known as the disk test, is used to test
the
resistance of a bacterial culture to various anti-infective
agents.
The method provides a basis for classification of a
bacterialstrain as susceptible, resistant, or intermediate
according to
the dimension of the inhibition zone.
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Antibiotics Susceptibility Test
Each point represents a bacterial strain. The size of the
inhibition zone is
determined in the agar diffusion test (disk test) and the
minimum inhibitory
concentration (MIC) in the dilution test. The MIC corresponds
logarithmically (log2)
with the diameter of the inhibition zone.
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Antibiotics Susceptibility Test
Interpretation of the results: the MICs or inhibition zonesare
brought into relation with the substanceconcentrations present at a
site of infection at standarddosage levels
This calculation is based on various pharmacokinetic(serum
concentration, half-life) and pharmacodynamicparameters
(bactericidal activity or not, postantibioticeffect, etc.)
Also based on clinical experience gained from therapy
ofinfections with pathogens of given suceptibility.
Such data are used to establish general guideline valuesdefining
the boundary between susceptible and resistantbacteria
Kayser, Medical Microbiology 2005 Thieme
