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Antimicrobial Drugs Measuring Drug Susceptibility … 1/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology Minimal Inhibitory Concentration (MIC)
Minimum Inhibitory Concentration is the lowest drug concentration that
prevents visible microorganism growth after overnight incubation. LEARNING OBJECTIVES Analyze data to interpret minimal inhibitory concentration
values KEY TAKEAWAYS Key Points Minimum inhibitory concentration (MIC) can be
determined by culturing microorganisms in liquid media
or on plates of solid growth medium.
A lower MIC value indicates that less drug is required
for inhibiting growth of the organism; therefore, drugs
with lower MIC scores are more e ective antimicrobial
agents.
By identifying appropriate drugs and their e ective
concentrations, MIC scores aid in improving outcomes
for patients and preventing evolution of drug-resistant
microbial strains.
Key Terms culture: The process of growing a bacterial or other biological entity in an arti cial medium.
minimum inhibitory concentration: This is the lowest concentration of an antimicrobial drug that prevents … 2/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology visible growth of a microorganism after overnight
incubation with media.
bacteriostatic: A drug that prevents bacterial growth and reproduction but does not necessarily kill them.
When it is removed from the environment the bacteria
start growing again. De nition and Measurement
In microbiology, minimum inhibitory concentration (MIC) is the lowest
concentration of an antimicrobial (like an antifungal, antibiotic or
bacteriostatic) drug that will inhibit the visible growth of a microorganism
after overnight incubation. MICs can be determined on plates of solid
growth medium (called agar, shown in the “Kirby-Bauer Disk
Susceptibility Test” atom) or broth dilution methods (in liquid growth
media, shown in ) after a pure culture is isolated. For example, to identify
the MIC via broth dilution, identical doses of bacteria are cultured in
wells of liquid media containing progressively lower concentrations of
the drug. The minimum inhibitory concentration of the antibiotic is
between the concentrations of the last well in which no bacteria grew
and the next lower dose, which allowed bacterial growth. There are also
several commercial methods available to experimentally measure MIC
values. Signi cance and
Applications
An MIC is generally regarded as
the most basic laboratory
measurement of the activity of an
antimicrobial agent against an
organism. Because a lower MIC
value indicates that less of the
drug is required in order to inhibit
growth of the organism, drugs
with lower MIC scores are more
e ective antimicrobial agents. Microbroth Dilution Method: To
identify the lowest concentration
required for a given antibiotic to
inhibit bacterial growth, an identical
amount of bacteria is introduced
into wells of liquid media containing … 3/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology Currently, there are a few webbased, freely accessible MIC
databases. MIC scores are
important in diagnostic
laboratories to con rm resistance
of microorganisms to an
antimicrobial agent and also to
monitor the activity of new
antimicrobial agents. Clinicians
use MIC scores to choose which
antibiotics to administer to progressively lower concentrations
of the drug. (Here, the dilution series
of the drug is set up from left to
right: for example, well E1 might
contain 100 units of drug; E2, 50
units; E3, 25 units; E4, 12.5 units;
etc.). Because bacterial growth
made the media in well E5 cloudy
and the media in well E4 is
indistinguishable from clear media,
this indicates that the minimum
inhibitory concentration is between
the drug concentrations in wells E4
and E5. (Image courtesy of Microrao,
Dept. of Microbiology, JJMMC,
Davangere). patients with speci c infections
and to identify an e ective dose
of antibiotic. This is important because populations of bacteria exposed
to an insu cient concentration of a particular drug or to a broadspectrum antibiotic (one designed to inhibit many strains of bacteria) can
evolve resistance to these drugs. Therefore, MIC scores aid in improving
outcomes for patients and preventing evolution of drug-resistant
microbial strains. Kirby-Bauer Disk Susceptibility Test
Kirby-Bauer testing measures sensitivity of bacteria to antibiotics by
culturing bacteria on solid growth media surrounding sources of drug. LEARNING OBJECTIVES Review the procedure for the Kirby-Bauer antibiotic test KEY TAKEAWAYS Key Points … 4/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology KB tests are performed under standard conditions, so
the minimum inhibitory concentration for a given
antibiotic can be calculated by comparing the observed
zone of inhibition ‘s size to known values.
Clinicians use KB test results to choose antibiotics
e ective against the speci c bacteria causing a
patient’s infection. Using speci cally-targeted antibiotics
helps decrease the frequency of drug-resistant bacteria
evolving.
Key Terms Kirby-Bauer antibiotic testing: This is a method to determine the sensitivity of microorganisms to speci c
antimicrobial drugs; greater drug e cacy yields larger
microbe-free zones surrounding drug-containing disks
after overnight growth on solid media.
zone of inhibition: This is an area of media where bacteria are unable to grow, due to presence of a drug
that impedes their growth.
minimum inhibitory concentration: This is the lowest concentration of an antimicrobial drug that prevents
visible growth of a microorganism after overnight
incubation with media. Kirby-Bauer antibiotic testing (also called KB testing or disk di usion
antibiotic sensitivity testing) uses antibiotic-containing wafers or disks to
test whether particular bacteria are susceptible to speci c antibiotics.
First, a pure culture of bacteria is isolated from the patient. Then, a
known quantity of bacteria are grown overnight on agar ( solid growth
media) plates in the presence of a thin wafer that contains a known
amount of a relevant antibiotic. If the bacteria are susceptible to the
particular antibiotic from a wafer, an area of clear media where bacteria
are not able to grow surrounds the wafer, which is known as the zone of
inhibition. A larger zone of inhibition around an antibiotic-containing disk
indicates that the bacteria are more sensitive to the antibiotic in the disk. … 5/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology KB tests are performed under
standardized conditions and
standard-sized zones of inhibition
have been established for each
antibiotic. KB test results are
usually reported as sensitive,
intermediate, or resistant, based
on the size of the zone of
Kirby-Bauer test: In Kirby–Bauer
testing, discs containing antibiotics
are placed on agar where bacteria
are growing, and the antibiotics
di use out into the agar. If an
antibiotic stops the bacteria from
growing, one can see circular areas
around the wafers where bacteria
have not grown. inhibition. If the observed zone of
inhibition is greater than or equal
to the size of the standard zone,
the microorganism is considered
to be sensitive to the antibiotic.
Conversely, if the observed zone
of inhibition is smaller than the
standard size, the microorganism
is considered to be resistant. The size of a zone of inhibition in a KB test is inversely related to the
minimum inhibitory concentration (MIC), which is the amount of antibiotic
required to prevent bacterial growth in an overnight culture. The MIC (in
µg/ml) can be calculated from known standard-curve (linear regression)
graphs based on the diameter of the observed inhibition zone diameter
(in millimeters).
Clinicians can use KB test results to choose appropriate antibiotics to
combat a particular infection in a patient. Administering antibiotics that
speci cally target the particular bacteria that are causing the infection
can avoid using broad-spectrum antibiotics, which target many types of
bacteria. Thus, clinical application of KB testing results can decrease the
frequency with which antibiotic-resistant bacteria evolve. Previous Next … 6/7 9/25/2019 Measuring Drug Susceptibility | Boundless Microbiology … 7/7 ...
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