The minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism. MIC testing determines if a microbe is susceptible, resistant, or has intermediate resistance to an antibiotic. MIC is measured through agar or broth dilution methods by incubating microbes in solutions with increasing concentrations of an antibiotic and observing growth. The MIC provides guidance on effective antibiotic treatment strategies.

1. MINIMUM INHIBITORY CONCENTRATION
The minimum inhibitory concentration (MIC) is the lowest concentration of a
chemical, usually a drug, which prevents visible growth of a bacterium or
bacteria. MIC depends on the microorganism.
Minimal inhibitory concentration (MIC) defines in vitro levels of susceptibility
or resistance of specific bacterial strains to applied antibiotic. Reliable
assessment of MIC has a significant impact on the choice of a therapeutic
strategy, which affects efficiency of an infection therapy.

2. UNDERSTANDING KEY TERMS
Susceptible- Microorganism is inhibited
by concentration of antimicrobial agent
Resistant – Microorganisms is resistant
to concentration of antimicrobial agent
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.

3. MINIMUM INHIBITORY CONCENTRATION
The MIC is determined by preparing solutions of the chemical in vitro at
increasing concentrations
 Incubating the solutions with separate batches of cultured bacteria,
 Measuring the results using agar dilution or broth microdilution.
 Results have been graded into susceptible (often called sensitive),increased
exposure, or resistant to a particular antimicrobial by using a breakpoint.
Regulatory Agencies:
Clinical and Laboratory Standards Institute (CLSI)
British Society for antimicrobialChemotherapy (BSAC)
European Committee on Antimicrobial Susceptibility Testing (EUCAST).

4. BROTH DILUTION METHOD
 The broth/tube dilution test is the standard
method for determining levels of
microbial resistance to an antimicrobial
agent.
 Serial dilutions of the test agent are made
in a liquid microbial growth medium
inoculated with a standardized number of
organisms and incubated for a prescribed
time.
 At the end of the incubation period
(generally 18-24 hours), the tubes are
visually examined for turbidity

5. RELATIONSHIP BETWEEN MIC AND ZONE OF INHIBITION
 The MIC and the zone diameter of inhibition are inversely correlated. The more susceptible the
microorganism is to the antimicrobial agent, the lower the MIC and the larger the zone of inhibition.
Conversely, the more resistant the microorganism, the higher the MIC and the smaller the zone of
inhibition.
 MIC is determined only in specific clinical scenarios under the instruction of a Microbiologist,
e.g., infective endocarditis. MIC gives valuable information, which will help to customize the treatment
to direct only the causative bacterium.

6. INTERPRETATION OF RESULTS
 Broth dilution susceptibility test (MIC 6.25μg/mL)E.g. in the image shown above, the breakpoint of
growth inhibition lies between 6.25 (no turbidity is seen) and 3.12 μg/mL (turbidity is still appearing)
of antimicrobial agents. However, by convention, the MIC is interpreted as the concentration of the
antimicrobial agent contained in the first tube in the series that inhibits visible growth. Therefore, in
this example, the MIC is 6.25 μg/mL
REPORTING MIC RESULTS
Laboratories have to report the interpretive category (susceptible, susceptible-dose-dependent,
intermediate, or resistant) to the physician with or without MIC value.

7. MIC, MBC
MIC is the lowest concentration of an antibacterial agent necessary to inhibit
visible growth.
Minimum bactericidal concentration (MBC)
The lowest concentration of an antibacterial agent required to kill a bacterium
over a fixed, somewhat extended period, such as 18 hours or 24 hours, under
a specific set of conditions.

8. MICROBROTH DILUTION METHOD
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
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.

9. RESULTS

10. SIGNIFICANCE 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 effective antimicrobial agents.
 Currently, there are a few web-based, freely accessible MIC databases.
 MIC scores are important in diagnostic laboratories to confirm 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 patients with specific infections and to
identify an effective dose of antibiotic.
 This is important because populations of bacteria exposed to an insufficient concentration of a particular drug or to
a broad-spectrum 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.

11. REFERENCES
 https://www.ncbi.nlm.nih.gov
 https://en.wikipedia.org
 https://pubmed.ncbi.nlm.nih.gov
 https://microchemlab.com/test/minimum-inhibitory-concentration-test-mic/