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Minimum Inhibitory Concentration Assay (MIC )
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Determine the minimum concentration of an antimicrobial agent required to kill 99.9% of a specific bacterial population
Qualitative assay used to evaluate the susceptibility or resistance of pathogenic bacteria to antibacterial agents
Quick understanding of the test
Minimum Inhibitory Concentration Assay
The purpose of MIC test is to determine the lowest concentration of an antimicrobial agent that inhibits visible growth of a microorganism.
- Various concentrations of the test sample are prepared.
- Test samples are inoculated with cultured bacterial strains.
- After inoculation, samples are evaluated for turbidity or visible microbial growth either using agar or broth dilution methods (macro or micro) to determine the lowest concentration that inhibits growth.
- MIC determines the precise antimicrobial concentration to inhibit bacteria, optimising treatment and reducing side effects.
- MIC testing promotes appropriate antimicrobial use, reducing misuse and resistance risk.
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Abstract
Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antimicrobial agent such as an antibiotic, disinfectant, or antiseptic that visibly inhibits the growth of a microorganism after a standardized incubation period. MIC values are typically determined using agar or broth dilution methods, in a controlled laboratory setting.
MIC plays a critical role in research and development (R&D), helping to evaluate the efficacy of new antimicrobial compounds, guide formulation decisions, and support claims for regulatory submissions or product labeling.
MIC tests can be followed by a few additional steps to check for germicidal (microbe-killing) activity in a test called the Minimum Bactericidal Concentration (MBC) test. This helps determine if the antimicrobial not only stops growth but also kills the microorganism.
The ESKAPE Pathogens
The term ESKAPE refers to six notorious nosocomial pathogens: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. These organisms are leading causes of hospital-acquired infections worldwide and often harbor multiple antibiotic-resistance mechanisms that let them โescapeโ standard treatments. Because ESKAPE pathogens cause severe, hard-to-treat infections in vulnerable patients, MIC testing of new antimicrobials against this panel is essential to demonstrate broad-spectrum efficacy and to monitor emerging resistance.
Methodology of Minimum Inhibitory Concentration (MIC)
MIC test for bacteria can be performed either in agar medium or broth medium.
Macrodilution Method
- Serial dilutions of the test sample are added to MuellerโHinton broth in sterile tubes.
- Each tube is inoculated with a fixed bacterial suspension (typically ~5ร10^5 CFU/mL).
- The tubes are intubated at 37 ยฐC for 18 hours.
- After incubation, the tubes are examined for turbidity (indication of growth).
Microdilution Method
- The microdilution test is performed using a 96-well microtitre plate. Approximately 50 – 100ยตL of broth medium is transferred into each well.
- Each well is inoculated with a defined volume of bacterial suspension, typically ~5ร10^5 CFU/mL.
- The plate is incubated at 37 ยฐC for 18 hours, then examined for growth by turbidity.
Control Tests
- Growth control – It has bacterial suspension without any antimicrobial agents. This is used to see the actual growth of bacteria without antimicrobial agents.
- Sterile control – It has only growth medium without any test bacteria and antibacterial agents. This is expected not to show any bacterial growth after incubation.
Result
The MIC is the lowest antimicrobial agent concentration (well/tube) with no visible turbidity (growth).
Strengths of MIC in Microbiology
- The MIC test is easy to set up and carry out, which subsequently enhances the outcome of the test.
- The MIC test can be performed on a small scale, thus reducing the quantity of the antimicrobial agent used.
- MIC assays yield exact concentration values (mg/L or ยตg/mL) at which growth is inhibited, allowing for comparisons between compounds.
- MIC methods are rigorously standardized. Thus, when properly controlled, MIC results are highly reproducible across labs.
- Broth microdilution in 96-well (or 384-well) plates and automation (liquid handlers, plate readers) enable testing dozens of isolates or compounds simultaneously.
- A paired tube (macrodilution) can also provide the minimum bactericidal concentration (MBC) for the compound, giving insight into bactericidal versus bacteriostatic effects.
Limitations relating to the use of MIC test values
- Small changes in MIC test parameters can significantly affect results.
- The MIC test shows that the antimicrobial agent may inhibit growth at certain concentrations, but it does not confirm complete eradication of microorganisms.
- Macrodilution uses large volumes of broth and antibiotic stock, increasing cost and waste. Microdilution reduces this but still involves multiple plates and controls per run.
- MIC testing only works for pathogens that can be readily cultured under lab conditions. Obligate intracellular bacteria or unculturable microbes cannot be evaluated by standard MIC assays.
- The MIC endpoint is based on visible growth, so factors like inoculum density and medium conditions can introduce variability.
Importance of MIC test
- The minimum inhibitory concentration (MIC) indicates the smallest amount of an antibiotic that can inhibit the growth of a pathogen and thus aid in the selection of the most effective treatment.
- MIC testing is crucial for determining effective treatments against ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species), which are known to cause serious infections in intensive care environments.
- Using antibiotics based on MIC data reduces the chances of underdosing, which can lead to resistant strains.
- The MIC values allow healthcare professionals to adjust antibiotic dosages for each patient in accordance with specific needs, and this enhances the therapeutic outcome.
Conclusion
At Microbe Investigations Switzerland (MIS), we deliver fast, reliable, and fully standardized MIC testing to support your antimicrobial innovations. Whether you need single-compound screening or full custom panels against high-priority ESKAPE pathogens, our lab ensures precise, reproducible results.
Contact us to arrange custom MIC testing or to learn how our services can accelerate your drug development and R&D efforts.
Frequently Asked Questions
DR. Martinoz Scholtz
MIC test determines the lowest concentration of an antimicrobial agent that inhibits the visible growth of the pathogenic bacterium.
This test applies to antimicrobial drugs, disinfectants, and other liquid formulations.
The turnaround time for the MIC is 3-4 days.
At MIS, we perform MIC test using bacterial strains such as Pseudomonas aeruginosa, E. Coli, Staphylococcus aureus, Salmonella enterica, and Candida albicans.
Common methods for performing MIC tests include agar dilution and broth dilution
No, MIC is not the same as Minimum Bactericidal Concentration.
The factors influencing the MIC values are inoculum size, incubation conditions, growth media, etc.
Small changes in MIC test parameters can significantly affect results.
The MIC value l determines how many antibiotics one needs to take since it describes the lowest concentration of antibiotics needed to inhibit the bacteria from growing, which enhances the effectiveness of the medication.
MIC is the quantitative method for determining the lowest concentration of an antimicrobial agent required to inhibit visible microbial growth. The disk diffusion method determines antimicrobial susceptibility by measuring the zone of inhibition around an antibiotic-impregnated disk.
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