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Minimum Inhibitory Concentration Assay (MIC)
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Summary
Minimum inhibitory concentration test provides the lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism. It is a crucial test method in clinical microbiology, antimicrobial susceptibility testing, R&D, Drug development to assess and compare antimicrobial efficacy.
Related tests for you
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.
- Test samples are serially diluted in different concentrations and inoculated with bacterial suspension.
- Inoculated samples are then incubated at specified temperature for a specified time period.
- After inoculation, samples are observed for presence of turbidity or visible growth of microbes to determine the lowest concentration of the antimicrobial agent that inhibits bacterial 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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What is Minimum inhibitory concentration (MIC)?
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.
After MIC, Minimum Bactericidal Concentration (MBC) test can be performed to understand if the antimicrobial agent can kill microorganisms (bactericidal effect).
Purpose of MIC testing in microbiology
MIC is performed for several important reasons:
- It evaluates the effectiveness of antimicrobial agents against specific microorganisms.
- It helps compare the activity of different compounds under standardized conditions.
- It supports dose selection during drug development and clinical research.
- MIC results provide insight into emerging antimicrobial resistance patterns.
- It is used to verify efficacy claims for antimicrobial formulations.
MIC test methods explained
MIC test can be performed using broth or agar dilution methods.
Broth Macrodilution method
- Test sample is serially diluted and added to Mueller-Hinton broth in sterile tube.
- Each tube is inoculated with bacterial suspension(usually ~5ร10^5 CFU/mL).
- The tubes are then intubated at 37 ยฐC for 18 hours.
- Post incubation period, the tubes are observed any turbidity as an indication of microbial growth.
Broth Microdilution Method (96- well plate)
- A 96-microtitre well plate is used in this test method. 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 any turbidity caused due to microbial growth.
Agar dilution method
To perform agar dilution method, varying concentrations of the antimicrobial agent are added into a molten agar medium, using serial two-fold dilutions. This is followed by the inoculation of a defined microbial inoculum onto the agar plates. Plates are incubated and MIC is determined by observing growth inhibition
Control tests
- Growth control – This control consists of bacterial suspension without any antimicrobial agents. It shows actual bacterial growth without antimicrobial agents.
- Sterile control – This control consists of only growth medium without the presence of any test bacteria or antibacterial agents. This should not show any bacterial growth after incubation.
MIC value interpretation (ยตg/mL or mg/L)
MIC values represent the lowest concentration of an antimicrobial agent that inhibits bacterial growth. The well/tube with no turbidity is identified as minimum inhibitory concentration.
Low MIC value – Higher antimicrobial potency
High MIC value – reduced susceptibility or resistance
MIC vs MBC
Parameter | Minimum Inhibitory Concentration (MIC) | Minimum Bactericidal Concentration (MBC) |
Definition | The lowest concentration of an antimicrobial agent inhibiting visible growth of a microorganism after overnight incubation. | The lowest concentration of an antimicrobial agent required to kill โฅ99.9% of the initial bacterial population |
What it measures | Inhibition of bacterial growth | Bactericidal property (ability to kill microorganisms) |
Methodology | Broth dilution or agar dilution techniques | Aliquots from MIC are subcultured onto antimicrobial agent-free agar. |
Result | Absence of visible turbidity indicates inhibition of growth | No colony formation after subculturing verifies bactericidal property |
Scope of application | Antibiotic selection, antimicrobial susceptibility testing, resistance surveillance | Differentiation between bacteriostatic and bactericidal agents, evaluation of disinfectants and antimicrobial products |
MIC Testing against ESKAPE pathogens
The term ESKAPE refers to six pathogens:
Enterococcus faecium
Staphylococcus aureus
Klebsiella pneumoniae
Acinetobacter baumannii
Pseudomonas aeruginosa
Enterobacter spp
These six microorganisms are the main causes of hospital-acquired infections around the world and often develop multiple antibiotic-resistance mechanisms that let them escape standard treatments. Since ESKAPE pathogens cause severe, hard-to-treat infections in vulnerable patients, MIC testing of new antimicrobials against these pathogens can help demonstrate broad-spectrum efficacy and to monitor emerging resistance.
Applications of MIC in R&D and regulatory submissions
- MIC test is widely used in R&D to screen and compare the antimicrobial activity of a new drug formulation. It helps optimize active compounds, establish the optimum dose during early-stage development.
For regulatory submissions, MIC helps in generating reliable data to verify antimicrobial effectiveness and can be used to support product claims and labeling. Regulatory bodies such as Clinical and laboratory standards institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) recognize MIC as a valid method for supporting antimicrobial activity claims.
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.
- Macrodilution can also give the minimum bactericidal concentration for the antimicrobial agent. This helps understand its bactericidal effect in addition to bacteriostatic effects.
Limitations of MIC test
- 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.
How MIS can help with MIC testing !
At Microbe Investigations Switzerland (MIS), our team delivers quick, reliable results in accordance with MIC testing to support your antimicrobial claims. Screening a single compound or full custom panels against highly critical ESKAPE pathogens, our lab helps generate precise, reproducible results.
Request MIC testing
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.
MIC takes 3-4 weeks to complete.
It involves exposing a standardized bacterial inoculum to serial dilutions of an antimicrobial agent in broth or agar. Post Incubation, the lowest concentration that prevents visible bacterial growth is identified as MIC.
MIC is quantitative as it provides a defined numerical value for antimicrobial activity.
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.
No, MIC measures growth inhibition while MBC measures the bacterial killing.
MIC values are reported as the lowest antimicrobial concentration in ยตg/mL or mg/L that inhibits visible bacterial growth.
The factors influencing the MIC values are inoculum size, incubation conditions, growth media, etc.
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