How-to guide: Minimum Inhibitory Concentration (MIC)
For many years, the Kirby-Bauer disk diffusion method accounted for most antimicrobial susceptibility tests. Using the Kirby-Bauer method, a zone of inhibition (where bacteria did not grow) could be identified, and the level of antibiotic resistance displayed by the bacteria could be determined [1]. However, because the Kirby-Bauer method is incapable of producing quantitative results, it has been largely replaced by the Minimum Inhibitory Concentration (MIC) method.
MIC testing generates a numerical value (in µg/mL) that identifies the lowest concentration of an antimicrobial agent that prevents visible bacterial growth. This method offers precise quantification and is a critical tool in antimicrobial efficacy testing.
At Emery Pharma, our microbiologists routinely perform MIC assays to evaluate antibiotic efficacy and resistance profiles. We frequently test a variety of antimicrobial compounds, including plant derivatives, proteins, and coatings, against known drug-resistant bacterial strains, including “ESKAPE” pathogens and Methicillin-resistant Staph aureus (MRSA). This guide summarizes MIC testing, result interpretation, and practical applications in antimicrobial research.
- About MICs
- Reporting MICs
- How MICs are Used
About MICs
The Minimum Inhibitory Concentration (MIC) is the lowest concentration of an antimicrobial agent that inhibits visible microbial growth after overnight incubation in broth [2]. This quantitative method of antimicrobial susceptibility testing provides exact concentrations (in µg/mL) of antibiotics—or other test articles—needed to halt bacterial proliferation.
By comparing an MIC value to CLSI breakpoint values, researchers can determine whether a bacterium is susceptible, intermediate, or resistant to a specific antimicrobial agent. This information is widely used in drug discovery and antibiotic screening to evaluate the in vitro activity of new therapeutic candidates.
Figure 1: Example MIC microtiter plate.
The image above shows a typical MIC microtiter plate. Wells in columns 1–10 contain decreasing concentrations of an antimicrobial agent (column 1 = highest, column 10 = lowest). Columns 11 and 12 include positive controls (no antimicrobial agent) and negative controls (no bacteria), which ensure the assay’s validity.
Rows A through H contain different test antimicrobials. White circles indicate no bacterial growth, meaning the antimicrobial was effective at that concentration. Blue circles indicate bacterial growth—darker shades suggest higher growth levels. The vertical black line represents the breakpoint, where bacteria transition from susceptible to resistant. Purple stars denote the MIC value for each compound—the lowest concentration at which bacterial growth is inhibited.
Reporting MICs
MIC results are reported as the lowest concentration (µg/mL) of an antimicrobial agent that inhibits visible bacterial growth. Comparing MIC values to CLSI breakpoints enables classification of bacterial susceptibility:
- Susceptible (S): The antibiotic is effective at inhibiting bacterial growth at achievable concentrations, suggesting potential clinical efficacy.
- Intermediate (I): The antibiotic inhibits growth only at higher concentrations, and its clinical effectiveness may be uncertain.
- Resistant (R): The bacterium is not inhibited by concentrations achievable in patients, indicating antimicrobial resistance and likely therapeutic failure.
Figure 2: Example MIC report.
The table displays MIC data for various Staphylococcus aureus strains against antibiotics such as ciprofloxacin, clindamycin, erythromycin, oxacillin, sulfamethoxazole/trimethoprim, and vancomycin. MIC values are shown alongside S, I, or R interpretations based on CLSI breakpoints. These standardized breakpoints vary by both antibiotic and bacterial strain.
For example, if an MIC value exceeds the breakpoint, the organism is considered resistant. If the MIC is equal to or less than the breakpoint, the organism is susceptible.
How MICs are Used
MIC testing is typically used as a high-throughput screening method in microbiology and antimicrobial development. MIC assays are performed on numerous client-submitted test articles to identify the most promising compounds. Candidates showing strong inhibition then undergo Minimum Bactericidal Concentration (MBC) testing.
MBC is defined as the lowest concentration of an antimicrobial that results in a 99.9% reduction of the original microbial population [3]. While MIC determines the minimum amount needed to inhibit bacterial growth, MBC confirms whether the antimicrobial agent can kill the bacteria [4].
Even if MIC testing shows inhibition, bacterial regrowth may occur if the compound isn't bactericidal. Therefore, MBC testing is essential to distinguish between bacteriostatic and bactericidal agents, providing a more complete profile of antimicrobial effectiveness.
If you are developing an antimicrobial contact Emery Pharma through our Contact Us page or call us at (510) 899-8814 for a free 1-hour consultation!
- Tan, T. Y. & Ng, L.S.Y. (2006). Comparison of three standardized disc susceptibility testing methods for colistin. Journal of Antimicrobial Chemotherapy, 58, 864-867.
- Babu, P. A. & Kumar, P. S. (2009). MIC database: A collection of antimicrobial compounds from literature. PubMed Central, 4(2), 75-77.
- Andrews, J. M. (2001). Determination of minimum inhibitory concentrations. Journal of Antimicrobial Chemotherapy, 48, 5-16.
- French, G. L. (2006). Bactericidal agents in the treatment of MRSA infections--the potential role of daptomycin.Journal of Antimicrobial Chemotherapy, 58(6), 1107-17.
