Minimum inhibitory concentration
In microbiology, minimum inhibitory concentration (MIC) is the lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation. Minimum inhibitory concentrations are important in diagnostic laboratories to confirm resistance of microorganisms to an antimicrobial agent and also to monitor the activity of new antimicrobial agents. A MIC is generally regarded as the most basic laboratory measurement of the activity of an antimicrobial agent against an organism.
MIC of an antibiotic is determined by using the following procedure:
- Preparation of antibiotic stock solution
- Preparation of antibiotic dilution range
- Preparation of agar dilution plates
- Preparation of inoculum
- Reading and interpreting results
MICs can be determined by agar dilution or broth microdilution, usually following the guidelines of a reference body such as the CLSI, BSAC or EUCAST. There are several commercial methods available, including the well established Etest strips and the recently launched Oxoid MIC Evaluator method.
The Etest system comprises a predefined and continuous concentration gradient of different antimicrobial agents, which when applied to inoculated agar plates and incubated, create ellipses of microbial inhibition. The MIC is determined where the ellipse of inhibition intersects the strip, and is easily read off the MIC reading scale on the strip.
Clinically, the minimum inhibitory concentrations are used not only to determine the amount of antibiotic that the patient will receive but also the type of antibiotic used, which in turn lowers the opportunity for microbial resistance to specific antimicrobial agents. Applying MIC testing to a number of bacterial strains in the same species provides an estimate of the concentration that inhibits 50% (MIC50) and 90% (MIC90) of bacterial isolates and can indicate shifts in the susceptibility of bacterial populations to antibiotics. Currently, there are a few web-based, freely accessible MIC databases.
- Bacteriostatic agent Rule of thumb the less the number(concentration) it gives better coverage.
- MBC (Minimum Bactericidal Concentration)
- Andrews, J. M. (1 July 2001). "Determination of minimum inhibitory concentrations". Journal of Antimicrobial Chemotherapy 48 (suppl 1): 5–16. doi:10.1093/jac/48.suppl_1.5. PMID 11420333.
- Turnidge JD, Ferraro MJ, Jorgensen JH (2003) Susceptibility Test Methods: General Considerations. In PR Murray, EJ Baron, JH Jorgensen, MA Pfaller, RH Yolken. Manual of Clinical Microbiology. 8th Ed. Washington. American Society of Clinical Microbiology. p 1103 ISBN 1-55581-255-4
- Davison, HC; Low, JC; Woolhouse, ME (December 2000). "What is antibiotic resistance and how can we measure it?". Trends in Microbiology 8 (12): 554–9. doi:10.1016/s0966-842x(00)01873-4. PMID 11115751.