We are actually talking about three different things, the minimal inhibitory concentration (MIC) itself, the MIC50 and MIC90. The MIC is the minimal concentration of an antimicrobial necessary to inhibit the growth of a target micro-organism. There are different ways of measuring the MIC, including disc diffusion testing according to Kirby–Bauer, E-test using gradient stripes and micro-dilution, which is usually performed in automates like Vitek, Phoenix or WalkAway (the list of methods and the list of automates is by no means complete, these are just examples). In any way, a MIC is determined for a given isolate.

If you, however, want to know, how good an antimicrobial works against a species, than you perform MIC testing on a large panel of isolates of that species. The MIC50, which requires testing at least 100 isolates, gives you the MIC, which inhibits 50% of your isolates, while the MIC90 (may be the result of different test panels in more than one laboratory) gives you the MIC which inhibits 90% of the isolates of the species, tested.

MIC50 and MIC90 are therefore interesting for R&D and for describing how effective a given antimicrobial is. The MIC50 provides the so-called “intrinsic activity” of an antimicrobial while the MIC90, which is calculated on the basis of larger, inter-centre studies, is a reflection of different resistance mechanisms of the species under investigation.

If you are going for testing susceptibility of clinical isolates in a laboratory under routine conditions, MIC testing of individual isolates is what you are doing.

We are actually talking about three different things, the minimal inhibitory concentration (MIC) itself, the MIC50 and MIC90. The MIC is the minimal concentration of an antimicrobial necessary to inhibit the growth of a target micro-organism. There are different ways of measuring the MIC, including disc diffusion testing according to Kirby–Bauer, E-test using gradient stripes and micro-dilution, which is usually performed in automates like Vitek, Phoenix or WalkAway (the list of methods and the list of automates is by no means complete, these are just examples). In any way, a MIC is determined for a given isolate.

If you, however, want to know, how good an antimicrobial works against a species, than you perform MIC testing on a large panel of isolates of that species. The MIC50, which requires testing at least 100 isolates, gives you the MIC, which inhibits 50% of your isolates, while the MIC90 (may be the result of different test panels in more than one laboratory) gives you the MIC which inhibits 90% of the isolates of the species, tested.

MIC50 and MIC90 are therefore interesting for R&D and for describing how effective a given antimicrobial is. The MIC50 provides the so-called “intrinsic activity” of an antimicrobial while the MIC90, which is calculated on the basis of larger, inter-centre studies, is a reflection of different resistance mechanisms of the species under investigation.

If you are going for testing susceptibility of clinical isolates in a laboratory under routine conditions, MIC testing of individual isolates is what you are doing.

Minimum inhibitory concentration is an important value to find activity in a antimcirobial compound. This activity will be more in a compound whose MIC value will be lower than the broad spectrum antibiotic. But when you wish to check a value to prove its intrinsic activity in suspension media, you can go for MIC 50 that is lower than the MIC values, similarly, MIC 90 indicate a value slightly lower than the MIC value. These are equally useful for identifying exact nature of biomolecules and their effectiveness in drug resistant microbes. MIC value is generally display about susceptibility of an antimicrobial agent when test clinical isolates in laboratory. But established strains those have drug resistance also need screening of toxicity of antimicrobial agent.

I agree with Oliver. However, I would like to add that in Clinical Practice you can predict 'MIC creep'  over a period of time with help of MIC 50 & MIC 90 data. MIC creep is  overall clinical isolates population drift  towards reduced antibiotic resistance 

I agree also with Oliver. However, this also depends on what you are interested in looking for as well in which antimicrobials and pathogens are you working on, as well the number of isolates available to test.

An excellent answer by Oliver! My only addition is to caution against the use of MIC values for various purposes. The MIC value is somewhat valuable to discern resistance development of strains in a population. However, the importance of an MIC value for any kind for potential therapeutic potential of an antibacterial agent is highly questionable. We all do MIC assays of antimicrobials but we also realize that these days most bacteria have been shown to form biofilms and the biofilm eradication concentration is usually 10-1000 times higher then the MIC for the same strain or isolate. Simiarly, to avoid resistance development in vivo, finding the serum concentration necessary to get sterilization over a treatment regimen is extremely important and based on the complexity of infectious diseases in their host environment, this concentration again will have nothing to do with the MIC. So MIC values are important for initial potency testing, to find potential activity but after that other testing including minimal bactericidal activity (MBC), activity against biofilms in vitro and in vivo studies will be more important for the ability of the agent to be used clinically. 

Dear All

Is it possible for any antimicrobial molecule having same MIC50 and MIC90 value.

Usually MIC50 and MIC90 values differ for any antimicrobials with respect to a specific  microorganism. There is a free online database for both MIC50 and MIC90 for about 4000 antimicrobials. Have a look at http://antibiotics.toku-e.com/. In case you really end up in finding same MIC50 and MIC90 value, please share with all of us.

Can we compare the MIC50 or MIC90 using difference methods, such as between microdilution and E-test?

Hey Pairat, the overall agreement between two different assay will depend on the microorganism tested and the threshold you decide to accept as agreement (e.g. ±2-fold dilution) here a couple of examples:J Clin Microbiol. Mar 1991; 29(3): 533–538. J Clin Microbiol. May 1995; 33(5): 1318–1321. J Clin Microbiol. Apr 1997; 35(4): 1021–1023. Now, especifically for MIC50 and MIC90, as an example; agreement between E-test and CLSI broth microdilution using Candida albicans has not been reached. 

Differences in the MIC value can differ according to the method you decide use. The choice of the method depends on the bacterial species you have to test and is regulated and standardized by some Reference International Laboratories (CLSI, EUCAST..). On this base also MIC50 and MIC90 can be different, in according with the method. 

For some bacterial species you can you use either broth or agar dilution method, but for some other species you are forced to use defined methods (eg. Clostridia must be tested by agar dilution method).

Hello everyone,

I would like to assess the MICs 50 and 90 of a bacterial species isolated from livestocks. Although the definitions of MIC50 and MIC90 are clear for me, I would like to find informations about the conditions for their assessment?  Do you know an official guideline (CLSI?) for these assessments? Is it  necessary to have test panels in more than one lab for MIC90 determination?

Thank you for your kind support.

Regards,

Kenny

Thanks a ton Mr. Oliver!

Thanks a lot Mr. Oliver........

Excellent information for MIC50 and MIC 90. Thank you Dr. Oliver Nolte

very well explained. Thank you Dr. Oliver

Thank you Dr. Oliver . very well explained.

DR.OLIVER . THANKS A LOT

Thanks to Dr. Oliver for his excellent and thorough explanation.

in medical mycology another MIC 50 exist for example : MIC 50 = 2.3 MICROGRAM/ML Explained that metabolite du 50% inhibition or reduced in growth with compared to positive control that consider (100%) if anyone explained it and what is efferent between this mic and another mic 50

MIC50 = Minimum Inhibitory Concentration required to inhibit the growth of 50% of organisms

MIC90 = Minimum Inhibitory Concentration required to inhibit the growth of 90% of organisms

thanks a lot apirom

The answer done by Oliver Nolte is correct. Minimum Inhibitory Concentration required to inhibit the growth of 50% of organisms is IR 50%

Thank you Dr. Oliver . very well explained.

Thanks a lot!

Oliver Nolte thank you