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I would like to know
1) How to calculate MIC (minimum inhibitory concentration) of bacteriostatic & bacteriocidal drug?
2) What is the difference between MIC and MBC.
3) What is MIC50 & MIC90.
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Dear rahul vats,
In vitro MIC (in bacteriology) are determined using several methods including:those used in research studies or in current clinical works:
1- Disc diffusion methods where filter paper disc impregnated with the antibiotic to be tested at certain concentration and is placed on agar where bacteria are grown. The drug will diffuse from the disc into the agar and prevent bacteria from growing (if they sensitive to this antibiotic) leaving a circle around the disc without bacteria. The mean diameter of this circle can determine if bacteria are sensitive or not.
2- Microdilution and macrodilution methods where antibiotic concentration is diluted in series of twofold dilution (usually from 32 mg/L to 0,5 mg/L). and a fixed quantity of bacteria ( cfu/ml) is added to each concentration . the lest concentration preventing bacterial concentration is the MIC (an example can be found in my paper published JAC In vitro antibacterial activity of aminosterols against multidrug-resistant bacteria from patients with cystic fibrosis)
3- There are also other methods largely used in clinical laboratories such as vitek http://www.biomerieux.com/servlet/srt/bio/portail/dynPage?lang=fr&doc=PRT_NWS_REL_G_PRS_RLS_33
However, bacteriostatic or bacrteriocidal activities can be determined using time-kill studies rather than susceptibility essays
MIC is minimal inhibitory concentration that can be bacteriostatic or bacrteriocidal while MBC is minimal bacteriocidal concentration that kills and not only prevent bacterial growth. Form some molecules MBC might be superior to MIC
Finally, MIC 50 is the concentration of the drug the kills 50 % of ,bacterial count and MIC90 is that the kills 90 of bacterial count. These can be evaluated using bacterial counting methods where a certain volume of bacterial suspension treated with a certain concentration of antibiotic is sampled and bacteria counts is determined
Reference documents about MICs determination methods can be found at the site of British society of antimicrobial chemotherapy
or the French society if microbiology
Hope this can answer you question
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Dear Rahul Vats,
In addition to what Kamel Alhanout has so kindly put together, further information on MIC, time-kill curves and their mathematical relationship can be found in e.g. the following publications:
Mueller M, de la Pena A, Derendorf H., Issues in pharmacokinetics and pharmacodynamics of anti-infective agents: kill curves versus MIC, Antimicrob Agents Chemother. 2004 Feb; 48(2): 369-77.
Schmidt S, Schuck E, Kumar V, Burkhardt O, Derendorf H., Integration of pharmacokientic/pharmacodynamic modeling and simulation in the development of new anti-infective agents - minimum inhibitory concentration versus time-kill curves, Expert Opin. Drug Discov. 2007; 2(6): 849-60
I hope this helps.
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