When measuring bacteria MIC(minimal inhibitory concentration), according to CLSI (clinical laboratory standard institute)-"Read the MIC as the lowest concentration of antimicrobial agent that completely inhibits growth of the organism in the tubes or microdilution wells as detected by the unaided eye", if using microplate reader, what is the inhibition rate will be defined as the MIC compare to un-treatment control? Thanks.
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.[1] A MIC is generally regarded as the most basic laboratory measurement of the activity of an antimicrobial agent against an organism.
https://en.wikipedia.org/wiki/Minimum_inhibitory_concentration
There is no rate information in an MIC measurement. It is a single time-point measurement made after a fixed incubation time. You can use an absorbance measurement with a microplate reader to measure the end-point absorbance, although visual inspection is just as good. With the reader, it will be clear that there is a substantial drop in absorbance (actually light scattering rather than absorbance) at the MIC compared to lower inhibitor concentrations.
If you are using the microplate reader to obtain growth curves by continuous absorbance reading during the incubation, you will see that there is little or no absorbance increase over time at the MIC and above. Lower inhibitor concentrations will allow growth of the culture and you will see a sigmoidal growth curve (flat at the beginning, increasing rapidly, then slowing down and reaching a stable final absorbance.)
The inhibition rate can be estimated as follows:
% Inhibition rate = 100*(ODuntreated-ODwell)/( ODuntreated-ODblank)
at MIC, the inhibition rate is close to 100% [ODwell = ODblank]. Usually, 95-100% is a good range for MIC.
OD = optical density recorded by the plate reader.
blank = culture medium without cells
untreated = only cells in medium without Antimicrobial agent
well = corresponds to a known concentration of Antimicrobial agent.
This estimation is based on fitting of the inhibition data. If the inhibition rate is plotted versus log[concentration], the fit is a sigmoid with three domains:
- domain I: at the beginning (slow to zero change in slope)
- domain II: rapid change in slope
- domain III: at the end (slow to zero change in slope)
The intersection between the tangents to domains II & III gives the MIC which usually falls within 95-100% range.
Filbert Totsingan Is there any references for the formula of the inhibition rate estimates ??? if so please provide the references
Thank you in advance
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