I will be working on the antimicrobial susceptibility testing for several potential plant extracts on bacteria, fungi, and yeasts. The problem I am facing now is I do not know what inoculum size to choose for the MIC test for yeasts and fungi. I have come across different inoculum size from various sources such as CLSI, EUCAST, and academic papers. Some of them used 10^6 cells/ spores , while some used 10^3 or 10^4 cells/ spores per well. I would like to know if anyone can advise me on how much inoculum to use.
In addition to that, I'm asked by my supervisor to find the IC50 for the extracts by plotting an inhibitory graph with the OD values obtained. The OD values will be read at 492nm after the addition of INT. And this is the formulation given by my supervisor:
% inhibition = [(blank - sample)/ blank)] x 100%
I find this formula weird because the value of (blank - sample) will be a negative value since the blank (INT+ solvent) is colourless and for sure it will have a lower OD reading compared to the OD reading of sample (bacteria + extract). So, I am wondering if anyone is doing the same experiment and can provide me with the correct calculation method?
Edited by Catherine10, 31 May 2013 - 08:46 PM.