I am determining antibacterial activity of isoniazid derivative against bacterial strains (mycobacterium smegmatis, E. coli & S. aureus) by turbidimetric method and I got good inhibition but unfortunately no zone of inhibition appeared when same compound was tested using well diffusion method.
First, a well diffusion method won't give you an MIC value unless you make some sort of standard curve by testing lots of strains with known MICs.
With that said, if your well diffusion doesn't give you any inhibition zones, you can start by increasing the amount of drug in the wells. As an example, the isoniazid discs provided by BD contain either 1 ir 5 µg of drug. How does that relate to the amount of drug in your wells? If your derivative has a potency similar to isoniazid, you could start out with these amount in your wells, or else just do a titration with increasing amounts of drug until you get a zone size that's reasonable to work with. If you still don't get any zones, you will need to start troubleshooting.
MIC by broth micro-dilution assay always better method than agar-well as diffusion of the drug will be a limitation in latter method. You need to study MIC and correlate the result with agar-well method. However, you will always get much higher MIC by agar-well which is not al all a standardize method. If you prepare agar plates with known concentration of drugs and check the bacterial growth, then you may found proper result (similar to the LJ propotional method).
I think the well-diffusion method with subculture for each dilution is the best.
THAT IS RIGHT. I use the tubidimetric methodology. The well diffusion method, only for screening but using high concentration of drugs.
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