I got a modified antibiotic , I checked the antibacterial activity by two different assays, first using disc diffusion test which showed less active than the WT. second, by minimal inhibitory concentration, which showed much more active than the WT!!! Anybody have an idea?? And which of these test are more accurate?
Thanks
agar diffusion test is a qualitative test and depends on size of inoculum, medium, age of the inoculum, incubation temperature etc. and handling person.
Thanks Anita
I run both the Test and control on the same plate, so there is no differences regarding what you mentioned. my question is why there is difference between disc diffusion and MIC??
What is your definition or meaning of 'less active' in disc diffusion and the meaning of 'more active' in MIC?
some antibiotics have different MICs when tested in agar plates (using antibiotic strips) or liquid media (containing progressively lower concentrations of the antibiotic) depending on the diffusion properties of the antibiotic. In your case it might be that the modified antibiotic doesn't diffuse through agar: you can incorporate different concentrations of your antibiotic in solid agar plates and test on these plates. Make sure to use the same inoculum with both methods.
Thanks Mihaela, may be it is the diffusion properties of the antibiotic, the modified one just has one hydroxyl group less than WT, do you think this does make this a big difference.??
Dear Robert
on disc diffusion plate, the inhibitory zone for the modified one is smaller than the WT, but in MIC the modified one inhibites the bacterial growth at 0.05ng/ml while the WT inhibites the growth at 3 ng/ml
I think difference is due to diffusion properties of antibiotic in agar medium if inoculum and all other conditions are the same , you can obtain MIC by E test this will help you to understand where is the difference.
Dr. Elymani, do you think the diffusion properties change vary on the same plate of agar? That seems unlikely to me. My suspicion is that the antibiotic in the disks is slightly different chemically than the antibiotic that is in the MIC. You could contact the manufacturers to double check.
One has to find out MIC ( that inhibits the bacterial growth by taking OD ) by using different concentrations of a test compound, and then same concentration can be applied on disc to observe zone not OD in a solidified and inoculated plate.
Dear Dr Robert,
I mean that diffusion properties differ from broth than agar medium, as in liquid medium we usually use powder or solution but in agar we use discs but in the same plat agar medium i do not think it differ .
Thank you Dr. Elymani, so we are still left with the question (as I understand it) as to why the difference. My guess is that the antibiotics in the disc and the MIC were somewhat different chemically.
It is almost always true that MIC in liquid are lower than in the agar. This is largely due to the differences in the diffusion of the antibiotic in the agar but also through the bacterial loan. When bacteria grow on agar, they grow in the biofilm mode, in which cells are embedded in extracellular matrix which protects cells by limiting the diffusion. Dead cells on the bottom of such biofilm play similar role. This is why it's much harder to treat biofilm-forming pathogens. In liquid culture, where all cell are dispersed, there is no biofilm effect and all cells are equally exposed to the antibiotic.
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