Our plant extract samples have shown good results in MIC estimation proving that the compound is antibacterial. But on disk diffusion no proper result was seen. What could be the possible reason or cause?
Firstly; I recommend repeating the diffusion assay using well diffusion method instead of disk diffusion method since the antimicrobial activity might be related to compounds with high molecular weight or having problem with distribution between the disk and the medium.
Also considering the solubility of the extracts, if you used microdilution assay to estimate the MIC because the extract constituents are hydrophobic it will be expected that the results with agar diffusion method will be negative.
Differences in mechanism of action, diffusion rates, chemical properties, interference, and concentration can contribute to the discrepancy between MIC values and disk diffusion results for certain antimicrobial agents. It's essential to consider these factors when interpreting antimicrobial susceptibility test results and to use complementary methods to validate findings whenever possible.
The differences in results between the Minimum Inhibitory Concentration (MIC) estimation and the Disk Diffusion method could be due to several factors:
You need to consider all these factors as you are interpreting your results. I hope this helps
Many good answers were given, I will add that the bacteria on agar and in liquid are not exactly the same, metabolically-wise. Bacteria on agar are more prone to starvation because diffusion of nutrients to the colony and to accumulation of growth 'toxins' near the colony, while il liquid diffusion is much faster
reduce inocum 0.5 Mc Farland DO 0.08 to 0.10 at 625 nm.
try with a high load of the impregnated disk.
is it not working check the medium used
temperature
the incubation period.
Hi,
In my opinion, the presence of insoluble compounds in the extract has caused the MIC test to work, but the disk diffusion test to fail. In the disk diffusion method, the substance must be in a soluble state to spread in the agar and demonstrate its effect. If substances are poorly soluble or insoluble, they will yield a negative result in this test.
Different bacterial strains may vary in their susceptibility to antibiotics, which can lead to differences in results between the two methods.