Currently I am working on the antimicrobial effects of the lipopeptide biosurfactant on E. coli. When I conducted disk-diffusion method, inhibition zones can be observed on the E. coli lawn.
However, when I conduct further antimicrobial testing, using broth microdilution method to determine the minimum inhibitory concentration (MIC), the growth of E. coli was not inhibited at all despite of being tested on various concentrations of the lipopeptide.
Thus, I am hoping for some insights on this matter and will it be the best if I concluded that this lipopeptide is not a good antimicrobial agent as it only showed intermediate sensitivity on E. coli and also gave a negative result (no growth inhibition) in MIC ?
There is something wrong there. Technically it is impossible. What was the concentration you used in the disc diffusion test? How much quantity did you use in each disc; which medium did you use in disc diffusion and MIC? These factors are very important. Which solvent did you use to dilute the sample? DMSO, methanol, etc.
Here is a thought , might be totaly off.
If your compund is hydrophobic (lipopeptide) and lower density then water (i.e. float on the liquid media) it might explain the problem.
Think about the fundumental diffeernt between disk difusion assay and liquid:
In disk difusion assay both the bacteria and your 'floating' compound are on the surface. Basically a 2-D matrix.
In liquid MIC the bacteria is suspended and your floating compound occupy a small layer on the top. If its highly hydropohobic it might even not reach the lower layers. The bacteria might grow happily there...
Where do you get "intermediate"? Testing of known antibiotics is calibrated to therapeutics. Disc diffusion of a new material only shows effect - and does not indicate use in application. Your compound may be (prob is) fairly insoluble but active - that could be useful in some applications.
As Kamal said, it doesn't make sense. Yoram 's idea might be relevant and also consider that material could be substantive, adhering to microtiter plastic (surface area to volume of fluid ratio is pretty high)and not available to suspended bacterial cells.
several questions need to be answered, what solvent did u use? did you use it as control to ensure the activity is not due to the solvent. and surely prof Gerchman raised an important issue about the solubility of your compound, you must be sure that is soluble (use a method of solubilization that doesn't include surfactant)
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