We are struggling with a phenomenon, a very good zone of inhibition on MH agar, Nutrient agar but no activity in a liquid medium while doing MIC using broth dilution methods for a novel (but known nutrient for many microbes) antimicrobial molecule, it is soluble in water at 10% concentration. It's 1 mg disc inhibits bacteria giving a zone of inhibition 20-30 mm on a nutrient agar plates.
Dear Sir
I think 1mg disc is very much higher concentration for testing. How much cells were spread/poured in agar plate. And second at what concnetration MIC is performed and how much cell concentration used to perform the experiment with broth.
In my opinion the your compound is inhibiting the microbial growth at very high concentration which might not be available in broth culture.
Thank you Shamsher Singh for your response.
While doing MIC we added the compound even @100mg/ ml without any inhibition but on agar plate 1 mg disc caused >18 mm zone, plates were swab inoculated from a broth culture having ãbout 12000 cfu/ml, while broth cultures were inoculated with 100 cfu each tube (having 3 ml medium with 300 mg of the compound.
maybe aeration / lack of it plays a role ( come next week I can check with a SEM such phenomenon)
Dear Bhoj,
I think, you should to be ensure for which agent could resoulble of this agent completely. We use very different chemicals (ethanol, methanol, formic acid etc) for solve antimicrobial agents. If you find the best solution for solve your agent, you should get good results in MIC analysis. You should ask disc company for chemical solution to solve this agent.
Dear sir,
We usually encounter the reciprocal, compounds act on broth but no inhibition in agar. But your case is different.
Some minor issues which I presume, may be escape your attention-
1. You are dealing with a compound with 10% water soluble and if you start with 100 mg/ml, the compound is beyond the solubility so it could not be active when you are testing in broth. As, it shows activity with 1 mg, which is too high dose but it can diffuse in agar, so showing activity. In my suggestion start the concentration with 1 mg/ml and go for further dilution till 1 microgram/ml.
2. Many times the zones formed in agar cup methods, are pseudozone, which is not clear but are shadow due to diffusion of compound. I do not think, it should be in your case.
3. Muller hinton broth is also not suitable for few bacteria, tryptic soy broth is the alternative.
I agree with Panda on the issue of concentration of the compound.. I understand that some compounds have the 'eagle effect'; meaning they are very potent at very dilute concentrations. I propose you keep testing at lower concentrations. you might get something good.
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