Hi there. I've read your questions but, I can't answer them, because I don't know what you're unclear on. Let's work together to get to the bottom of it all.
I have 100 mg of colistin sulphate salt (Sigma). Potency is > 15000 U/mg and solubility is in water (soluble 50 mg/ml according to the label on antibiotic).
OK. Some facts: how much you have (?) The potency in units your application needs, and solubility (maybe we need later.)
So far, so good.
I want to check the MIC (minimum inhibitory concentration) of this antibiotic against micro organisms. EUCAST has given the guidelines that microbial growth at >2mg/ml would be considered resistant. Now, I am kind of confused how to make the stock solution and in what volume should I make.
And this info doesn't seem to help you. OK. But maybe you're confused as to what you want? You have potency, this reference is in mg. But we can work with other info.
Ok, I have been reading papers and different protocols on colistin resistance.
OK, we can use different info.
I'll be using broth microdilution for MIC determination.
I don't recognize this term, it could be jargon for your specific application. But we can still work with what we have.
I'll be making a 10 mg/ml stock solution for this purpose. The potency of antibiotic is 15000 U/mg, when I used online converters, I got the information that 1U=731.7 ug.
OK, that's a simple division, don't know why you use a converter. But fine, whatever. No question here, yet.
I have also read that W=C*V/P formula does not apply on peptide antibiotics.
OK. You've used jargon again. These abbreviations for units aren't used everywhere. Also, why would they not be used for peptide antibiotics. Some sort of relationship here with concentration and potency, that applies to things other than protein antibiotics? I'm not following.
So, should I make 14 mg/ml stock solution which would be equal to 10243.8 ug/ml? and use it as stock solution? I'll make further dilutions from its for using in assay.
That could work, as a starting point, depending on what you want to do next.
Kindly guide.
I'll appreciate if anyone can guide about it.
We're glad to help. And one of our rules is, you have to show your work. Sometimes, when we tell someone that, they dump random facts, and say "There. Sort out my work and write out my protocol." In some ways, that's what you've done here. But we need a better view of what you know, what you don't know, and what you want to do, in order to help.