antibiotic stock solution

[Silvia_84]

Hi all,

 

I am new with experimental stuff in Microbiology. I need to do some MICs (first time I do it) and I am trying to read up a little bit. I saw that the first step to do is making antibiotic stock solutions. The protocols that I saw suggests to get the purity of the antibiotic and calculate the amount of it to be weighted with the following formula: volume x concentration/potency. My question is: how can I get the potency of antibiotic? and which volume is better to use?

 

Any suggestion would be really appreciated.

 

Thank you in advance,

 

Silvia

[hypervalent_iodine]

I have no idea what is meant by potency, but I do know that volume x conc. will get you number of moles rather than a mass. Converting that to mass of antibiotic would mean you would divide by molar mass. Where are the antibiotics coming from (i.e. are they bought as pure compound or as the marketable item) and how are you calculating purity?

 

My guess is that potency refers to the combination of molar mass and w/w purity. For example, if you have a sample of an antibiotic that is 150 g / mol and you know the mass % purity of your sample to be say, 50%, then obtaining 1 mole of your antibiotic would mean you need 300 g of your sample. Thus, if you wanted to make a 1M solution, you would dissolve 300 g into 1 L. Or, as per your calculation, mass = volume x conc. / potency = 1 L x 1 mole / L / 300 g / mol. Within this example and using this reasoning, 300 g would be the 'potency,' and this would obviously change with molar mass and % purity (I calculated it by dividing molar mass by % purity x 100). I should also add that I have not encountered this term in this context before and I could be wrong in my thinking.

[Silvia_84]

Thank you very much for your reply. Well, as far as I know, they come from pharmaceutical companies. For example we have Meropenem bought from ZenecaPharma. My issue is that I don't know how to proceed for the first steps of MIC. for example I need to do MIC with Meropenem whose

suggested range for MIC determinations (mg/L) is 0.015-4 and I don't have an idea of how to do the antibiotic stock solution and which volumes I should use both for the stock and for the working solutions..

I thank you again,

Silvia

[CharonY]

Assuming it is not just a hobby, I strongly suggest first to talk to your advisor about the experiments you are going to conduct. It is very important for them to assess which parts of a project you are comfortable doing and where they need to coach you or at least which reading material they should provide. Otherwise it is very tricky to figure out what went wrong until it is far too late.

[BabcockHall]

I agree with CharonY. For many common antibacterial solutions, there are tables that provide concentration ranges at which the compound is typically used. These tables may be found in reference books such as Sambrook et al. However meropenem is unfamiliar to me, and I don't recall having seen it in any table with which I am familiar; therefore, some of these tables may not be what your need.