[dashnia]

 Hello
please i have problem in determining the right potency for my antibiotic...i am working on (oxacillin sodium monohydrate) from Himedia....its potency is from(815-950). i am using it for doing MIC ....i have used the average of this no. but not giving resut 
so i want to know how to take the right potency ?
please help me 

[Babcock_Hall]

What is MIC?  What are the units of the number range you provided?  For what purpose are you using oxacillin?

[dashnia]

MIC is minimum inhibitary concentration ....iam working on the susceptibility testing of oxacillin against bacteria ,the bacteria is s.aureus .
and for that iam preparing the stock solution of the antibiotic by this equation weight= volume(ml) x concentration(mcg/ml) / potency (mcg/mg)
the potency that given by the company was (815-950 mcg/mg) .i took the average which was 883 but its not working ,,,,i hoped that u could give me an advice that can help me to find the right potency.

[Arkcon]

So what's going wrong?  Did you try using the minimum value?  Or is it too inhibitory, and maybe you want to use the maximum?  Generally, when a chemical supplier give you a range, they're expecting the range to have adequate resolution for a typical application.  So it shouldn't matter.  But maybe you've gotten a bad batch.

[Yggdrasil]

How are you testing susceptibility?  Are you adding bacteria to solid or liquid media containing a certain concentration of the antibiotic, then letting the cultures grow?  What is the final concentration of antibiotic in the media?

The CDC quotes 2µg/mL as the MIC for susceptible S. aureus strains (http://www.cdc.gov/MRSA/lab/index.html).  I'm not sure why you're giving a MIC in units of µg/mg.

[dashnia]

I am using macro dilution method. Its by inoculating the bacteria to the liquid media and after 15min  adding the antibiotic in concentration 16 and then making serial dilution till 0.125 mcg/ml
This unit (mcg/mg) is for the potency not for the concentration.