Final concentration of prepared testing solution - (Jun/10/2013 )

In one of the studies I read on MIC, it reports:

One hundred microliters of Mueller-Hinton broth (MHB) were added in each well of a microtiter plate. The 100-μl aliquot of stock solution of crude ethanolic extract (400 mg/ml) was added, and subsequently two-fold serially diluted with MHB.

The inoculum suspenion (20 μl) of each bacterial strain was then added in each well containing crude ethanolic extract and MHB.

The final concentrations of the extract were 166.7, 83.3, 41.7, 20.8, 10.4, 5.2, and 2.6 mg/ml.

My question is : I did my calculation for reaching rhe final concentration reported it was never the same however bit more.

Any idea how this concentration was reached?? it was 400 mg/ml in the 100 microliter stock solution before adding it to another 100 microlieter of broth and 20 microliter of inoculum. Therefore the total volume is 220 microliter.

I did the simple calculation and did not work..

Appreciate a help again as I am working on MIC and need to dilute my extract and got to understand this information! I attach the file just in case needed.

Yeah, I don't get their concentrations either - I worked out that they added 40 mg (100 ul of 400 mg/ml) in a total volume of 220 ul (0.22ml) which gives c=n/v c=40/0.22 = 181.8.

If they were adding 40ul of bacteria (maybe 2 strains at 20 ul each?) their calculations were OK.

You didn't add an attachment, but that may be because your post count is too low - I think you need a minimum of 5 posts before you can add them.

Oh i am thankful!! that proved I am not slow in calculation !! i got your same answer too and wondered what did they do. I believe some published materials may be not accurate then !

Really thankful. It is a wonderful helpful forum . Bear with my many quesitons as I am new in this field and through a learning process.

Best!