# alcohol in antibiotic solutions?

Wed Nov 1 09:53:38 EST 2000

```hroychow at nmsu.edu ("Dr. Hiranya S. Roychowdhury") wrote:
:
:Microbicidal conc. of etoh is 40-70%.
:By your admission, the conc. of etoh in the medium would be 0.01%.

25 ul into 10000 ul is 0.25%, 25X more than you say.

:At even
:room temp, 25uL etoh should not take long to evaporate off the medium.

Diluted ethanol evaporates a lot slower than pure ethanol.

:BTW, could you please illuminate me about the "molarity" of 200proof EtOH?
:May be then I will get the "50mM" figure for etoh in the medium.

What's so difficult? I don't remember specific density of EtOH but
for some reason I have a number 0.7 g/ml sticking out. With this:
1 l of EtOH has 700 g in it. MW = 24+6+16 = 46. Thus molarity of
200 proof ethanol would be 700/46 ~ 15.2 M. Therefore 400X dilution
into medium makes it 15200/400 ~ 38 mM (Emir's calculation
assumed density of 1.0 in which case you'd get ~ 50 mM).

:Also, I was wondering what would happen if I had just 100uL of the 20mg/mL
:soln of Cm in EtOH? Should I still be adding 25uL to 10mL of LB? Should I
:add 2.5uL to 1mL of LB? Should I make up 900uL of the 20mg/mL soln of Cm in
:EtOH to bring the volume of the antibiotic stock solution to 1mL? It is
:rather confusing ...

- Dima

:At 09:49 PM 10/30/00 -0600, Emir wrote:
:>Let's just calculate. Say, you have 1 ml of 20mg/ml soln of Cm in EtOH. You
:>add 25ul of that soln to 10ml of LB to make up to 50ug/ml Cm. Your finally
:>EtOH concentration in the medium is ~50mM then. How miniscule it is? 2g/l of
:>glucose for example is just about 10mM. The only good thing about it is that
:>to my understanding E.coli cannot utilize EtOH when grown aerobically.
:>Still, the presence of 50mM alcohol should have a certain effect on, say,
:>the length of the lag phase when you re-inoculate your o/n culture into
:>fresh LB-antibiotic medium.
:>
:>Emir
:>
:

```