First, several overnight growth curves are generated and the log-phase doubling time for the incubator and strain are calculated using the following formula:
(time final - time initial (in hours))/(1.44 ln (OD final /OD initial))
where time final - time initial is the number of hours between t = 0 and t= n time points and OD final and OD initial are OD 600 at t= 0 and t=n time points, respectively. This number should be something like 1.45 hours (1 hour 25 minutes at 30°C).
The linear range for some spectrophotometers is between .05 and .7 at the 600 nm wavelength so dilutions may be essential for obtaining good readings. Always mix your culture well before withdrawing a sample, as well.
Then the number of mls of a log phase culture of OD600 = X required to obtain a culture of volume (V) with an OD 600 of Y at time (T) hours later can be calculated from:
(V*(Y/X)) / (2**(T/DT)) = # of mls to add.
where Y is the target OD 600
X is the current OD 600
T is the number of hours of growth
V is the culture volume in mls
DT is the doubling time in hours--about 1.45 for our incubator.
So if your current culture has an OD 600 of 10 and your 100 ml overnight culture should have an OD 600 of 1.0 15 hours later, then you would add 2.4 µl.
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