by  using spectrophotometer at 650 nm  you can plot the growth curve than you came to know log, lag and stationary phase. 

Thank You. I need to make a calibration curve at 600 nm with a spectrophotometer. I want to correlate absorbance with CFU.

For, this I must be sure at the moment, that bacteria are in log phase. If I find out in the stationary phase when they were in log phase it will be too late.

Don't You know a faster method?

I think for any particular culture, if you make a growth curve under your standard conditions once, you can use the particular OD / culture-time corresponding to mid-log phase (under similar culture conditions) for further experiments. At least for E.coli it works.

Did you try flow cytometry?

Hi Otilia,

If the growth rate is relatively fast, you could check the difference in culture turbidity and utilize the doubling time calculation. You don't really need to know the exact CFU (at least for estimation). For some m.o the difference in OD between 30 minute reading may give us the picture whether it is in log phase.

Hope this helps

Thank You a lot.