LiAc transformation from SC media instead of YPD

Denys Proteau dp2p+ at andrew.cmu.edu
Wed Jul 14 15:36:13 EST 1993


    In response to many posts that make use of the concept of cell
density, I'd like to point out that the ratio of OD to cell density
varies greatly from one spectrophotometer to another so that the cell
density one person is using at an OD600nm of 0.5 is not likely to be
quite the same as the cell density for a culture with the same OD
measured but measured with a different instrument.  Therefor use caution
when comparing cell densities over the net, particularly if people only
mention an OD but don't know how many cells per ml that OD corresponds
to for their spectrophotometer.

     This is because cell density OD's are not primarily the result of
true absorbance but instead are largely the result of light scattering
away from the angle that would let the light enter the photometer.  The
geometry of the size of the opening to the photometer, and the position
and distance of the sample chamber relative to the opening to the
photometer are different for different models of spectrophotometers, but
these geometric parameters greatly affect how much scattered light can
and can't get to the photometer.

    I must thank Rob Preston who opened my eyes to this phenomenon, or
in other words "Showed me the Light" :)
    Denys



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