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OD units/specs!

S L Forsburg nospamforsburg at salk.edu
Fri Mar 26 17:29:21 EST 1999

Gavin Sherlock (sherlock at genome.stanford.edu)
>  There's no simple conversion factor from OD600 to
> cells/ml, as it depends on many factors, eg haploid vs diplod, cell
> size, medium, strain background etc, so for any strain in any given
> growth condition you should make a curve by counting cells under the
> microscope with a hemocytometer.

A frequent source of variation in calculating OD is the spectrophotometer.
Depending upon the position of the detector and the geometry of the
light path, the OD from spec to spec can vary several fold for turbid
samples (eg, yeast cultures), even though clear samples (eg,
beta gal assays) don't vary.  It is important,  as Gavin suggests,
to calibrate an unknown spec or compare it over an entire
growth curve  to the old familiar one down
the hall.   For example, my lab has had to adjust our calculations
on a new spec in the lab which is off by a consistent x1.62 from the 
one we have used for years.  For this reason, if you are writing 
protocols, it is best to translate your OD into cells/ml, which is
not variable from lab to lab.

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S L Forsburg, PhD  forsburg at salk.edu
Molecular Biology and Virology Lab          
The Salk Institute, La Jolla CA 

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