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Cell size/OD

S L Forsburg nospamforsburg at salk.edu
Wed Dec 30 16:11:56 EST 1998

Measuring optical density of a culture is a measure of cell mass.  if your
 cells are larger, they will have greater mass, and there will be
relatively fewer cells per OD unit.  This is particularly apparent for 
striking size differences, for example cdc mutants that continue to grow
without division.  Whether it makes a meaningful difference for a
 particular set of strains can only be resolved by comparing OD to 
cell number (determined by coulter or hemocytometer).  My feeling is 
 that the effect  is relatively minimal for normally growing 
wild type haploid and diploid cells,  where the size difference is ususally
not that great. 

Incidentally as a general rule of thumb, its a good idea to calibrate
new or different specs with a growth curve comparing OD to
cell number.  Different specs can vary in measuring the turbidity of
cultures which of course changes the conversion factor for cell number.
Best to remember  that OD at best is only an estimate of cell number.
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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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