sysop at microbiol.org sysop at microbiol.org
Fri Mar 31 05:42:25 EST 1995

IK>Try the following:

IK>OD550 = (1.0315e-8)(cell number/ml)+1.7633e-3

IK>The r-squared for this regression is 0.9997 and the data are for the
IK>genus Vibrio (taken from Primrose & Wardlaw, 1982, Sourcebook of
IK>experiments for the teaching of microbiology, p. 151).  This equation is
IK>valid only for ODs from 0.02 to 0.50, and in general one should be use
IK>ODs less than 1 for accuracy (so dilute).

I would reinforce an earlier reply - OD's are related to cell mass, not
cell numbers, and so must be determined for each species and wavelength.
 The above was correctly stated as specific for Vibrio, and may not work
for E coli or Pseudomonas, etc.  It is important to correlate OD to
colony forming units, as some organisms tend to cluster or chain, and so
will not yield a strict relationship of 1 CFU = 1 viable cell.

Scott Sutton
The Microbiology BBS


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