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OD units

Gavin Sherlock sherlock at genome.stanford.edu
Tue Mar 23 14:17:17 EST 1999

In article <7d6fr4$co5 at net.bio.net>, Peter Mastrangelo
<peter.mastrangelo at utoronto.ca> wrote:

> When a certain number of OD600 units is specified in a protocol are theses
> units calculated from a certain OD range (ex. 0.4 to 0.6) and if so, how do
> OD units convert to cell concentration (cell/mL)?

In general I feel comfortable with the linearity of an OD600
measurement in the 0.1 to 0.3 range.  You should dilute down if you're
(far) above this.  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.  That having been said I almost always
say that an OD600 of 0.1 is roughly equivalent to 1x10^6 cells/ml.  If
you're just transforming a plasmid, or making extract this is fine.  If
you're doing plasmid loss assays, or viability testing you need a
curve, or even better use a Coulter Counter if one's available,
Gavin Sherlock

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