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Yeast cfu:OD ratios

S L Forsburg forsburg at salk.edu
Mon Oct 7 09:46:27 EST 1996

stanyon at wehi.edu.au wrote:
>         Recent posting suggested that 1 OD unit of S.cerevisiae was equivalent
> to 50x10^6 cfu. Having worked with the strain HF7c for a couple of years now, I
> can safely say that the faster tha yeast are growing, the more there are per
> OD600 unit: from 1x10^7 with a stationary culture, up to 3-5x10^7 for a log
> growth phase culture. Careful observation will reveal that the number of cells
> per OD600 unit does not increase smoothly - even when in log phase, the cell:OD
> unit ratio tends to fluctuate, probably due to the asymmetric division of the
> cells, combined with the extended G1 of the daughter cell population.

Indeed.  And don't forget that OD really measures cell mass; cell size
can be enormously affected by strain background.  For an extreme
 example, just think of those cdc mutants at the restrictive temp,
but a lot of cerevisiae strains vary quite a bit in overall cell size.

Also, don't forget the old problem that not all specs are alike. The
best thing to do in a new lab, or with a new spec, is take your
standard strain under standard conditions of temperature and media
and do a curve comparing OD and cell number.  That will give you 
a solid figure for YOUR conditions, as well as show you how accurate 
your spec is at high and low ends.  And a hemocytometer isn't that
hard to use.  

-- susan

Susan L Forsburg PhD
MBVL, The Salk Institute
forsburg at salk.edu

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