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 L Forsburg PhD
MBVL, The Salk Institute
forsburg at salk.eduhttp://flosun.salk.edu/~forsburg