From: Dace (firstname.lastname@example.org)
Date: Mon 05 Dec 2005 - 20:40:20 GMT
> From: Derek Gatherer <email@example.com>
> Exactly, the selection medium simply gets living cells dumped in on
> top of the dead ones. The same flask is used, not a clean one. In
> the unselected medium there is passage (which means spinning down
> cells, taking an aliquot and seeding it into a clean flask with fresh
> medium). So tell me then, where's the control? What is being
> compared with what?
Here's the relevant passage, from the bottom of page 213:
"The serial assay was redesigned as follows. A cell line was grown from
small inocula in a culture medium containing no selection agent. At each
passage, one or more cell samples were withdrawn and seeded into separate
flasks (instead of a depleted monolayer) containing the selection medium.
In these flasks, because of selection pressure, the wt cells died while
drug-resistant cells gave rise to visible colonies. Mutation frequency at
each passage level of the cell line was then calculated from the total
number of colonies divided by the number of cells assayed for resistance
and corrected for cloning efficiency."
What Hill has demonstrated is that when a colony of cells is subjected to a
toxin and develops resistance to this toxin (over the course of
generations), the same mutation enabling this resistance will also begin
appearing in a closely related but physically separated culture. This
indicates a nonrandom pattern of mutations in direct contradiction to
Incidentally, Hill has responded to your initial complaint, namely that if
his results were true, researchers would already have spotted this
Dear Ted Dace,
Researchers don't see nonrandom mutations in unexposed cultures because
they don't look for them in a good way. To my best knowledge, no one ever
tried a serial assay though such assays are known from 1986 (see below).
Unlike a standard procedure the serial assay "extends across several
passages". This may last for several months, showing that during this time
period the cell population contains no resistant mutants.Then at further
passages these mutants are found at small but increasing numbers. The
absence of such mutants at low passages and their occurrence at high
passage levels, taken together, provide evidence of nonrandomness.
See also Hill M and Hillova J., Non-random acquisition of ethionine
resistance by Chinese hamster fibroblasts. Appendix: acquisition of
thermoresistance. Biomedicine & Pharmacotherapy 40, 11-19, 1986.
Yours sincerely, Miroslav Hill
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