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THE UNIVERSITY OF WISCONSIN

COLLEGE OF AGRICULTURE

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DEPARTMENT OF GENETICS

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9

Br. Sol SpiegeLmn
2epar bent of Bac ter iology
University of Illinois
Urbana, Illinois.

Dear Sol:

Madison 6

November 25, 1951

Seymour relayed your request for more details on preservation of microbes

on silica gel. The basic principle is obvious: to use a chemically inert desi-
cant so that it need not be separated from the microbes. It was hoped that
adeyuateiy dried bacteria could not be influenced by the presence of oxygen,
so the procedure has consisted simply of adding a sinall volme of concentrated
suspension to a pre-sterilized tube af the gel, then sealing off directly.
If the tubes have to be evacuated, several extra steps (constricting tubes
with a torch,; evacuating, and then sealing) are needed, and there would be
no advantage over the technique used by Brown, Uershey, Lindegren, etc.

I wish I could rgport a well-developed method, but a good deal of experhen-

tation of a rather odious bxs sort (with long waiting periods) is still needed.
Xth cells sus-ended in peptone, there is a process loss of 1-2 bels,  and over
a 90 dsy period a continued attenhation amounting to about 5 bels per year.
At this rate, a year would be the mxizlum, but all depends on whether the
attenuation curves continue, or fall off. I have been using several types of
silica gel from the Davison Coinpany, Raltinore, idd. Their purest grade is
"Code &Off, 6-16 aesh, and sells for about .70 per pound.

There are at least ttvo innovations that should be tried: a protein menstruum

rather than peptone, and perhaps adding the suspension to a layer of filter
paper or glass or the like just over the silica column. Host of all, I don't
know the optimum moisture conditions: I suspect that it is rather easy to
obtain preps. that are @ dry. Published work refers to 1-2; residual mokiture
as optimal by the stahdard nethods, but it is rather difficult to caQculate
the vapor pressure4 over the cell mass, which &&.IC is the critical and controllable
parameter when silica is used. The usual methods do not establish a true equili-
briuin betweer. the cells and the dessicant or vapor trap, and the dessicants
used (z.g. CaCl  probably have to be cakculated at tie vapor pressure of a
saturated solutfon, considering the quahtities usualljz prescribed.

Here is a good exanple  of a technical innovation that has a theoretically
sound basis ,  and that would be very worthwhile (especiilly to geneticists with
thousands of cultures), but which ,nay require an anount of empirical xork that
would be prohibitive for one person (like mysell). In some respects, it would
aake ail excellent problem to be aywoached fro% a biophysical point of view,
and one that would have Laportant Lrplications for general biology: what is a
viable, dried cell? Jeanwhile, I have been qlaylng with the system a bit,
in rather discouraged fashion. dilica ?el is excellenthfor short-tern nressrvation

and aailing cultures. Perishability, esgpcially with

greatly reducsd.

Sincereiy,