Department of Genetics
University of Wisconsin
Madison 6, Wisconsin

January 1, 1952
Preservation of Bacterial Cultures on Silica Gel

This circular was written in response to a number of inquiries. Judging
from these, present methods for preservation of bacterial cultures are not
entirely satisfactory, and it would be a real contribution to laboratory tech-
nique to work out a better one. Unfortunately, I can only suggest a principle
that seems very plausible, but that has not yet been empirically justified.

The working principles are (1) that suitably dried bacteria should survive
just as well in a sealed tube under air as in vacuo, and (2) that if this is
correct, chemically inert desiccants such as anhydrous silica gel could greatly
facilitate the practice, The following arrangement has been tried: small vials
or tubes are filled nearly full with silica gel (Davison Company, Baltimore;
Grade 40, 6-16 mesh). About 1 to 1.5 ems. of silica fits well into the tubes
used. The tubes are plugged, then baked in a sterilizing oven at 160-180 C.,

2 hours, to dry and sterilize the tubes. These are stored in a desiccator. The
bacteria to be preserved are suspended in 2% peptone. About -95 ml. is pipetted
directly to the silica, and the end of the tube sealed off. The tubes were then
stored in a refrigerator, The water disappears very quickly. To regenerate the
cultures, the tubes were broken, and the silica poured into broth. Considerable
gas is liberated. After about an hour to allow redispersion, viable counts were
made on the broth. The 24-hour survival, in apparently dried condition, was qutite
high (about 1€%), but this was more encouraging than long-term experiments. After
four ~- five months, the survival has been low, of the order of 10-5, and some
tubes are inviable. In its present form, the method is not a success, and can-
not be recommended for long-term preservation and storage. I think that it could
be greatly improved, without complication, by experiments leading to a better
suspending fluid, and possibly by drying the cells on a layer of glass bead over
the silica. What is most needed is an improved theoretical understanding of the
biology of suspended animation in successfully dried cultures.

Despite its shortcomings, the silica gel tubes provide an ideal method for
mailing cultures, ‘They are not affected by undue cold in the way agar slants
are, and probably ought to be more resistant to high temperatures as well. The
mechanical strength of small sealed tubes allows them to be sent with simple
padding in an ordinary envelope, and the absence of any liquid minimizes pos-
sible hazards from breakage and leakage,

I hope that other workers with suitable facilities to study preservation
problems, or with a potential interest in the biophysics of suspended animation
may be able to make some use of this suggestion.

Joshua Lederberg

ee