November 12, 1949.

Dr. S. A. Wakeman,

Dept. MierobLology,
Rutgers University,
New Brunswick, N. J.

Dear Dr. Waksman:

We have been cénducting some experiments lately on the use of
antibiotics for the selection of genetic recombinants in E. coli
and related bacteria. For example, from a mixture of azide-sensitive
streptomycin-resistant with azide-resistant streptomycin-sensitive
E. coli, it is possible tc select doubly resistant bacteria by
plating into agar containing both inhibitors. An exeminution of
other differential characters, e.g. fermentations, nutritional re-
quirements, phage resistance, etc., shows that most of the double
resistants are the result of recombination: ssfiboxtioocass|cn
ag’ St® x Az® se? -—— Azt se’, although a small fraction
probably are merely new resistance mitations. The results obtained
here are precisely analogous t thase previously published, with
nutritional selective methods.

So far, streptomycin has been the enly entirely suitable inhi-
biter for this purpose. Azide 1s usable, but it is not so sharply in-
hibitory, and spontaneous mutations occur uncomfortably frequently.

We badly need other inhibitors, of biological origin ur otherwise,
with the following properties: a) complete inhibition of sansitive
bacteria in agar plates, b) lack of cross-resistance, or of synergism
with streptomycin or with other inhibitors, ¢) the production of
resistant gutations, at a single step, at a relatively low frequency
ica 10-10 pur division], the resistant mutants being very nearly
unaffected by ths inhibitor. As you can judge, streptomycin fits
these requirenents beautifully for E. coli.

I am writing this letter firstly to ask for your suggestions for
other antibiotics that might be used, with information on how they
might be obtained. Secondiy, I would judge from published reports
that neomycin, in particular, and actd&nomycin as well, should be satis—
factory. ould you be in a position to make avallable small amounts
of these substances. 1 would need about 10° FE. coli units for extensive
work, but could use about che-tenth this amount for preliminary work.
Your cooperation will be greatly appreciated.

Yours sincerely,

Joshua Lederberg
Assistant Professor of Genetics