November 12, 1946.

Dr. T.M. Rivers,
Director, Rockefeller Inst. Hosp.,

New York, N.Y.
Dear Dr. Rivers,

Some years ago, a paper appeared by Dr. Valentine and
yourself, on symbiosis between two spp. of Hemophilus, involving
the interchange of X- and V- factor respectively. This work has
been of very great interest to me, as we have been conducting
similar experimenss using different nutritional variants or
mutants of a strain of Escherichia coli. A similar type of nutri-
tional symbiosis (or syntrophism) takes place: for example, a.
mutant strain deficient in methionine and one deficient in proline
will grow quite well together in a synthetic medium which will
not support either of the original strains to any extent.

Further work on thiis phenomenon however, using strains
which were marked with several nutritional requirements and other
distinctive characters has led us to the conclusion thet another
process may take plave in such mixed cultures, complicating the
situation. This is the appearance of new types in the modiua, which
can only be adequately interpreted by assuming that recombination
of genes controlling these chamacters takes place, that i: to say
that there is a sexual mechanism in this strain of Escherichis coli.
Aivong these new types are some (prototrophs) which have no nutri-
tional requirements and can grow in the unsupplemented synthetic

medium. If you are interested, furtiier details are given in a short

note in tne Oct. 19 issue of Nature.
Attempts, thusfar,to demonstrate the occurrence of
recombination of characters found in natural populetions of
bacteria have thusfar been unfruitfudl. For this reason
the interaction of Hemophilus parainfluenzae and of H.
(henoglobinophilus) canis is of particular interest to us.

It is not apparent from the data published in your
paper that & recombinational mechanism can be. excluded;
this would require that the original X~ or Ve requiring
types be re-isolated from the mixed culture, and that atteppts
to obtain a pure culture, requiring neither X- nor V-, fail. |
Dr. A. Lwoff, who has repeated the experiments in the manner
in which they are published, has informed me that he did not
attempt to re-ifsolate the origin:1 components either.

Dr. Lwoff also informs that H. canis is not available
at any jJocation with which he is familiar. I would appreciate
it very much if you could let me know where a strain of this
organism can be obtained, or even better, if you could send me
transfers of the canis and parainfiluenzae cultures which were
used in your experiments. On the other hand, we would be e:ually
delighted to hear that you might be interested to conduct such
experiments on your own part, since we have had very little experi-
ence with these fragile organisms, and their nutrition. Any other
information would be helpful, as would reprints of your relevant
publications, and would be greatly appreciated.

Very sincerely yours,

Joshua Lederberg.