July 13, 1951.

Dear Stan:
I hope the new set of 58-161 and W1177 arrived ok.

If you've still kept them, I wonder if you would be willing to send me some
sete of your Aerobacter mtants that you said were not crossable, Norton Zinder
has gotten some leads on the mechanism of recombination in Salmonella that I'd
like to follow up in other related groups of bacteria, and your mterial would
save some time. If you'd rather hold on to it to follow it up yourself, just say
go: it'll be perfectly all right with me. What I should like to have where possible
is a wild type strain plus two non-overlapping double mitants from it. I have the
impression that you have made up such seta in a number of distinct strains.

Do you remember the problem you started to work out in the lab here~ making
mitants in other coli strains for cross teste’? By crossing "-]177 with wild types
on minimal streptomycin agar (S$ T-L- x 8° T+L+, selecting for S° T+L+) we have
been able to save most of the work, and have isolated about 25 new strains (out
of 750 tests) $hag will oroas with K-12, They seem to be of all different antigenic
types, and there is no obvious pattern so far with which to recognize a cross-fertile
strain without testing it directly. I'm hoping to have a postdoctoral fealow come
in and do the iamnogenetiés.

If you haven'’ heard about it already, and are still working on isolating
mutants of other genetic work, we've developed a new technique that saves a tremendous
amount of work for some needs. It involves using sterilized velvet or velveteen
dises to print bacterial colonies from one plate to an unlimited series of others.
The velvets are mounted on a wooden cylinder, nap up, with the help of q circular
hoop. Then, a plateful of colonies is brought down on the velvet, transferring
them to its flat surface. This can then be used to print a series of other plates
(EMB different sugars} complete and minimal medium; et@.) One general applic tion
has been to show that phage- (and, we hope also drug-) resistant mitants occur in
preexisting clones in films of bacterial growth on agar plates. Transferring the
films to successive phage-coated plates results in a large portion of congruent
resistants, which can only mean that resistant clones had been theref to begin
with. This may help to @lear up that vexatious problem of ixkwmml directed ys.
selected mittations to resistance, in words of few syllables. I have Always had
trouble teaching this, especially to students who have had quite some bacteriology
before.

Sincerely,

Joshua Lederberg