March 20, 1950

Dr. @rich Seligmann

Bepartment of Bacterlology & Serdlogy
Beth Israel Hospital

Stuyvesant Park Fast

New York 3, New York

Dear Dr. Seligmann:

Thank you for the two cultures of ccliform bacteria which
you sent me some time ago. I hope tc continue study of these
organiama but ao far I have not been able to observe the
variation from lactose-positive to lactose-negative that you
describe as occurring in vivo, and rarely in vitro. I realize,
however, from yourdescription that this will be an infrequent
oceurrance. Until a mors positive result can be obtained it
will be difficult to determine whether this variation represents
a segregation or a mutation.

In my hands, both of these cultures appear to be negative
or very slow on xylose, although mutations to xylose-nositive
do cccur. Is this experience in accord with your own? I had
the impression that the lactoge-negative variant obtained in
wivo had simultancously acquired the capacity to ferment xylose,

Singerely yours,

Joshua Lederberg
Ascistant rrof. of Genstics

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