TeLePHone PADDINGTON 148

1662

DR/ GC

Dr. J. Lederberg,
Department of Genetics,
University of Wisconsin,
Madison,

Wisconsin,

U.S.A.

Dear Dr. Lederberg,

THE WRIGHT-FLEMING INSTITUTE OF MICROBIOLOGY,
ST. MARY’S HOSPITAL MEDICAL SCHOOL,
PADDINGTON, LONDON, W.2.

25th February, 1954.

Thank you for sending me the six B. coli strains which
outeross with K12, Unfortunately none of them was mouse virulent.
In the meantime we have been examining some hundreds of coli strains

for recombination with Kl.

We have found several strains which are

fertile and one of them is extremely mouse virulent.

We have crossed this with avirulent W677S2/F- on SM By but
of 100 prototrophs only one was virulent, the nex} had the same virulence

as the WF- parent.

The numbers of recombinants are very greatly reduced with
the other selective media which we have tried. We are now developing
some auxotrophic markers in our virulent strain. .

Do you have any other auxotrophic F- strsins other than
58 yW? if so We Would be most grateful for any such strains. Thank you

for the strains,

Kind regards,

Yours sincerely,

. *
naw ea

we OE pet

~~

Derrick Rowley.