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I have delayed so long to write to you that you have probably forgotten our original correspondence. If you remember I was
unable to obtain any mutant cultures from strain of Pseudomonas pyocyanen following xray treatment and obtained some helpful
suggestions from you.
I might mention at the start that I had tried an incubation period (after irradiation) in complete medium before looking for
mutants without any success.
The penicillin technique was one which was unfamiliar and on trial has proved very successful. I have had to use pencillin
at 10,000 units/ml in the technique, a value which is considerably higher than the 1000 units/ml which you quoted you had
used with Px. fluorescence. Probably the only reason for continuing with the organism has been pure stubbornness. The work
was originally started as a means of investigating the high resistance of the Pseudomonas to most of the common chemotherapeutic
agents. It was felt that there might well be changes in resistance to some of the agents associated with changes in metabolic
I have now obtained about eight singly mutants and from them I have prepared a number of double mutants giving about 18[?]
Following a treatment with about 20,000, a period of incubation in complete medium and penicillin treatment I obtained 1-3
mutant cells from about 20,000 survivors from the x rays. Thus my mobility to restart them before probably arose because
of their so few numbers.
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About nine mutations are about all that have appeared. In other words the same mutations keep occuring over and over again.
One or two mutants are of interest in [. . . ] in a single step double requirements appear. A need for adenine and thiamine
appeared together and (anginine and urasil) has appeared on several occasions. In another case a strain requiring methanine
was irradiated. On investigation of the double mutants one was found to need thiamine and tryptophane but not methionine.
This latter may be due to an impure culture at the start since I did not start at that stage with a single cell.
I now intend to investigate each of these mutant cultures and to see if there are any differences in t heir susceptibility
to various drugs. I am very grateful to you for your suggestions which have helped me to reach this stage.