MUTATIONS IN METABOLIZING BACTERIAL POPULATION BY LONG-TERM CONTINUOUS IRRADIATION WITH RADIOISOTOPES, P³², S³⁵ AND Sr⁹⁰

Abstract

The lethal and mutagenic effects of continuous irradiation with radioisotopes, P³², S³⁵ and Sr⁹⁰, on the growing and metabolizing population of a Mycobacterium were studied.
Continuous irradiation with P³²-isotopes added as p³²-orthophosphate at a concentration of 0.5μc/ml to a medium exhibited lethal and mutagenic effects during a four week observation period, but the effects became insignificant after five or six weeks of incubation. The effects were more prominent in the Sauton medium, in which the organism grew abundantly, than in the phosphorusdeficient medium, in which it grew only poor and slowly. However, greater amounts of P³² were incorporated into the cells growing in the phosphorusdeficient medium than these cells growing in the Sauton medium. The effects on the growing and metabolizing bacterial population were not too affected by the content of P³²-atoms in the DNA or in the total cell as it affected the frozen and resting cells. On the other hand, the population was considerably affected by the physiological condition of the cell. The lethal effect was less in the metabolizing population than in the frozen population.
The addition of 5μc of S³⁵-sulfate per ml to sulfur-deficient medium, in which the organism grew abundantly, could cause neither mutagenic nor lethal effect on the metabolizing population at least during a six-week observation period, whereas the addition of 0.5μc of P³²-phosphate per ml to a medium exhibited a significant change.
A continuous irradiation of the organism, which grew slowly in a phosphorus-deficient medium containing 2μc of Sr⁹⁰-nitrate per ml for a 285-day irradiation period, caused a significant increase (a 27-fold increase) in mutation frequency to isoniazid resistance but caused a slight increase (a 3-fold increase) in viability.
It is suggested that, in the growing and metabolizing population also, lethal and mutagenic effects are produced chiefly by transmutation and that a longterm irradiation with beta-ray (Sr⁹⁰) also caused an increase of mutation frequency without the accompaniment of a significant loss in viability.