¹ From the Laboratory of The Rockefeller Institute for Medical Research, Department of Bacteriology, University of California, Berkeley

Cultures of megatherium 899a, growing under different conditions, were exposed to ultraviolet or white light.

1. Cultures exposed to ultraviolet light and then to white light continue to grow at the normal rate. Cultures exposed to ultraviolet light and then placed in the dark grow at the normal rate for varying lengths of time, depending on conditions, and then lyse with the liberation of from 5 to 1000 phage particles per cell, depending on the culture medium.

2. Increasing the time of exposure to ultraviolet light results in an increase in the fraction of cells which lyse in the dark. The lysis time decreases at first, remains constant over a wide range of exposure, and then increases. The lysis can be prevented by visible light after short exposure, but not after long exposures.

3. The time required for lysis is independent of the cell concentration.

4. Effect of temperature. After exposure to ultraviolet the cell concentration increases about 4 times at 20°, 30°, or 35°C., but only 1.5 to 2.0 times at 40–45°. This is due to the fact that the growth rate of the culture reaches a maximum at 38° while the lysis rate increases steadily up to 45°.

5. Terramycin decreases the growth rate and lysis rate in proportion.

6. At pH 5.1, the cultures continue to grow slowly in the dark after exposure to ultraviolet light.

7. Megatherium sensitive cells infected with T phage lyse more rapidly than ultraviolet-treated 899a, and visible light does not affect the lysis time.

The results agree with the assumption that exposure to ultraviolet results in the production of a toxic (mutagenic) substance inside the bacterial cell. This substance is inactivated by white light.

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