The Killing of Moulds by an Ordinary Electric Bulb. ∗

Darkness provides optimum condition for the growth of moulds, and ultraviolet rays exert a rapid and violent fungicidal effect on them, but one does not usually suspect the extreme sensitiveness of the Mucoraceae to the feeble light of an ordinary electric lamp. Our attention has been drawn to this point by the fact that Mucoraceae cultures, while on our experimenting table, awaiting examination, have been killed by the light of the microscope lamp.

Spores of Rhizopus nigricans, in suspension in sterilized water, were spread, by means of a soft brush, on the surface of agarized Coon's medium,† sterilized and distributed in a series of Petri dishes. These cultures were irradiated, through the glass covers of the dishes, by the light of an ordinary bulb, labeled: “Westinghouse, Mazda, 60 W, 115 V.” and working on direct current. There was a distance of 5 cm. between the cultures and the luminous filament. An electric fan cooled the dishes and excluded the influence of heat.

Irradiations were made as follows: 1. Half an hour after the inoculation with the spores, that is, before any germination. 2. At the end of 15 hours, when the sprouts were some mm. long. 3. At the end of 30 hours, when the length of the mycelium exceeded 1 cm. 4. At the end of 40 hours, when the sporangia have developed. (These stages of growth are attained in the indicated intervals at a temperature of 22°C.)

Under these conditions we have observed, in 15 different experiments, that 20 to 30 minutes exposure destroyed the spores, that 5 to 8 minutes killed the mycelium of 15 to 30 hours, but that 5 hours irradiation were not sufficient to cause the death of sporangia.