THE RESISTANCE OF MOLD SPORES TO THE 
ACTION OF SUNLIGHT 
JOHN WEINZIRL 
The present paper is one of a serie§ on the action of light 
on microorganisms, and grew directly out of the work on 
the action of light on bacterial spores. In that paper it was 
shown that bacterial spores resist the action of direct sun- 
light from two to eight hours, i. e., they show about sixty times 
the resistance of non-spore-bearing bacteria. Having found 
bacterial spores to be highly resistant, it was quite natural 
to inquire how this resistance compares with that of mold 
spores. The simple method devised for making the exposures 
has not been tried on mold spores so far as the writer is aware 
and it seemed worth while to extend it into this field. The 
method has previously been described at length;? briefly 
stated, it consists in first making a suspension of the spores 
in water or physiological salt solution; from this suspension 
a droplet is spread upon a small slip of sterile paper (1 x 3 
em.) contained in a sterile petri dish; for this purpose the 
standard platinum loop used by bacteriologists works very 
well; the infected paper slips are first dried, then exposed to 
the sun in a petri dish without the presence of any medium. 
This avoids all disturbing factors such as germination, forma- 
tion of disinfectants, and the excessive absorption of the chem- 
ical rays by the medium. The plates are exposed on wire trays 
raised about 16 inches above the floor, and are held so that 
the sun’s rays fall as nearly vertically as practicable. After 
being exposed a given length of time the inoculated slips are 
transferred by means of a sterile forceps to a suitable medium, 
nutrient agar being very satisfactory. If the surface of the 
medium is dry, then it is well to add a few drops of bouillon. 
1American Journal of Public Health, 4, 11, p. 969, 1914. 
2 Journal Infectious Diseases, Sup. No, 3, p. 128, 1907. 
