Conditions Prejudicial to Growth of Bacteria 61 



Reaction. Should the pabulum supplied contain an 

 excess of either alkali or acid, the growth of the micro- 

 organisms is inhibited. Most true bacteria grow best in a 

 neutral or feebly alkaline medium. There are exceptions 

 to this rule, however, for Bacillus butyricus and Sarcina 

 ventriculi can grow well in strong acids, and Micrococcus 

 urea can tolerate excessive alkalinity. Acid media are 

 excellent for the cultivation of molds. Neutral or feebly 

 alkaline media serve best for the cultivable protozoa. 



Light. Most organisms are not influenced by the presence 

 or absence of ordinary diffused daylight. The direct 

 rays of the sun, and to a less degree the rays of the electric 

 arc-light, retard and in numerous instances kill bacteria. In 

 a recent careful study of this subject Weinzirl* found that 

 when bacteria were placed upon glass or paper, and exposed 

 to the direct rays of the sun, without any covering, most 

 spore-bearing bacteria, including Bacillus tuberculosis, B, 

 diphtheriae, B. typhosus, s. cholerae asiaticae, B. coli, B. pro- 

 digiosus, and others are killed in from two to ten minutes. 

 Certain colors are distinctly inhibitory to the growth, blue 

 being especially prejudicial, 



Treskinskajaf found that sunlight had a marked destruc- 

 tive effect upon the tubercle bacillus, and varied according 

 to altitude. By direct sunlight at the sea-level they were 

 destroyed in five hours: at an altitude of 1560 meters, in 

 three hours. In winter the time of destruction was about 

 two hours longer than in summer. In diffused daylight the 

 time required for destruction was about twice as long as in 

 direct sunlight. His experiments were performed with 

 pure cultures dried in a thin layer upon glass. 



Certain chromogenic bacteria produce colors only when 

 exposed to the ordinary light of the room. Bacillus mycoides 

 roseus produces its red pigment only in the dark. The 

 virulence of many pathogenic bacteria is gradually attenuated 

 if they are kept in the light. 



Molds and yeasts grow best in the dark, so that in general 

 it can be said that the vegetable micro-organisms, belonging 

 to the fungi and having no chlorophyl, need no light and are 

 injured rather than benefited by it. 



The pathogenic protozoa have not been particularly studied 



* " Centralbl. f. Bakt. u. Parasitenk. Ref.," xi/ra, Nos. 22-24, P- 68l 

 f " Jour. Infectious Diseases," vol. iv, 1907, Supplement, No. 3, 

 p. 128. 



