EFFECT OF DELETERIOUS INFLUENCES UPON BACTERIA 103 



pletely destroyed vitality in from five to six hours. Electric arc-light 

 inhibited growth in five hours and destroyed vitality in eight hours. 

 Incandescent light inhibited growth in from seven to eight hours and 

 killed in eleven hours. Similar results have been obtained with B. coli, 

 B. typhosus, and B. anthracis. According to Koch, the tubercle bacillus 

 is killed by the action of direct sunlight in a time varying from a few 

 minutes to several hours, depending upon the thickness of the layer 

 exposed and the season of the year. Diffuse daylight also had the same 

 effect, although a considerably longer time of exposure was required 

 when placed close to a window, from five to seven days. 



Only the ultraviolet, violet, and blue rays of the spectrum seem to 

 possess bactericidal action ; green light has very much less so ; red and 

 yellow light not at all. The action of light is apparently assisted by the 

 admission of air; anaerobic species, like the tetanus bacillus, and facul- 

 tative anaerobic species, such as the colon bacillus, are able to withstand 

 quite well the action of sunlight in the absence of oxygen, the B. coli 

 intense direct sunlight for four hours. 



According to Richardson and Dieudonne, the mechanism of the 

 action of light may be at least partially explained by the fact that in 

 agar plates exposed to light for a short time (even after ten minutes' 

 exposure to direct sunlight) hydrogen peroxide (H 2 O 2 ) is formed. This 

 is demonstrated by exposing an agar plate half covered with black 

 paper, upon which a weak solution of iodide of starch is poured, and 

 over this again a dilute solution of sulphate of iron; the side exposed 

 to the light turns blue-black. In gases containing no oxygen, hydrogen 

 peroxide is not produced, and the light has no injurious effect. Access 

 of oxygen also explains the effect which light produces on culture media 

 which have been exposed to the action of sunlight, as standing in the 

 sun for a time, when afterward used for inoculation. The bacteria 

 subsequently introduced into such media grow badly far worse than 

 in fresh culture media which are kept in the shade. 



Influence of Radium. Radioactive fluids have a slight inhibiting 

 effect on bacterial growth, but nothing decided enough to be used for 

 therapeutic purposes. 



Influence of X-rays. These rays have a slight inhibiting effect on 

 bacteria when they are directly exposed to them. 



Influence of One Species upon the Growth of Another. While it is the 

 custom of bacteriologists to have pure cultures to work with, we should 

 never forget that in nature bacteria often occur in mixed cultures. If 

 we examine water, milk, or the contents of the intestines of either sick 

 or healthy persons we shall always find several species of bacteria occur- 

 ring together. This admixture may, perhaps, seem to us at first merely 

 accidental, but on further investigation it will appear also that in the 

 department of bacteriology there exist synergists and antagonists, or 

 at least bacteria which assist or oppose one another mutually or one- 

 sidedly. Nencki speaks of symbiosis and enantobiosis. 



The existence of antagonisms can be demonstrated experimentally 

 by inoculating gelatin streak cultures of various bacteria. It is found 



