48 CONDITIONS OF BACTERIAL LIFE. 



The action of the light seems to occur under the coopera- 

 tion of the oxygen of the air ; obligate anaerobes (tetanus) 

 and facultative aerobes (B. coli), when oxygen is com- 

 pletely excluded, withstand the sunlight very well; for ex- 

 ample, B. coli withstood direct intense sunlight for four 

 hours. (Compare also Wesbrook, Journal of Pathology 

 and Bacteriology, iv, 352. ) 



Regarding the mechanism of the action of light, the 

 observations of Richardson and later of Dieudonne appeal 

 important, if not furnishing a complete explanation. They 

 assert that in illuminated agar plates, and indeed only in 

 blue to ultra-violet light, in a short time (even after ten 

 minutes in direct sunlight) peroxidof hydrogen 1 appears. 

 For its demonstration one exposes to the light an agar 

 plate, half covered with dark paper, then pours over 

 the same a weak iodid of potassium paste, and upon 

 this a weak solution of ferrous sulphate; the illumi- 

 nated side becomes dark blue. (With gases that do not 

 contain oxygen there is no formation of H 2 2 , nor in- 

 jurious action from light. ) This also explains what has 

 been often observed, that one may obtain a slight attenua- 

 tion of bacilli if they are inoculated upon agar plates that 

 have previously stood in the sunlight. 2 Bacteria previ- 

 ously exposed to light develop especially badly upon, media 

 that have been illuminated, much more so than upon good 

 media. 



According to Rieder, strong Rontgen rays injure bac- 

 terial growth in a way similar to light (Munch, med. 

 Woch., 1898, No. 4, 101). 



9. THE EFFECTS ON BACTERIAL GROWTH OF 

 OTHER BACTERIA* 



Although it is the endeavor of every bacteriologist to 

 always obtain bacteria in pure culture, we must not forget 

 that in nature bacteria often occur in combination. If 



1 With gelatin it is hours before H 2 2 can be recognized. 



2 Also other decompositions of the nutrient medium by sunlight 

 may occasionally render difficult a subsequent growth of fungi ; for 

 example, the origin of formic acid from tartaric acid (Duclaux). 



