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 appear 

 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 i 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 H2O2, 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. ^ 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 (Miinch 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 foro-et 

 that in nature bacteria often occur in combination, ''if 



^With gelatin it is hours before H2O2 can be recognized. 



^Also other decompositions of the nutrient medium by sunlieht 

 may occasionally render difficult a subsequent growth of funo-i- for 

 example, the origin of formic acid from tartaric acid (Duclauxf ' 



