26 CLINICAL BACTERIOLOGY. 



Light is required only by such species of bacteria as 

 contain chlorophyl. For the far greater number, the 

 action of sunlight and even of diffuse daylight is directly 

 injurious ; their growth is inhibited, and after a time the 

 bacteria are entirely destroyed by the action of the light. 

 This sterilizing influence of light is most manifest when the 

 rays of the sun fall vertically upon the surface of the cul- 

 tures. Not only the growing forms, but also the perma- 

 nent forms, the spores, succumb to this action of light. 

 The spores of anthrax, for instance, are destroyed in a 

 moist substratum by the action of light in a somewhat 

 shorter time than the mature bacilli. In the dried state, 

 however, the spores prove more resistant to the rays of the 

 sun. The metabolic products of the bacteria also become 

 materially attenuated under the influence of strong light, 

 especially when there is no obstacle to access of oxygen. 

 Nutritive media that have been exposed to light are not, in 

 consequence, unsuitable for the cultivation of microorgan- 

 isms. Not all rays of the spectrum are bactericidal, but 

 only the blue, the violet, and the ultraviolet. 



Of great importance is the influence of oxygen. Many 

 bacteria are capable of developing only in the presence of 

 this gas (aerobic bacteria] ; others, on the other hand, 

 thrive only in complete absence of oxygen (anaerobic bac- 

 teria) ; a third group occupies an intermediate position : 

 they grow as well in the presence as in the absence of oxy- 

 gen {facultative anaerobic bacteria). 



In their growth and multiplication the bacteria generate 

 metabolic products upon and from their culture-medium. 

 Thus the whole range of fermentative processes is an ex- 

 pression of bacterial activity, and the substances that are 

 thereby formed are to be viewed as the metabolic products 

 of fermentative bacteria. The same statement applies to 

 putrefaction and decomposition. 



A number of mostly innocuous bacteria are characterized 

 by the formation of pigment. These pigment-bacteria pro- 

 duce most varied coloring-matters, which appear in all pos- 

 sible shades of color only when access of air is entirely 

 unobstructed. Some bacteria give rise in their culture- 

 media to beautiful fluorescence, others to phosphorescence 

 that is, they appear luminous in the dark. 



The chemic combinations to which bacteria give rise in 

 artificial nutritive media are at present of preeminent in- 



