NUTRITION OF BACTERIA. 41 



The poisonous soluble products of bacterial growth 

 are known as toxins and ptomains : toxins being, in gen- 

 eral, uncrystallizable poisons, secreted or excreted by 

 the bacteria, of whose intimate chemical nature little or 

 nothing is known ; while ptomains are crystallizable prod- 

 ucts of their metabolic activity which, physically speaking, 

 are analogous to the ordinary vegetable alkaloids. 



NUTRITION OF BACTERIA. We have said that 

 through the agency of chlorophyll, in the presence of 

 sunlight, the green plants are enabled to obtain the 

 amount of nitrogen and carbon which is necessary to 

 their growth from such simple bodies as carbon dioxide 

 and ammonia, which they decompose into their ele- 

 mentary constituents. The bacteria, on the other hand, 

 owing to the absence of chlorophyll from their tissues, 

 do not possess this power. They must, therefore, have 

 their carbon and nitrogen presented as such, in the form 

 of decomposable organic substances. 



In general, bacteria obtain their nitrogen most 

 readily from soluble albumins, and to a certain extent, 

 but by no means so easily, from salts of ammonium. In 

 some of Niigeli's experiments it appeared probable that 

 they could obtain the necessary amount of nitrogen 

 from inorganic nitrates. At all events, he was able 

 in certain cases to demonstrate a reduction of nitric to 

 nitrous acid and ultimately to ammonia. Neverthe- 

 less, in all of these experiments circumstances point to 

 the probability that the nitrogen obtained by the bac- 

 teria for building up their tissues in the course of 

 their development was derived from some source 

 other than the nitric acid or the nitrates, and that the 

 reduction of this acid was most probably a secondary 

 phenomenon. It must be borne in mind, however, that 



