50 BACTERIOLOGY 



ptomains play any part in either the causation or course 

 of the definite, infective diseases. 



Ptomains have been isolated from decomposing cadavers, 

 from putrid meat, milk, cheese, and from a number of 

 bacterial cultures. Poisonous ptomains occasionally develop 

 in improperly preserved food. True toxins and dangerous 

 bacteria have also been found in such substances. 



Nutrition of Bacteria. — ^We have said that through the 

 agency of chlorophyl, 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 decom- 

 pose into their elementary constituents. The bacteria, on 

 the other hand, owing to the absence of chlorophyl from 

 their tissues, do not possess this power. They must, there- 

 fore, 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 proteins, and to a certain extent, but by no 

 means so easily, from salts of ammonium. In some of 

 Nageli'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 ulti- 

 mately to ammonia. Nevertheless, in all of these experi- 

 ments circumstances point to the probability that the 

 nitrogen obtained by the bacteria 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. We must bear in mind, however, the specific 

 group, the nitrifying bacteria, which increase and mul- 



