INFECTION AND IMMUNITY. 639 



vatea substances which, when separated from the 

 bacteria by which they were produced, possess tiie 

 power of causing in animals all the constitutional 

 symptoms and pathological tissue-changes that occur 

 in the course of infection by the organisms themselves. 

 In some instances these poisons — toxins,^ as they are 

 collectively called — appear to be the direct result of 

 metabolic changes brought about by bacteria in the 

 medium or tissues in which they may be developing — 

 i. e., they are products of nutrition that pass readily into 

 solution, as is conspicuously seen in the case of the 

 bacillus of diphtheria and of tetanus when under both 

 artificial cultivation and in the animal body. Many 

 bacteria which do not possess the power of generating 

 or secreting such poisons may, nevertheless, have inti- 

 mately associated with their protoplasmic bodies poison- 

 ous substances that can only be isolated by particular 

 methods ; thus the toxins of bacillus tuberculosis and of 

 spirillum oholeroe Asiatioce are much more conspicuously 

 present in the protoplasm of these bacteria than in the 

 fluids in which they have grown ; and Buchner has 

 isolated from several species of bacteria "bacterio- 

 proteins" having the common properties of solubility 

 in alkalies, resistance to the boiling temperature, attrac- 

 tion of leucocytes (positive chemotaxis ^), and pyogenic 

 powers. 



There is as yet little agreement of opinion as to the 

 chemical nature of toxins ; but it is probable that the 



' The term "toxins" is commonly applied to amoiylious, nitrogenous 

 poisons pvoduced by bacteria in both living tissues and dead substances ; 

 while, on the other hand, " ptomains " relates to crystallimble, nitrog- 

 enous poisons that are formed in dead tissue, and "leucomains" to 

 poisonous and non-poisonous alkaloidal bodies that occur in living 

 tissues as a result of physiological metabolism, 



2 See Chemotaxis, 



