INFECTION AND IMMUNITY. 565 



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. On the 

 other hand, as said above, certain bacteria do not possess 

 the power of generating or secreting such poisons ; they 

 have, nevertheless, intimately associated with their pro- 

 toplasmic bodies poisonous substances that manifest 

 themselves only when these organisms gain access to 

 living susceptible tissues ; thus the toxins of bacterium 

 tuberculosis and of microspira comma are much more 

 conspicuously present in the protoplasm of these bacteria 

 than in the fluids in which they have grown. 



Buchner has isolated from several species of bacteria 

 " bacterioproteins " having the common properties of 

 solubility in alkalies, resistance to the boiling tempera- 

 ture, attraction of leucocytes (positive chemotaxis T ), and 

 pyogenic powers. 



There is as yet little agreement of opinion as to the 

 chemical nature of toxins ; but it is probable that the 

 group comprises diiferent bodies of the nature of globu- 

 lins, nucleo-albumins, peptones, albumoses, and enzymes 

 or ferments. 



Toxic ptomains are probably not conspicuously con- 

 cerned in producing the characteristic symptoms of 

 infection, as they are absent from cultures of certain 

 highly pathogenic bacteria. 



In particular instances the production of poisonous 

 principles, even under artificial conditions of cultivation, 

 1 See Chemotaxis. 



