PRODUCTS OF MICROBIAL ACTIVITIES 249 



bodies, but even this has not been proved. A pure toxin has never 

 been obtained and we have no criterion for its purity. The presence 

 of a toxin is recognized only by an animal test and in this way the com- 

 parative concentration can be determined approximately. Such 

 standardization of toxin solutions is only comparative, however, and 

 gives no clue as to the actual amount of toxin present. Not all ani- 

 mals are sensitive to all toxins. It is quite possible that all bacteria 

 produce compounds with chemical qualities similar to toxins, and only 

 a few of them happen to react upon men or animals. 



Toxins are not always the product of microbial action. Vegetable 

 toxins or phytotoxins are known, among which the ricin of the castor- 

 oil bean is perhaps the most studied representative. The best-known 

 zootoxin is the rattlesnake poison. These non-microbial compounds 

 have the same quality as the microbial toxins they are extremely 

 poisonous. Toxins are the cause of disease in diphtheria, tetanus and 

 botulism. If a culture of these organisms is filtered through a porcelain 

 filter which removes all bacterial cells, the filtrate injected into an 

 animal will cause the disease with all its accompanying symptoms 

 though there are no microorganisms introduced into the animal body. 

 If the filtrate is heated, however, no effect will take place after the in- 

 jection, because heat destroys the toxin. The amount of toxin that will 

 kill an animal is extremely small. .000005 m g- f the purest tetanus 

 toxin will kill a mouse, .0007 mg. of ricin will kill a rabbit, less than 

 .23 mg. of tetanus toxin will kill an adult man. The body of an animal 

 ,or man forms an anti-body against the toxin which neutralizes its 

 poisonous action. Anti-bodies are also formed against enzymes 

 injected into an animal. 



Toxins are very sensitive to heat. A short exposure to temperatures 

 between 80 and 100 will inactivate them. They are also very sensi- 

 tive to light. While some toxins are secreted, others are retained within 

 the cells of microorganisms, and never leave them until the cells die or 

 disintegrate. Ptomains, which are also metabolic products of micro- 

 organisms and sometimes cause poisoning, differ from the toxins in their 

 resistance to heat and light (page 241). Ptomains differ in no way 

 essentially from ordinary organic compounds; the animal or human 

 body produces no anti-ptomains to counteract their poisonous effects. 

 There is no chemical relation whatever between toxins and ptomains, 



