PRODUCTS OP METABOLISM 1 79 



If the filtrate is heated, however, no efiect 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 ™g- of 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 171). 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, 

 and. the physiological effects are also quite different, though they both 

 cause poisoning. 



Toxins are not essential products of the metabolism of pathogens. 

 Strains of pathogenic bacteria can be bred which do not produce toxins 

 as chromogens can be bred without pigment, or lactic bacteria which 

 do not produce acid. The strains which lose their pathogenicity grow 

 better on artificial media but are less able to produce disease in the 

 animal. They may regain the power of producing toxin if passed 

 through the body of the animal. The real object of toxin production 

 by microorganisms is not known; the microorganisms derive no ap- 

 parent benefit. 



Factors Influencing the Type of Decomposition 



In the chapter on products of metabolism, it has been shown 

 that the same compound can be decomposed in many different ways, 

 and the question may well be asked what decides the type of decomposi- 

 tion. Since bacteria are widely distributed, it must be expected that 

 there are certain conditions which are most favorable to a given type 

 of fermentation, while under changed conditions, other types are more 



