160 PRINCIPLES OF BACTERIOLOGY 



Practical Value of Injections of Bactericidal Sera. The use of serums 

 having specific protective properties has been tried practically on a large 

 scale in man as a preventive of infection. In susceptible animals 

 injections of some of the very virulent bacteria, as pneumococci, 

 streptococci, typhoid bacilli, and cholera spirilla, can be robbed of all 

 danger if small doses of their respective serums are given before the 

 bacteria have increased to any great extent in the body. If given 

 later they are ineffective. For some bacteria, such as tubercle bacilli, no 

 serum has been obtained of sufficient power to surely prevent infection. 

 Through bactericidal serums, therefore, we can immunize against an 

 infection, and even stop one just commencing; but as yet we cannot 

 cure an infection which is already fully developed, though even here 

 there is reason to believe that we may possibly prevent an invasion of 

 the general system from a diseased organ, as by the pneumococcus from 

 an infected lung in pneumonia. On the whole, the serums which 

 simply inhibit the growth of bacteria have not given, as observed in 

 practice, conclusive evidence of great value in already developed disease. 

 This type of serum loses much of its bactericidal properties quickly 

 and should not be used when kept for more than a few weeks. 



Development of Antitoxins together with Bactericidal Substances. 

 Although the serum of animals which have been infected with any one 

 of many varieties of bacteria is usually both antitoxic and bactericidal, 

 still one form of these protective substances is usually present almost 

 alone; thus antitoxic substances are present almost exclusively in animals 

 injected with two species of bacteria which produce powerful specific 

 poisons viz., the bacilli of diphtheria and tetanus. When the toxins 

 of either of these are injected in small amounts the animals after com- 

 plete recovery are able to bear a larger dose without deleterious effects, 

 and these doses in the more suitable animals can be gradually increased 

 until a thousand times a previously fatal dose may be administered 

 without any serious results whatever. To Behring and Kitasato we 

 owe the discovery that this protecting substance accumulates to such 

 an extent in the blood that very small amounts of serum are sufficient 

 to protect other animals from the effects of the true extracellular toxins. 

 Except the diphtheria and tetanus bacilli a few only of the important 

 parasitic bacteria attacking man produce these toxins and thus become 

 capable of causing the production in the body of antitoxins, and even 

 these do it to a far less extent than those of tetanus and diphtheria. 

 Following them is the plague bacillus, and then the cholera spirilla, 

 the typhoid bacilli, the streptococci, etc. These latter bacteria when 

 injected excite more of the substances which inhibit bacterial growth 

 than of those which neutralize their toxins. 



Antitoxin a Preventive. Antitoxin prevents the poisonous action of 

 toxin. It does not restore the cells after they have been injured by 

 the toxin; it is, therefore, like the bactericidal substances, a preventive 

 rather than a cure. We find, experimentally, that a very much smaller 

 amount of antitoxin will neutralize a fatal dose of toxin in an animal, 

 if given before or at the same time, than if given only shortly after it. 



