' 



24 VACCINES OF DEAD BACTERIA 



cholera has been referred to already, and it is the one usually 

 employed in the laboratory, where old cultures are used in pre- 

 ference to more virulent ones in the early stages of immunization. 



(e) By the use of very small doses of living cultures of full 

 virulence. This has been proved possible in anthrax, symptomatic 

 anthrax, and some other diseases. At present the process is more 

 interesting than practically useful, but it has been used clinically 

 in the case of tubercle, treatment being commenced by the injec- 

 tion of a single living bacillus, and promising results have been 

 obtained. 



3; A third class of methods consists in the use of vaccines 

 composed of dead bacteria. The advantages are obvious : the 

 dose is under accurate control ; the disease which it induces is 

 self-limited, so that it is impossible for a general infective process 

 to be produced when used on a person of deficient natural 

 immunity ; and the vaccine is easy to keep in a condition ready 

 for immediate use. Hence this method is mostly used in human 

 medicine, whereas the use of mitigated or unmitigated viruses is 

 mainly confined to veterinary work. The methods used in the 

 preparation of the vaccines varies greatly in the different cases, 

 and here we can only glance at some of the general principles. 

 In preparing the cultures, the most careful precautions have to be 

 taken to insure the purity of the microbe used and absence of all 

 other pathogenic forms, especially perhaps the spores of the 

 tetanus bacillus. The age of the culture has to be determined by 

 the necessities of the case, but as a rule young cultures are 

 preferable. The method by which the bacteria is killed also 

 varies, but heat is generally employed, and as a rule the shorter 

 the exposure and the lower the temperature the better. In other 

 cases the bacteria are emulsified in saline solution and allowed to 

 undergo autolysis at the body temperature, sterility being ensured 

 subsequently by means of heat or chemical antiseptics ; or they 

 may be killed with a minimum of heat, and submitted to autolysis 

 at 37 C. subsequently. 



There are numerous methods of determining the dose to be 

 used, (a) A definite fraction of an agar or other culture of 

 known age may be taken, or, what comes to much the same 

 thing, the growth from so many square centimetres or millimetres 

 of surface of the culture medium. (b) The amount may be 

 judged by the weight, and this is the method used in the case of 

 tubercle. When it is employed with other bacteria it is usually 



