WRIGHT'S METHOD OF COUNTING BACTERIA 133 



THE PREPARATION OF VACCINES. 



During recent years, in consequence of the work of Sir 

 Almroth Wright, the principle of treating bacterial disease by 

 vaccines has been very much developed. The general principle 

 is to inject into the infected individual an emulsion of dead 

 bacteria. In certain cases the bacteria are subjected to dis- 

 integrating processes before being used, but most frequently the 

 vaccines simply contain killed bacterial cells, and the preparation 

 is comparatively simple. 



In the case of pyoyenic cocci, either bouillon cultures or a 

 growth off sloped agar emulsified in normal saline is taken and 

 killed by heat. The temperature employed should be the 

 minimum at which death occurs, say 65 C., applied for half an 

 hour. In the case of certain staphylococci, we have found, 

 however, that a higher temperature is necessary. After any 

 sterilisation procedure, tubes of agar must be inoculated from 

 the presumably dead vaccine, and incubated for twenty-four 

 hours in order to ascertain if the sterilisation has been effective. 

 As the dosage of a vaccine is of great importance, it is necessary 

 to count the bacteria present. This is done by one of the 

 methods given below. Appropriate doses (see Chapter VII.) are 

 then with all aseptic precautions measured by means of a sterile 

 graduated pipette, and placed, along with an equal volume of 

 5 per cent, lysol, in little glass bulbs drawn out to a capillary 

 tube at one end. These when charged are sealed off, and for 

 use the sealed end is broken off, the contents are sucked up into 

 a sterile hypodermic needle, and injected fairly deeply into the 

 -kin, usually in the region of the flank. 



In the case of the typhoid bacillut, organisms are used of such 

 virulence that a quarter of a twenty-four hours' old sloped agar 

 culture, when administered hypodermically, will kill a guinea- 

 pig of from 350 to 400 grams. Flasks of bouillon are inoculated 

 with such a culture for forty-two hours at 37 C. The bacteria are 

 then killed by the flask being put into a water bath at 62 C. 

 for fifteen minutes ; '5 per cent, lysol is added, and the bacteria 

 in the vaccine are counted. By such methods, vaccines against 

 any of the pyogenic cocci and against any members of the coli- 

 typhoid group can be made. 



The vaccines used in tuberculosis, cholera, and plague will be 

 described in the chapters on these diseases. 



Wright's Method of counting the Bacteria in Dead 

 Cultures. In the making of vaccines it is, as indicated above, 

 necessary to know the total number of bacterial cells, whether 



