BACTERIOLOGICAL TECHNIC. 41 



when the temperature is raised above 100 C, or even if kept at 100 C. for 

 a long time or for a short time, if oft repeated. The autoclave is convenient 

 for sterilizing discarded cultures, test-tubes, and glassware generally, and 

 such media as beef broth and agar. 



In many instances it is desirable to sterilize at a temperature lower than 

 100 C. Albumen and blood serum, for instance, will coagulate at that 

 temperature. Again, it is desired to kill the microbes without destroying 

 the toxins which they form, as in the manufacture of bacterial vaccines. In 

 the sterilization (pasteurization) of milk, a lower temperature is employed. 

 In the sterilization of these and other substances the temperature ranges 

 from 50 C. to 85 C. The discontinuous method is employed, differing 

 from the method already described in that the period of exposure is much 

 prolonged, about one hour. The number of daily exposures ranges from one 

 to six. For example, milk exposed to a temperature of 60 to 70 C. for one 

 hour is considered sufficiently sterilized, whereas blood serum is subjected 

 to hourly exposures of a temperature of 60 C. for six successive days before 

 it is pronounced completely sterilized. 



5. Preparation of Culture Media. 



The pharmacist should give especial attention to the preparation of 

 bacterial culture media, as in this he may be of service to the physician. 

 The busy general practitioner who is not eqipped with a suitable bacterio- 

 logical laboratory, or who does not have time to prepare culture media, would 

 no doubt consider it a very decided advantage should the pharmacist offer 

 to assist him. This will be more fully set forth in the last chapter. 



In brief, it may be stated that microbes feed upon the same substances 

 that we feed upon. In the presence of adequate warmth and moisture they 

 attack all organic substances. This being the case, it may readily be as- 

 sumed that there are many substances or media which can be used as food 

 for bacteria. Such is the case, and the number of media which have been 

 used is legion. Almost any organic substance may be used, provided it is 

 not aseptic or antiseptic in its properties. 



Culture media are liquid or solid, simple or compound. In the case of 

 liquid or liquefiable solid media, the following physical properties are de- 

 sired, in so far as it is possible to attain them: 



a. Culture media should be perfectly clear. There should be no sedi- 

 ment, no opacity or flocculent suspension, and no floating matter. In the case 

 of broths, extracts generally, gelatin media, and blood serum, these require- 

 ments are easily attained. Perfectly clear agar is difficult to obtain. Milk 

 is normally opaque. 



b. Media should be neutral or very slightly alkaline to litmus, which is 



