BACTERIOLOGICAL TECHNIC 



69 



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 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 

 desired, 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 

 requirements are easily attained. Perfectly clear agar is difficult to 

 obtain. Milk is normally opaque. 



FIG. 17. Arnold Steam Sterilizer. Boston Board of Health Form. This sterilizer 

 is square,* and constructed with a side-door all in accordance with the recommendation 

 of the Boston Board of Health. Its large size makes it well suited to the requirements of 

 Board of Health laboratories, and it has been found to be very serviceable and conven- 

 ient. It is made of copper throughout, following the same principles as employed in the 

 construction of the other sterilizers. 



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

 equivalent to a slightly acid reaction to phenolphthalein, at a temperature 

 of about 20 C. Most microbes develop best in media of such reaction. 



c. They must be free from living microbes and their spores, and from 

 other organisms. This requirement is attained by sterilization as already 

 described. Culture media contaminated with living organisms are not 

 usable in bacteriological work. 



