Microbes Must Eat 83 



The peptone and the beef extract are heated in the distilled 

 water just sufficiently to dissolve them. After determining the pH 

 of the broth, either electrometrically or colorimetrically, 10% sodium 

 hydroxide (NaOH) is added to raise the pH to the proper level 

 (about neutrality), before the carefully-weighed agar is intro- 

 duced. The percentage of agar added to the broth varies from 

 lVo% to 27c, depending upon the hardness desired in the finished 

 medium. Several minutes of a rolling boil (avoid burning or boil- 

 ing over) will be required to dissolve the agar. The hot medium 

 is transferred to flasks or to tubes, depending upon the ultimate 

 use of the medium, and each container is plugged with non-ab- 

 sorbent cotton. After autoclaving^ to sterilize the nutrient asjar, the 

 tubes or flasks are kept in the refrigerator until readv for use. If 

 the medium is to be used within a few days, storage at room 

 temperature is satisfactory, but refrigeration will materially reduce 

 dehydration during prolonged storage. 



The refinements of blending the ingredients, adjusting the pH, 

 plugging the tubes, and autoclaving the finished product are 

 covered more fully in laboratory manuals, and hence will not be 

 discussed in detail here. 



From these observations on the preparation of a suitable 

 medium, it should become evident that feeding microbes is not a 

 simple matter of throwing them a bone, the way one might satisfy 

 the desires of a hungry canine blessed with sharp incisors. Run- 

 ning a restaurant for microorganisms is a scientific endeavor based 

 upon table service rather than cafeteria style of mass feeding. In 

 devising a menu for microbes, the microbiologist attempts to pro- 

 vide an adequate supply of available food, served in the same stvle 

 to which the organisms are accustomed in nature where thev are 

 eking out their own livelihood in a highly competitive environment. 



