70 METHODS USED IN THE STUDY OF BACTERIA 



would be designated by the per cent represented in figures 

 preceded by the minus sign; and media which is acid to 

 phenolphthalein is represented by the per cent of acid pre- 

 ceded by the plus sign. Most of the bacteria, for instance, re- 

 quire a medium from o to + i ; while certain bacteria, such 

 as those commonly found in water, reach their maximum of 

 development when the medium is + 1.5. In the preparation 

 of bouillon, as well as that of many other media, it is necessary 

 to clear it, and this is done by adding an egg, or some similar 

 albuminous material, while the medium is cool, at least 

 below the coagulation point of the proteins i.e. 60 C., 

 then heated sufficiently to thoroughly coagulate the albu- 

 minous substance. This draws the fine particles of the pre- 

 cipitate into larger clumps, or masses, which will be held back 

 by the filter material, and the medium which goes through the 

 filter under these conditions will be perfectly clear. 



Bouillon may be modified in various ways by the addition 

 of glycerine (6 per cent) for the Bact. tuberculosis and Bact. 

 mallei ; by the addition of dextrose or lactose (i per cent) for 

 fermentation tests ; carbolic acid (.025 per cent) for B. ty- 

 phosus, etc. 



Sugar-Free Bouillon (Smith). For certain work, such 

 as the detection of indol, a special medium is used from which 

 the muscle sugar has been removed. Since muscle always 

 contains a small amount of sugar (inosit) it is necessary to 

 remove this, and it is done at Smith's suggestion by inoculat- 

 ing a bacterium capable of splitting up this sugar and doing 



