18 EXPERIMENTAL DAIRY BACTERIOLOGY 



is not so easily decomposed, and media containing it may be 

 sterilized in the autoclave even when acid. When it is desir- 

 able to test the properties of an organism with reference to 

 its conduct towards certain sugars, care should be taken to 

 have no other sugar present in the medium than the one 

 added, and to have this present in an unchanged form. 



Media prepared from meat infusion will contain some 

 muscle sugar (inosite). This may be removed from the in- 

 fusion by seeding the same with an organism able to fer- 

 ment this sugar (B. coli communis), incubating from sixteen 

 to twenty hours at 37 C. and then proceeding as usual in 

 the preparation of the medium. 



Preparation of litmus solution. The litmus ordinarily 

 purchased varies so widely in its content of the pigment 

 desired, azolitmin, that it is impossible to give definite 

 directions for its preparation. Ten grams of Kahlbaum's or 

 Schuchardt's litmus powder are extracted with 500 cc. of 

 water by placing over night in a warm place ; then filter and 

 tube. Sterilize by the discontinuous method. 



Preparation of water blanks. For dilution purposes in 

 quantitative work sterile water must be employed. It is cus- 

 tomary to use the so-called physiological salt solution, pre- 

 pared by the addition of 0.6 per cent of common dairy salt 

 to distilled water, in place of the distilled water which may 

 exert an injurious effect on the bacteria by reason of its 

 osmotic properties. 



Various modifications of the above media are used in bac- 

 teriological work, as well as many other types of media, such 

 as potato, blood serum, egg, etc. These, however, are not usu- 

 ally employed in ordinary dairy work, and for their prepara- 

 tion and use the student is referred to books on general and 

 medical bacteriology. 



