242 BACTERIOLOGY. 



to the method given in Chapter XV. Litmus is usually ad- 

 ded to the glucose, or lactose media, or to milk in order to 

 ascertain whether a given organism produces acid products. 

 The medium should be colored decidedly blue. A very con- 

 centrated aqueous solution of litmus should be employed in 

 order not to dilute the medium too much. Excessive steril- 

 ization of sugar media, which contain litmus, results in the 

 decoloration of the latter. The color may, in part, return 

 on standing*. It is advisable, therefore, not to steam for 

 more than 15 minutes at a time; or better still, to sterilize 

 the litmus solution by itself and then add this, by means of 

 a sterile pipette (Fig. 61), to the sterile nutrient medium. 



Serum media. The preparation of blood-serum, and of 

 serous exudates will be described later. In this connection 

 it will be sufficient to state that the addition of blood-serum 

 from animals, or of highly albuminous exudates obtained 

 from man, makes an agar or bouillon specially valuable for 

 the cultivation of certain bacteria. 



Thus Lfiffler's blood-serum which is an excellent medium 

 for the diphtheria bacillus, consists of 3 parts of blood- 

 serum and 1 part of a 1 per cent, glucose bouillon. 



The addition of 1 part of ascitic fluid to 2 parts of agar 

 gives a medium well adapted for cultivating the gonococcus. 

 The fluid, must of course, be collected under sterile condi- 

 tions, otherwise it should be filtered through porcelain 

 (Fig. 66). Such a mixture can be used for plating purposes, 

 in which case, however, the agar should be first melted and 

 allowed to cool to about 50. The ascitic fluid, or blood- 

 serum, previously warmed to this temperature, should then 

 be added to the liquid agar. The mixture can now be inocu- 

 lated, dilutions made and Petri dishes poured as in the case 

 of ordinary agar cultures. 



Laboratory work. One half of the class will prepare agar and 

 bouillon according- to the methods just given. One liter of bouillon is 

 prepared first, and from this the agar and the modified media are pre- 



