THE CULTIVATION OF MICRO-ORGANISMS II 9 



become accustomed to their artificial surroundings, as it were, 

 they may be propagated easily afterward; this is especially true 

 of the tubercle bacillus. After they are developed, stock cultures 

 are best kept in a refrigerator, and it is well to seal them so as to 

 prevent drying. Rubber caps or rubber stoppers are useful for 

 this purpose (Figs. 46 and 47). 



Some bacteria flourish better on one culture-medium than 

 another. The tubercle bacillus grows best on blood-serum and 

 glycerin-agar; the bacillus of diphtheria grows best on Lofiler's 

 blood-serum; the gonococcus on human s^rum-agar or ascitic- 

 fluid-agar. 



The virulence of most pathogenic bacteria becomes diminished 

 after prolonged cultivation upon media. In some forms the viru- 

 lence is lost very quickly, for example, the streptococcus and 

 pneumococcus. 



Regulation of Temperature 



High-temperature Incubator. — Many bacteria flourish best 

 at a temperature about that of the human body, 37° C. Some 

 species will grow only at this temperature. The pathogenic bac- 

 teria in particular, for the most part, thrive best at a point near 

 the body temperature. 



The ordinary incubator is a box made of copper, having 

 double walls, the space between the two being filled with water. 

 The outer surface is covered with some non-conductor of heat, 

 such as felt or asbestos. At one side is a door, which is also double. 

 The inner door is of glass, the outer door is of copper covered 

 with asbestos. At one side is a gauge which indicates the level 

 at which the water stands in the water-jacket. The roof is per- 

 forated with several holes, some of which permit the circulation 

 of the air in the air-chamber inside the box; some of them enter 

 the water-jacket. A thermometer passes through one of these 

 holes into the interior of the air-chamber, and often another into 

 the water standing in the water-jacket. A gas-regulator passes 

 through another hole, and is immersed in the water standing in 



