22 BACTERIA 



an e gg- Glycerine and grape-sugar may be added as else- 

 where. Blood agar is ordinary agar with fresh sterile blood 

 smeared over its surface. Blood serum is drawn from a jar 

 of coagulated horse-blood, in which the serum has risen to 

 the top. This is collected in sterilised tubes and 

 coagulated in a special apparatus (the serum in- 

 spissator). Potato is prepared by scraping ordin- 

 ary potatoes, washing in corrosive sublimate, and 

 sterilising. They may then be cut into various 

 shapes convenient for cultivation. Upon any of 

 these forms of solid media the characteristic 

 growth of the organism can be observed. Of the 

 nutr ^ ent elements required, nitrogen is obtained 

 PREPARED FOR f rom albumens and proteids, carbon from milk- 

 SU g arj cane-sugar, or the splitting up of proteids ; 

 salts (particularly phosphates and salts of potas- 

 buibofthetube s j um ) are reac jily obtainable from those incorpor- 

 ated in the media ; and the water which is required is 

 obtainable from the moisture of the media. 



There are two common forms of test-tube culture, viz. : on 

 the surface and in the depth of the medium. In the former 

 the medium is sloped, and the inoculating needle is drawn 

 along its surface ; in the latter the needle is thrust vertically 

 downwards into the depth of the solid medium. Plate cul- 

 tures and anaerobic cultures will be described at a later 

 stage. When the medium has been inoculated the culture 

 is placed at a temperature which will be favourable. Two 

 standards of temperature are in use in bacteriological labora- 

 tories. The one is called room temperature, and varies from 

 18 C.-2O C. ; the other is blood-heat, and varies from 35 

 .-38 C. It is true, some species will grow below 18 C., 

 and others above 38 C. The pathogenic (disease-producing) 

 bacteria thrive best at 37 C., and the non-pathogenic at the 

 ordinary temperature of the room. The different degrees 

 of temperature are regulated by means of incubators. For 



