SPECIAL CULTUKE MEDIA 39 



made faintly alkaline to phenol-phthalein ; 1 per cent, of glucose 

 is added as above. 



These bouillon, gelatin, and agar preparations constitute 

 the most frequently used media. Growths in bouillon do not 

 usually show any characteristic appearances which facilitate 

 classification, but such a medium is of great use in investigating 

 the soluble toxic products of bacteria. The most characteristic 

 developments of organisms take place on the gelatin media. 

 These have, however, the disadvantage of not being available 

 when growth is to take place at any temperature above 24° C. 

 For higher temperatures agar must be employed. Agar is, how- 

 ever, never so transparent. Though quite clear when fluid, on 

 solidifying it always becomes slightly opaque. Further, growths 

 upon it are never so characteristic as those on gelatin. It is, 

 for instance, never liquefied, whereas some organisms, by their 

 growth, liquefy gelatin and others do not — a fact of prime 

 importance. 



Special Culture Media. 



An enormous variety of different media has been brought 

 forward for use in cases either where special difficulty is ex- 

 perienced in getting an organism to grow, or where some special 

 growth characteristic is to be studied. It is impossible to do 

 more than give the chief of these. 



Peptone Solution. 



A simple solution of peptone (Witte) constitutes a suitable 

 culture medium for many bacteria. The peptone in the propor- 

 tion of 1 to 2 per cent., along with - 5 per cent. NaCl, is dissolved 

 in distilled water by heating. The fluid is then filtered, placed 

 in tubes, and sterilised. The reaction is usually distinctly 

 alkaline, which condition is suitable for most purposes. For 

 special purposes the reaction may be standardised. In such a 

 solution the cholera vibrio grows with remarkable rapidity. It 

 is also much used for testing the formation of indol by bacteria ; 

 and by the addition of one of the sugars to it the fermentative 

 powers of an organism may be tested (p. 79). Litmus may be 

 added to show any change in reaction. 



Robertson's Bullock's Heart Medium. 



This medium was introduced for the cultivation of anaerobes 

 and is made as follows : 8 oz. of bullock's heart is minced very 



