4 GENERAL BACTERIOLOGY LESSON 



38 0. will show a copious white growth, while 

 the other tube will show a pink or red growth. 

 Now keep both tubes at 20 C. The growth 

 originally kept at 38 C. may gradually develop 

 pigment, but at times it has permanently lost its 

 chromogenic properties. 



(2) Inoculate two sloped agar-agar tubes with a single 

 loop of a broth culture of Bacillus pyocyaneus (eighteen 

 hours old), distributing the material over the entire surface 

 of the agar-agar. 



Expose one tube to the light and keep the other at 

 the ordinary temperature of the room protected 

 from the light. 



Examine them on successive days : the tube exposed 

 to the light shows restricted growth, or perhaps no 

 growth at all, in any case limited production of 

 pigment, while the other tube will show a copious 

 growth, the culture medium at the same time 

 becoming bright green. Light, therefore, has an 

 inhibitory action on the growth and activity of 

 the Bacillus pyocyaneus, which is more marked 

 the longer the rays of the sun are allowed to act, 

 and the less material is used for inoculation. 



(3) Repeat the experiment with the Bacillus prodigiosus. 

 Inoculate two sloped agar-agar tubes with a single loop 



of a pigmented broth culture of Bacillus prodigiosus (about 

 a week old), distributing the material over the entire 

 surface of the agar-agar. 



Keep one tube opposite a sunny window at the ordinary 

 temperature of the room, and the other at the same tem- 

 perature but protected from the light. 



