100 LABORATORY GUIDE IN BACTERIOLOGY 



The plates prepared in the above manner should be 

 studied after 24 hours, or, if not sufficiently developed, 

 after 48 hours, according to directions in Section 9. 



EXERCISE 2. STUDY OF PIGMENTS 



On the sixth day take agar slant or potato cultures 

 of the four chromogenic bacteria, and proceed as 

 follows: Pour 96 per cent alcohol on the cultures of 

 B. prodigiosus, B. violaceus, and Sar. lutea. The 

 pigment should dissolve. Filter the liquids into clean 

 test tubes, and, by adding a few drops of 5 per cent 

 hydrochloric acid, note the change in color. Then 

 add an excess of a 2 per cent solution of sodium hydrate, 

 and note whether or not the color returns and is 

 changed again. Then pour chloroform on a culture of 

 B. pyocyaneus. This will dissolve the bluish-green 

 pigment (pyocyanin). Filter, and evaporate on the 

 water bath. When almost dry, place a small amount 

 on a slide, and observe the small crystals of pyocyanin 

 under the microscope. Note also the aromatic odor 

 given off by the pigment as the solvent evaporates. 



SECTION 8 

 STUDY OF MICROCOCCI 



Make transfers from laboratory cultures of Staphylo- 

 coccus aureus and Streptococcus lacticus. 



Follow the outline of routine work and make descrip- 

 tions of these two organisms. 



