78 LABORATORY GUIDE IN BACTERIOLOGY 



6. Allow to remain in alcohol, with occasional agita- 

 tion, for at least 4 minutes, or until no more stain is 

 taken up by the alcohol. 



7. Dry without washing, and mount. 



This stain is an important means of differentiating 

 species of bacteria. 



It is a positive Gram stain if by application of this 

 method either the organism loses none of the stain or 

 the stain is dark blue or dark slate blue. It is a 

 negative stain if either the coloration is completely 

 gone or only a light bluish tinge is left. 



The preparation before mounting may be washed in 

 water and counterstained with Bismarck brown. This 

 method shows all foreign matter brown in contrast to 

 the bacteria, and is especially adapted for staining bac- 

 teria in tissues, sputum, etc. 



By mounting a Gram stain and a gentian violet 

 stain of the same organism on the same slide both 

 time and material are economized. 



SECTION 3 



STUDY OF MOLDS, YEASTS, AND TORULAE 



EXERCISE I. CULTURAL STUDIES 



Yeasts, torulae, and molds grow better in a medium 

 of acid reaction than in a neutral or alkaline medium. 

 Media prepared from hopped beerwort are generally 

 used for this purpose. 



Make transfer of a stock culture of Saccharomyces 

 cerevisiae or any other species of yeast which may 

 have appeared on the plates prepared in the previous 

 section. Also transfer two species of molds from these 

 plates to slanted wort agar. 



