54 LABORATORY GUIDE IN BACTERIOLOGY 



/) 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. 



g) Dry without washing, and mount. 



This stain is an important means of differentiating 

 species of bacteria. 



In order to be able to judge of the effect of Gram's 

 iodin solution on the stain, 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 of bacteria in tissues, sputum, etc. 



It is a positive Gram's stain if by application of this 

 method either the organism loses none of the stain, or 

 the st.ain 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. 



All stained preparations and all Gram stains must be 

 preserved for inspection. 



3. Transfer from agar-culture to the following media: 

 Glucose-agar. 



Gelatin. 

 Potato. 

 Broth. 



Litmus milk. 

 Dunham's solution. 

 These transfers, excepting gelatin, are to be placed 

 in the thermostat, unless otherwise ordered. 



4. An accurate description is to be made of each 

 individual culture (see Chap. X), and outUne sketches 

 especially of the gelatin and the milk cultures. These 

 descriptions should be made complete after 24 hours, 



