APPEARANCES OF CULTURES ON SOLID MEDIA 173 



other hand, Saccharomyces inlermedius, validus and Pastorianus, 

 varieties equally closely related, have the same temperature limits. 

 With regard to the time necessary for the scum to appear, some equally 

 interesting differences are brought out. 



Macroscopic Appearances of Cultures on Solid Media 1 



The various varieties of yeasts do not develop after the same 

 manner on solid media (agar and gelatin). They offer vegetative 

 growths which we may use as differential characteristics. 2 Certain 

 varieties liquefy gelatin rapidly, others slowly or not at all. This is 

 an important characteristic. It is important to inoculate the yeast 

 into agar, gelatin, carrot or potato, and examine the 

 microscopic appearance of the growth after the yeast 

 has developed. The following determinations may also 

 be made. 



Plate Culture: This is prepared by putting a little 

 of the yeast in dilution into a Petri dish. The dish is 

 partially filled with gelatin which serves as a food. Fig. 71. Plate 

 When the gelatin has solidified, each cell will develop 

 into a colony which, for each yeast, will have some differential 

 characteristic. 



Streak Culture: A test tube or Petri dish containing a solid me- 

 dium with a large surface is streaked with a little of the yeast. The 

 yeast will develop by growing along this line of inoculation. 



Stab Cultures: The yeast is stabbed into a solid medium by means 

 of a stiff platinum wire. This introduces the yeast into 

 an environment which has a reduced air supply. 



One may thus obtain many characteristics which 

 will serve in the differentiation of the yeasts. The 

 colonies will possess special forms. Hansen, for instance, 

 has shown that on beer wort gelatin, S. cerevisiae, 

 ellipsoideus, Pastorianus, validus, and intermedius when 

 Fig. 71- A. inoculated in streak cultures, present very different 

 appearances to the naked eye. The same was found 

 out with regard to the stab cultures. 



1 The Descriptive Chart of the Society of American Bacteriologists has been 

 used by some investigators in America for recording the salient characters of 

 yeasts. It has the advantages of offering a uniform method of procedure and 

 of recording concisely in a small space the data for each yeast. The comparison of 

 characteristics of yeasts is thus made easy. 



2 According to the investigations of Orsos, the form of the colonies is a func- 

 tion of the elasticity of the medium upon which the yeasts are. The state of 

 cohesion of the substrate is one of the determining factors and also, to a lesser 

 degree, the activity of the yeast (Orsos, Die Form, der tierfliegenden Bakterien 

 und Hefencolonien. Cent. Bakt. 54, 1910). 



