GIANT COLONIES 65 



After growth, the usual color tests for nitrites were made. It is quite 

 possible that the ordinary peptone-glucose solutions such as 

 Sabouraud's with added nitrates would serve as well as the above 

 medium. 



According to Zimmermann ^^ and Mrak and McClung,^° the aux- 

 anographic method cannot always be relied upon, and no doubt 

 liquid media are to be preferred. The ability to utilize ethyl alcohol 

 is also an important character in the identification of yeasts. This 

 is determined by inoculating the organism into a liquid medium: 



The alcohol will have to be added after sterilization. If growth 

 occurs, as manifested by turbidity or sediment, the test is positive. 

 Growth-promoting substances often have to be added to liquid media. 

 In the auxanographic method, the large inoculum provides these 

 substances. 



Giant Colonies. The form and texture of colonies of yeasts are 

 of diagnostic value, especially those of large isolated colonies (giant 

 colonies) which have been allowed to grow for some time (4 to 6 

 weeks). See Fig. 114. Since lack of oxygen may modify growth, 

 the containers cannot be sealed, and to prevent too much evapora- 

 tion a large volume of agar is required. For this purpose round 

 narrow-mouthed bottles 5 cm. in diameter and 12 cm. high are suit- 

 able. These are filled with 50 ml. each of the American Sabouraud 

 agar described above and autoclaved. They are inoculated by touch- 

 ing a straight wire to the stock culture, then touching the tip of the 

 wire to the center of the agar surface in the bottle. It is important 

 to hold the bottle in an inverted position while inoculating to prevent 

 air contamination. The agar surface should be allowed to dry a day 

 or two before inoculating. The bottles are incubated at room tem- 

 perature. For clear observation or for photographing, the bottles 

 may be cut at the level of the agar surface by a short cut with a 

 file and pressing the molten end of a glass rod against the file mark. 

 The glass will usually crack neatly around the bottles at this level. 

 The use of Petri dish cultures for photographic surfaces is more con- 

 venient and economical if contamination can be avoided and the 

 plates do not dry too much in the long incubation period. Instead 

 of giant colonies, Stelling-Dekker and Lodder used agar slant growth. 



