68 ESSENTIALS OF BACTERIOLOGY 



CHAPTER XI 

 THE GROWTH AND APPEARANCES OF COLONIES 



Macroscopic. Depending greatly upon the temperature 

 which should be about 65 F. (20 C.) for gelatin, and 40 C. 

 for agar, the colonies ordinarily develop so as to be visible to 

 the naked eye in two to four days. Some require ten to four- 

 teen days, and others grow rapidly, covering the third dilution 

 in thirty-six hours. The plate should be looked at each day. 



The colonies present various appearances from that of a 

 small dot, like a fly-speck, to that resembling a small leaf. 



Fig- 35- Staphylococcus pyogenes aureus: colony two days old, seen upon 

 an agar-agar plate (x 40) (Heim). 



f 



Some are elevated, some depressed, and some, like cholera, cup- 

 shaped umbilicated. 



Then they are variously pigmented. Some liquefy the gela- 

 tin speedily, others not at all. The appearances of a few are 

 so characteristic as to be recognized at a glance. 



Microscopic. We use a low-power lens, with the Abbe 

 nearly shut out that is the narrowest blender. The stage of 



