WHAT ARK BACTERIA? 3 



thing, most of the methods of bacteriological study do not even 

 involve the use of a microscope. 



The unit with which the student commonly starts is not a 

 bacterium, but a colony. By reference to experiment No. 6 it 

 will be seen that a colony is a mass of bacteria in some solid 

 jelly medium. (Fig- 2.) Such a colony is composed of thou- 

 sands of bacteria, all of which are supposed to have risen from 



FIG. 



ARIOUS TYPES OF COLONIES OF BACTERIA GROWING ON 

 GELATIN 



a single bacterium by multiplication. The whole makes a 

 cluster big enough to be seen without any lens, and since all 

 the bacteria in it have come from a single one they are all 

 alike. The colony may be picked out of the jelly, placed in some 

 medium in which the bacteria will thrive, and then, after 

 proper growth, the bacteria can be studied by the microscope. 

 Such cultures form the basis of bacteriological study. 



Form of Bacteria. Bacteria are of three quite different 

 shapes, but all very simple. I. Simple spheres. (See Fig. 3, 

 b, c, d.) Such spherical forms are called cocci, and in 



