72 LABORATORY GUIDE IN BACTERIOLOGY 



observed that a number of spots of different sizes, 

 shapes, colors, etc., have formed on the surface of the 

 medium. 



The low power of the microscope will reveal a dark 

 part in most of these spots. This dark part is a dust 

 particle, which has carried micro-organisms to the 

 surface of the agar, and thus opportunity is given for 

 multiplication and formation of a "colony." A colony 

 originating in this manner may or may not consist of 

 one species of organism. By studying the organisms 

 we will find that the colonies are composed of bacteria, 

 yeasts, torulae, or molds. All or any of these may be 

 carried by dust particles in the air. 



EXERCISE 2 



Inoculate two or three colonies on agar slants. 



Method of inoculation — 



1. Singe the cotton plug of a tube containing the 

 medium. Organisms in the air are constantly alighting 

 on the cotton, and bacteria may also be depo§ited on the 

 cotton by handling it with the fingers. If these organ- 

 isms are not killed by the process of singeing, they may 

 drop on the medium after removal of the stopper, and 

 thus ruin a pure culture. 



2. Hold the tube (or, if a transfer is made, both 

 tubes side by side) between the thumb and the fore- 

 finger, so that the end of the tube rests on one edge of 

 the hand (Fig. 27), holding them at an angle of about 

 45°. If held horizontally, the condensation water, 

 usually present at the lower end of the agar slant, will 

 moisten the surface and destroy a characteristic growth 



