Laboratory Outline for General Bacteriology 



EXAMINATION OF UNSTAINED BACTERIA 



The purpose is to study bacteria in a living condition ; to dem- 

 onstrate their form, arrangement, and motility. 



Bacteria have two kinds of motion, the so-called Brownian or 

 molecular movement, and true motility. Brownian movement is 

 shown more or less by all small particles of insoluble matter (in- 

 cluding living non-motile or dead bacteria) in suspension. It is 

 characterized by a vibratory movement; the relative positions of 

 the particles or bacteria are not changed. This type of movement 

 may be distinguished from true motility which is characterized 

 by progressive movement, more or less rapid, of an organism across 

 the field of a microscope, and which changes its position in the 

 field independently of and in a direction contrary to other organisms 

 present. If large numbers of bacteria are moving in one direction 

 it is an indication that they are being carried by currents in the 

 liquid. 



Method 



Place a drop of water on a cover glass held in cover-glass 

 forceps. Sterilize platinum loop by heating to redness in gas flame. 

 When cool transfer a small amount of the culture to be examined 

 to the drop of water. Use only enough to cloud the water slightly. 

 Take the culture from the edge of the growth in the lower part of 

 the tube. The bacteria are on the surface of the agar, so do not 

 allow the loop to cut down into the agar. In opening the tube 

 remove the cotton plug and hold it so that the inner part will not 

 touch anything. Never lay it on the table. This is necessary to 

 prevent the culture in the tube from becoming contaminated with 

 other kinds of bacteria or molds. After removing loopful of cul- 

 ture replace cotton plug. Invert the preparation onto a slide and 

 examine with the 1/6 objective. No oil is used with this objective. 

 With the coarse adjustment lower the objective until it nearly 

 touches the cover glass, being careful not to touch it. Then with 

 the eye at the ocular, focus up with the fine adjustment until the 

 bacteria come into view. This focal point will be passed without 

 noticing it if passed too quickly, if the light is too intense, or if 

 the light is too dim. Regulate the amount of light by raising or 

 lowering the condenser and by adjusting the mirror. Unstained 

 bacteria are nearly colorless and appear as faint shadows. 



Bacteria lose their motility very quickly so if there is a delay 

 of two of three minutes in getting the focus a wrong determination 

 may be made. In such a case it is better to make a new prepara- 

 tion. Do not make but one preparation at a time. 



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