CHAPTER IX. 



MICROSCOPIC EXAMINATION OF BACTERIA. 



MOTILITY : The first step in the microscopic examination of 

 bacteria is to determine whether the organism is motile. This 

 is done by means of the hanging drop. The living bacteria 

 can be observed for days at a time, if necessary, in their char- 

 acteristic grouping, and their multiplication and formation of 

 spores watched. 



A cover-glass is cleansed thoroughly and a drop of sterile 

 water or bouillon placed in its centre. With a sterile pla- 

 tinum loop some of the culture is transferred to this drop. 

 The cover-glass is then inverted over a glass slide having a 

 round excavation in its centre (Fig. 32), a concave slide. In 



FIG. 32. 



Hanging drop. 



order to increase the size of this little chamber, and also to 

 exclude the air from the drop, the edge of the depression is 

 rimmed with vaseline, the cover-glass being dropped on 

 the vaseline. The drop may be examined with a powerful 

 hand lens or with a high-power lens ( or 1) of the micro- 

 scope. An oil-immersion lens cannot be used unless it is 

 focussed simply on the cover-glass without changing the field. 

 Any attempt at changing the field is apt to spoil the drop. 

 The lens must be brought down very slowly, so as not to 

 break the cover-glass. The iris-diaphragm is closed almost 

 completely, as the bacteria are very refractile. 



The bacteria are seen as minute shining bodies that either 

 remain stationary or scamper across the field with either a 



80 



