THE EXAMINATION OF CULTURES 19 



28. Making Hanging-drop Preparations. — (i) From a bouil- 

 lon culture. Place a clean cover-glass on the tray. With the 

 loop, remove a drop of the liquid culture and place it on the 

 middle of the cover-glass. With a pair of fine forceps invert 

 the cover-glass over the glass ring fixed to a slide for this 

 purpose. The surface of the ring should previously be moist- 

 ened with liquid vaseline to prevent the cover-glass from slid- 

 ing. The preparation is then ready for examination. Examine 

 it first with the high power dry lens and then with the oil 

 immersion objective. (For directions in the use of the micro- 

 scope, see "The Microscope" by Professor S. H. Gage.) 

 (2) From cultures on solid media. On account of the very 

 large number of bacteria in the growth on solid media it is 

 necessary to separate them in a clear liquid. Take a cover- 

 glass as before and place a loopful of sterilized water or bouil- 

 lon on the centre. With the platinum needle touch the surface 

 growth very gently with the end of the needle and carefully 

 rinse it in the drop of liquid on the cover-glass. From this 

 point the examination is the same as with the liquid culture. 

 Upon examination, if the bacteria are so numerous that the 

 individual organisms cannot be clearly distinguished, i.e. 

 separated from each other, the preparation must be rejected 

 and another one made, using a smaller quantity of the growth. 

 After examination, the cover-glasses should be placed at once 

 in a glass jar containing a strong disinfectant (5% carbolic 

 acid, I to 1000 corrosive sublimate solution, or a strong solution 

 of a mineral acid). 



29. Suggestions for the Microscopic Examination of Living 

 Bacteria. — In examining the bacteria, as they appear under 

 the microscope in the hanging-drop preparation, the following 

 features should be observed : Are the individual bacteria 

 spherical, rod-shaped, or spiral in form? Are they single or 

 united in pairs, masses or clumps, or in shorter or longer 

 chains? For this determination it is better to examine the 

 organisms near the edge of the drop. Are they motile, that 



