THE EXAMINATION OF CULTURES I9 



29. Making hanging-drop preparations, (a) From a bou- 

 illon culture. Place a clean cover glass on a 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 mois- 

 tened 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 on the use of the 

 microscope, see The Microscope, by Professor S. H. Gage.) 

 (U) 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 bouillon or sterilized 

 water on the center. Touch the surface growth very gently 

 with the end of the platinum 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 indi- 

 vidual 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 % carbohc acid, 

 I to 1000 corrosive subhmate solution, or a strong solution 

 of a mineral acid). 



30. 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 is. 



