62 PURE CULTURE OF BACTERIA 



The growth upon agar and potato or in broth and 

 other media can also be given in a small space by 

 judicious selection of terms for such a table. 



Ex. 35. Determine if the organisms forming the colonies on 

 any of the agar or gelatine plates which you have prepared are 

 motile or non-motile. 



To do this, take a hollow-ground " hanging drop " slide and 

 sterilize it by passing it four or five times through the Bunsen 

 flame. Now smear a thin ring of vaseline round the edge of the 

 hollow in the slide. 



Then take a cover-glass and, after passing it through the flame 

 two or three times, transfer a drop of sterile broth to the centre 



with a platinum loop, 

 taking care that the drop 



FIG. 22. Section of hanging-drop slide. does not flow tO the 



margin of the glass. Now introduce a small portion of the 

 culture to be tested into the drop with a platinum needle. Lift 

 the cover-glass with forceps and turn it over very rapidly so that 

 the drop is hanging on the under surface ; then lower it on to 

 the ring of vaseline. The drop should now be hanging in the 

 enclosed space without touching the slide, as in Fig. 22. As 

 communication between the hollow space and the outside air is 

 prevented by the vaseline, the drop does not readily dry up. 



Keep the whole at room temperature or 22 C., and examine 

 with the microscope every half hour for two or three hours. 

 If the organisms are motile their movement will be readily 

 perceived, especially if a small diaphragm opening is used. 



Care should be taken to avoid bringing the objective down on 

 the cover-glass : if difficulty arises in focussing, make a small 

 dot on the surface with ink and focus this first. 



Ex. 36. Test some of the organisms which you have 

 isolated for gas production 



(a) With Durham's Tubes. 



First add about 10 c.c. of nutrient beef broth containing 

 glucose (see 2, Ex. 25) to several Durham's fermentation tubes 



