10 
Exercise 4. Microscopic Study of Living Bacteria 
MATERIALS: 
Carmine powder in water 
Hay infusion 
Whole milk 
Pure cultures of bacteria in meat broth 
Hanging-drop slide 
Cover glasses 
Vaseline 
1. Clean a hanging drop slide. Place a ring of vaseline around the edge of 
the depression. Clean a cover glass and lay it on a clean piece of paper. 
2. With the wire loop place a small drop, about the size of a pin head, of a 
suspension of carmine in water on the center of the cover glass. 
Note. —Whenever the wire loop or needle is used, it must be sterilized 
immediately before and after using, in the manner shown by the instructor. 
3. Invert the slide and carefully lower it until the cover glass is touching 
the ring of vaseline on all sides. 
4. Quickly turn the slide right side up so that the drop is suspended (hangs) 
FIG. 6-CORRECT HANGING DROP 
XX 
FIG. 7-WRONG OR “FALLEN” HANGING DROP 
over the depression in the slide. (See Fig. 6.) Carefully press down on the 
edge of the cover glass so that the vaseline makes a seal. Be careful not to 
cause the drop to fall or touch the bottom of the depression. (See Fig. 7.) 
In case this happens clean the slide and cover glass and repeat. 
5. Examine the drop with the 16 m. m. objective. Shut off most of the 
ight in order that the drop may be seen distinctly. Find the edge of the drop. 
6. Raise the 16 m. m. objective a little and revolve the 4 m. m. objective 
into place. Regulate the light (usually by admitting more of it) until the field 
is grayish. 
7. Looking at the objective sideways, carefully lower it until it nearly 
touches the cover glass. 
8. Looking into the eyepiece, focus upward very slowly with the fine 
adjustment until the particles of dye are plainly visible. Try regulating the 
diaphragm until the best light is secured. 
9. Examine the smaller carmine particles and groups of particles. 
(a) What is their color and shape? 
( b) Do they move? 
(c) Is the movement vibratory, or do the particles move from one 
place to another in the preparation? 
The vibratory form is called pedesis or brownian movement. 
10. Examine some diluted milk (1 part whole milk to 2 parts water) in a 
like manner. 
