24 
Gelatinase 
1. Prepare stab cultures in gelatin using the pure cultures 1, 2, and 3 from 
Exercise 10 and the culture of B. graveolens. 
2. Be sure the gelatin is solid. If it has the slightest tendency to become 
soft, the tubes should be placed in a glass of ice-water until solid. 
3. Make the stab culture in the gelatin by thrusting the inoculated straight 
wire into the center of the solid medium, nearly to the bottom of the test tube. 
Be sure the wire is perfectly straight. 
4. Incubate the cultures in the 20° C. incubator to prevent melting of the 
gelatin. Examine after one week. 
Observation of Cultures 
1. Flood the starch agar plates evenly with Lugol’s solution and drain dry. 
2. Examine both the starch agar and milk agar plates for halos around the 
colonies. Notice the clear, yellow or wine red color of the halos in starch agar. 
3. Examine, draw and describe the gelatin stab cultures. See Appendix A, 
Part B, pages 77 to 80. 
(a) What causes the clear halos around the colonies? 
( b) How do you account for the enzyme action so far from the 
colonies in some instances? 
(c) What is the value of these enzymes to the bacteria? 
( d ) How are these bacterial enzymes of value to man? 
(e) Were any molds observed to produce enzymes? 
(0 What are the common names applied to the enzyme changing 
starch to maltose? 
( g ) What kind of chemical change is produced by the enzymes 
studied in the exercise? 
( h ) Name three other types of changes caused by enzymes? 
( i ) State your conception of an enzyme? 
O') Why is agar used for streak cultures more than gelatin? 
( k) Why is gelatin preferred to agar for stab cultures? 
