Exercise VI 



STUDIES IN MICROBIOLOGY (1) 35 



of bacteria now present. Label this series B. 

 Take the agar plates home with you. Keep 

 them in a fairly warm spot. Within a few days 

 colonies should appear. Count them. Calculate 

 the number of bacteria in your culture before 

 and after the two hours of growth. (Remember 

 that a drop contains 0.05 and a loop contains 

 O.OOI ml.) How many divisions did each cell 

 undergo in this period? How much time was 

 needed for a cell to duplicate itself? 



Bacterial calalase 



While your culture is growing, continue with 

 other parts of the experiment. Rinse out the 

 small tubes and dropper pipets which were used 

 for dilution series A. These will now be used 

 in the study of catalase. It is not necessary to 

 use sterile procedure in this part of the experi- 

 ment. 



Obtain 5 ml (1 inch) of the old Serratia cul- 

 ture in a 6-inch test tube. The old culture is 



more convenient for enzyme studies since it con- 

 tains more cells and hence more enzyme than 

 the young culture. Add 10 drops of this culture 

 to a small tube. Add 10 drops of hydrogen 

 peroxide. Wait a few minutes. Observe. What 

 happens when you dip a glowing splinter into 

 an empty test tube? into the reaction tube? 

 Why? 



We shall now measure catalase activity with 

 a "volumeter," a device that measures gas ex- 

 changes in terms of changes in the total volume 

 of gas at constant pressure (see illustration). 

 One of the volumeter test tubes will contain the 

 experimental material. The other is left empty, 

 to act as a thermobarometer — a means of cor- 

 recting for changes of gas volume owing to 

 trivial changes of temperature or barometric 

 pressure in the course of the experiment. Such 

 changes should be equal in both tubes if both 

 contain about the same volumes of gas. 



Working in pairs, place 3 drops of the bac- 

 terial suspension in one of the volumeter test 



