28 ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 
EXERCISE 43 
HYDROLYSIS OF STARCH 
Prepare starch agar plates and inoculate them by streaking with the cul- 
tures furnished. Incubate at 37° C. and test. _ 
Starch agar is made by adding 0.2 per cent of water soluble starch to nu- 
trient agar. 
The diastatic action of the organisms is tested by flooding the surface of the 
starch agar plates with a saturated solution of iodine in 50 per cent alcohol. A 
clear zone around the area on which the organisms have grown indicates 
hydrolysis of the starch. 
EXERCISE 44 
RELATION OF FREE OXYGEN TO GROWTH 
Melt tubes of meat infusion agar and hold for a few minutes at 100° C. to 
expel the free oxygen. Cool to a temperature of 42° to 45° C. and inoculate with 
cultures of Bacterium coli, Bacillus subtilis, and Clostridium sporogenes. Be care- 
ful not to agitate the tubes so as to incorporate unnecessary air in the agar. After 
inoculation, solidify the agar by placing the tubes in water below 40° C. In- 
cubate at 37° C. for two days and observe the types of growth. 
EXERCISE 45 
PIGMENT PRODUCTION BY BACTERIA 
Make glucose agar slope and plain nutrient agar slope cultures in duplicate 
from the chromogenic organisms furnished. After inoculation, incubate one cul- 
ture of each organism on each medium at 20° C. and at 37° C. Observe the 
color and nature of pigments produced. Make observations after two and four 
days’ incubation. 
