Exercise 27. Milk Fermentation 
Acid Fermentation Caused by Streptococcus Lactis 
MATERIALS: 
a. 1 tube of plain broth 
b. 1 Durham tube of lactose broth 
C. 1 Durham tube of saccharose broth 
d. 1 Durham tube of glycerin broth 
e. 1 fermentation tube of dextrose broth 
f. 1 lactose agar slope 
g. 3 tubes of lactose agar for plates 
h. 1 tube lactose gelatin 
i. 1 small tube of plain milk 
j. 1 small tube of litmus milk 
k. 1 large tube of plain milk 
l. 4 sterile Petri dishes 
m. Ice water for gelatin 
n. Culture of Streptococcus lactis 
0. Outfit for determination of acidity 
The souring or formation of acid in milk is the most common change ob" 
served in it and may be caused by a great variety of bacteria. The souring o^ 
.milk without the formation of gas and with the production of the pleasant 
buttermilk flavor and odor is due to a group of bacteria of which Streptococ¬ 
cus lactis is the principal representative. 
1. Label each of the culture tubes and the Petri dishes with the number of 
the exercise and the letter indicating the kind of culture medium. 
2. Prepare loop dilution plates for colony formation. Label sterile Petri 
dishes number 1, 2, and 3. Melt three tubes of lactose agar and cool them not 
lower than 45° C. Label one tube number 1; one, number 2; and the remain¬ 
ing one, number 3. Transfer one loopful of the culture to the agar tube num¬ 
ber 1. Mix the organisms and agar well. Transfer 3 loopfuls of the agar and 
organisms from tube number 1 to tube number 2. Mix well. Transfer 4 
loopfuls from tube number 2 to tube number 3. Pour the agar in tube num¬ 
ber 1 into Petri dish number 1, and the agar in tube number 2 into Petri dish 
number 2, and that of tube number 3 into Petri dish number 3. This gives a 
dilution of the organisms and enough scattered colonies on one of the plates 
so that the colonies may be studied in detail. 
3. Make a stab culture in the gelatin. 
4. Inoculate all of the remaining tubes of media by means of the wire loop. 
5. Incubate the gelatin culture in the ice box. Incubate all of the other 
cultures 5 to 7 days at room temperature. 
6. Record the morphological, cultural, and biochemical properties of the 
organisms according to the outline in Appendix A. The description should be 
recorded on the regular note paper, following the order given below: 
A. Physiology 
1. Relation to Oxygen: aerobic, anaerobic, faculative. 
2. Acid production: from dextrose, lactose, saccharose, glycerin. 
3. Gas Production: from dextrose, lactose, saccharose, glycerin. 
4. Production of pigment: fluorescent, violet, blue, green, yellow, orange, 
red, brown, pink. 
