ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 25 
EXERCISE 39 
GELATIN LIQUEFACTION 
Make two gelatin stab cultures each of Streptococcus lactis, Bacterium coli, 
Bacillus subtilis, and Proteus vulgaris. Incubate one culture of each organism at 
20° C. and the other set at 37° C.; include with each set one sterile tube as a 
control. 
After four days, examine the cultures incubated at 20° C. for liquefaction. 
Chill the tubes, incubated at 37° C., in cold water and observe whether the solidi- 
fying power of the gelatin has been destroyed. Care should be taken not to agitate 
the tubes incubated at 37° C. while in the liquid condition, as in cases of partial 
liquefaction the reaction would be obscured by the mixing of the liquefied portion 
with the unchanged gelatin. 
EXERCISE 40 
REACTIONS IN MILK 
Using cultures of Streptococcus lactis, Bacterium coli, Bacillus subtilis, and 
Proteus vulgaris, inoculate tubes of: 
(1) Plain milk 
(2) Litmus milk 
(3) Brom cresol purple milk 
Incubate at 37° C. for four days and observe the changes which have taken 
place. 
Plain Milk Medium 
Plain milk medium consists simply of fresh skimmed milk. Fresh milk should 
be used; milk in which the acidity has begun to rise is apt to coagulate when 
sterilized. 
Milk media may be sterilized in the autoclave or by the intermittent method. 
Milk, especially milk of poor quality which is heavily contaminated with bac- 
teria, is frequently difficult to sterilize, hence the autoclave method is more sure. 
On the other hand, milk is browned somewhat at high temperatures due to the 
caramelization of the milk sugar. 
Litmus Milk 
Litmus milk consists of plain skimmed milk to which sufficient litmus solution 
has been added to impart a light purple color. For this purpose, a saturated 
aqueous solution of litmus should be used. Because of the great variation in the 
commercial litmus preparations, a definite rule as to amount necessary cannot be 
given. 
Besides being of value to indicate the production of acidity or alkalinity in 
milk by bacteria, litmus is reduced by many bacteria to a colorless compound. 
‘The reduction of litmus is of some differential value in the study of bacteria. 
