ELEMENTARY BACTERIOLOGY LABORATORY EXERCISES 53 
1.1 gms. of the dry dye (medicinal methylene blue) in 500 c.c. of distilled water. 
When needed, 1 c.c. of the stock solution is measured out and diluted with dis- 
55 
tilled water to 40 c.c. One c.c. of this diluted portion contains —-———— gms. 
1,000,000 
of the dye. This amount added to 10 c.c. of milk gives a concentration of approx- 
55 1 
imately = in the final mixture. 
11,000,000 200,000 
The stock solution keeps for long periods due to the toxicity of methylene 
blue at that concentration. Nevertheless, it is advisable to heat the dilute solu- 
tion to 100° C. before use. 
EXERCISE 78 
A STUDY OF STREPTOCOCCUS LACTIS 
From the culture of Streptococcus lactis furnished, inoculate: agar slope; 
glucose, lactose and sucrose broth Durham tubes; gelatin stab; and plain milk, 
litmus milk, and brom cresol purple milk. 
By the loop dilution method plate the culture on lactose agar in order to 
study the type of colony produced. Incubate the cultures and plates at 37° C. 
until the next period. 
Make a Gram stain preparation from the agar slope culture and make 
drawing. Is it Gram positive? Does it produce spores? 
Note: The type of growth produced on the agar slope; the type of colony 
produced on the Petri plates; which sugars are fermented, and whether gas is 
produced in the fermentation of sugars by this organism; and whether gelatin 
is liquefied. 
From the brom cresol purple and litmus milk cultures, note reaction changes 
caused by the production of acid or alkali. Note whether litmus is reduced. Ob- 
serve the plain milk tube for the presence of coagulation, type of curd pro- 
duced, and whether there is any digestion of the casein. From this culture deter- 
mine the amount of lactic acid produced in milk. 
Determination of Lactic Acid in Milk 
Take 5 c.c. of milk from the culture to be tested and dilute with 5 c.c. of dis- 
tilled water. Using phenolphthalein as indicator, titrate with N/20 NaOH. 
Calculate the per cent of acid as lactic by use of the formula: 
c.c. N/20 NaOH & .0045 >< 100 
Per cent as lactic acid = 
c.c. milk used 
