86 A MANUAL OF BACTERIOLOGY 



1. Fill a test tube one-third full of raw fresh milk. Set it for 

 fifteen, minutes in boiling water which rises higher than the level 

 of the milk in the tube. Cool and place the tube in the incubator. 

 Note whether the milk shows curdling. What is the reaction of 

 the curd ? Test with litmus paper. 



2. Inoculate tubes of sterile milk and litmus milk with B. suh- 

 tilis. Set in the incubator. Note whether the milk curdles. What 

 happens to the litmus ? Is it acid ? 



3. Make stains of the bacteria ia the pure cultures of B. subtilis 

 and in the heated milk, using carbol-fuchsin and Gram's stain. 

 How do the bacteria in the two cultures compare ? 



Exercise 124. Study of Bacillus cyanogenus 



This organism sometimes occurs in the dairy. It is the cause 

 of " blue milk." Its growth is quickly inhibited if lactic-acid 

 bacteria are also present, since it is quite sensitive to the presence 

 of lactic acid. 



Make cultures and study the growth of the organism on 

 gelatin stab, agar slope, milk, and litmus milk. Make stains with 

 carbol-fuchsin and Gram's stain. 



Exercise 125. Study of Bacillus lactis viscosus 



Ropy milk may be produced by a group of organisms of which' 

 this is perhaps the most common. These organisms are so resist- 

 ant to heat that they often survive the ordinary methods of 

 cleansing dairy utensils. 



Make cultures on agar slope and in flasks of sterile milk. 

 Test the viscosity of the milk from time to time with a glass rod. 



Exercise 126. Study of Microspira tyrogena 



This organism, according to Chester, is sometimes found in 

 cheese. It has been previously noted as a type of spiral bacteria. 



Make cultures and study the growth on gelatin stab, agar 

 slope, potato, milk, and litmus milk. Make cover-glass prepara- 

 tions stained with carbol-fuchsin and Gram's stain. 



