X-2 A MANUAL OF BACTERIOLOGY 



2. The rack full of tubes is then immersed in a water bath 

 kept at a temperature between 37 C. and 40 C. 



3. Take out the rack and examine the tubes at the expiration of 

 six hours. Note the appearance, odor, and taste of each sample. 

 Replace the rack in the water bath and continue for another six 

 hours, when the condition of the milk in each tube is again noted. 



According to Gerber ("Die praktische Milchpriifung ") good 

 milk properly handled should not coagulate in less than twelve 

 hours when kept under the conditions described. 



Peter interprets the five types of curds as follows : 



I. Fluid (slight coagulation) : germ content low, with cocci 

 predominating ; bacteria of the lactic-acid, Coli, and Aerogenes 

 types few. 



II. Gelatinous: lactic-acid bacteria predominating; few Aero- 

 genes and Coli varieties. Cocci and Fluorescent bacteria may be 

 present. Gas bubbles indicate presence of Coli and Aerogenes 

 groups. 



III. Granular : lactic-acid, Coli, and Aerogenes types of bac- 

 teria present ; sometimes cocci numerous. 



IV. Cheesy-curdy : lactic-acid bacteria along with cocci, Coli, 

 and Aerogenes ; sometimes only the latter are active. 



V. Gassy : Coli and Aerogenes types predominating, along 

 with lactic-acid bacteria, cocci, and B. mesentericm. 



Exercise 118. Study of Curds Formed by Different Organisms 



Prepare and fill with milk a number of small bottles or flasks 

 holding about 150 cc. Sterilize fractionally and inoculate dif- 

 ferent bottles with Back lactis-acidi, B. coli, B. factis aerogenes, 

 Streptococcus factious, Oidmm lactis, a good dairy starter, and 

 some separator slime. Incubate at 30 C. and examine the curds 

 formed in each case. What does a gassy curd indicate ? 



Exercise 119. The Reducing Action of Milk of High and Low Germ Content 



Milk in which bacterial development has occurred has a greater 

 or less reducing action upon methylene blue. This action is due 

 to the development of bacteria and not to some substance present 

 in the milk at the time it is drawn. Oxygen must be excluded 



