BACTERIA OF MILK 61 



and skin of cows at pasture. The manure does not con- 

 tain very many and few are carried into milk on this 

 substance (Weigmann). The hay and potato bacilli 

 grow best at 23 to 37 C. (73 to 99.6 R), but will de- 

 velop at any temperature between 10 and 45 C. (50 to 

 113 F.) . The spores are very resistant to heat and will 

 survive several hours boiling. 



Other peptonizers include the bacteria of the proteus 

 group of putrefactive organisms, which are often present 

 in milk, although more frequently found in water (Fig. 

 8). They are long, thin bacilli which grow in colonies 

 of various forms. The most common representative of 

 these organisms is the Bacillus pfoteus vulgaris, which 

 grows in colonies with branches or ray-like projections. 

 They get into the milk principally through the water used 

 to wash the milk vessels and utensils. The Bacillus 

 proteus vulgaris grows best at about 25 C. (77 F.) 



4. Alkali-forming Bacteria. Some of the bacteria 

 commonly found in milk render the milk alkaline without 

 producing any change for a time in its appearance, taste, 

 or odor. When the alkalinity has attained a certain 

 degree, the fat is saponified and the neutral calcium 

 caseinate compound becomes basic, in consequence of 

 which the milk is changed to a yellow, translucent, whey- 

 like fluid (Jensen). Within the ordinary life of milk, 

 however, the bacteria of this group are practically without 

 effect. Usually they get into the milk in the same man- 

 ner as hay bacilli (page 60) , their source being the soil, 

 but occasionally they are derived from the faeces 

 (Rogers). 



5. Inert Bacteria. A large number of the common 

 milk bacteria produce no change in the appearance, odor, 

 taste, or reaction of milk and are consequently said to 



