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° F.), 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 proteus 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 feces 
(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 
