TYPES OF BACTERIA FOUND IN MILK 163 
milk curdles, but remains sweet. ‘This is due to a class of bac- 
teria that secrete enzymcs. The bacteria in question really 
secrete two enzymes, one of which is similar to rennet, secreted 
by the stomach of a calf, and the other is similar to irypsin, 
secreted by the pancreatic gland of man and other animals 
When this class of bacteria grow in milk, both of these enzymes 
act upon it. The rennet enzyme shows its effect first, and causes 
the milk to curdle; but since no acid is produced by these organisms 
this curd is not sour. The curd is also softer than that produced 
by the lactic acid bacteria. The phenomenon is sometimes called 
sweet curdling. After a short time, usually two or more days, the 
second enzyme begins to show its effects. This, acting like a 
digestive fluid, changes the nature of the casein from an insoluble 
to a soluble condition, and as fast as this occurs the curd is dis- 
solved in the liquid of the milk. The curd thus disappears, as 
the casein is dissolved, and, finally, the whole curd may be dis- 
solved so that the milk becomes liquid again (Fig. 31, c, d). 
But it is a totally different product from the original milk, since 
it no longer contains casein, but only the digested and dissolved 
products of casein. This softening and dissolving of the curd is 
so much like the digestion that goes on in the intestine of animals, 
that it has commonly been called digestion. The bacteria that 
produce it are also sometimes called the peptonizging bacteria, since 
they produce peptones among the soluble products that come 
from the digested casein. 
Still another term is applied to these bacteria. One of the 
common culture media used in bacteriological work is solidified 
with gelatin. Now, many kinds of bacteria cause the solidified 
jelly to become liquid (Fig. 34, e.f.g.). The liquefaction of the 
gelatin is due to the same enzyme that causes the digestion of the 
milk curd. Hence, we find that the bacteria that liquefy gela- 
tin commonly have the power of dissolving milk curd. The term 
liquefiers is applied to them, since they liquefy both curdled milk 
and gelatin. This type of bacteria is very abundant, and in the 
