EFFECT OF TEMPERATURE ON BACTERIA. 35 
never multiply to any considerable extent. They have little 
or no effect upon milk, and do not contribute to the changes 
which occur in milk after it has been kept for some time. 
Group VI. Rapid liguefiers. ‘This group includes several 
species, all characterized by a very rapid liquefaction of gelatin. 
Frequently a single colony may spread so rapidly as to destroy 
a plate in twoorthreedays. Itincludes 2. subtilis, B. mycordes, 
and their allies, and comprises species which are commonly 
putrefactive in their action. It isa group of considerable sig- 
nificance because of its rapid decomposing action on milk. 
Fortunately it is never present in very great abundance. 
Group VII. Slow liquefiers. The members of this group 
are not always clearly separated from those of Group VI. They 
liquefy gelatin plates slowly and incompletely, the colonies even 
after several days being rarely more than I centimeter in diam- 
eter, and frequently even smaller. ‘Their putrefactive power is 
less, as indicated by a less offensive odor. All, of course, produce 
digestive enzyms, and consequently show a decomposing action 
upon milk, although this action is not always very noticeable. 
Several species of bacteria are here included. In each sample 
of milk, however, this group commonly includes only a single 
species, although sometimes two or three species of slow lique- 
fiers are found in the same sample of milk. 
Because of their decomposing action upon milk and their 
tendency to produce putrefaction, we regard the last two types 
of bacteria as of much significance. They always tend to pro- 
duce decomposition of the proteidsin the milk, and their presence 
in milk is always undesirable. Any conditions that encourage 
their growth are to be avoided, while conditions that check them 
are to be desired. 
Group VIII. Pale thin colonies. These do not represent 
a single species, but in nearly every case where they have been 
tested they prove to be very difficult to cultivate in artificial 
media and to have no effect.on milk. They oecur only in small 
numbers. | 
Group IX. ed brown colonies.: These were found in small 
numbers only, and in only a few samples of milk. 
