PROTECTIVE ACTION OF LACTIC ACID BACTERIA 173 
them. Whatever this may be, its influence ceases after a few hours. 
This power of checking bacterial growth is sometimes called the 
germicidal power of milk, and it lasts from three to twenty-four 
hours, according to temperature, being less at higher tempera- 
tures. After it has passed, the bacteria begin to increase rapidly 
and the number present at any later period is more dependent 
upon the extent of their multiplication than upon the original 
contamination. The rate of multiplication of all bacteria de- 
pends upon temperature. The majority of milk bacteria grow 
best at temperatures between 60° and 1oo°F., and, generally 
speaking, they grow more rapidly at the higher temperatures. 
The effect of temperature is shown by the following example: 
Fresh milk contained.......... 0.0 ccc cece nce e eee 6,525 bacteria per c.c. 
After 25 hours at 50° the same milk contained...... 6,425 bacterla ‘“ 
After 25 hours at 70° the same milk contained...... 6,275,000 bacteria “ 
In this example it is seen that for twenty-five hours the bacteria 
in milk kept at 50° did not multiply at all, while in that kept at 70° 
they multiplied one thousand fold. It is not common to find such 
a, striking difference, but in all cases there is a very marked 
contrast. 
PROTECTIVE ACTION OF LACTIC ACID 
BACTERIA 
If ordinary proteids, like eggs or meat, are left undisturbed to 
the action of bacteria, they will putrefy. Milk also contains a 
proteid, casein, which is just as liable to putrefaction as other 
proteids. But under ordinary conditions it does not undergo this 
unpleasant change. Milk sours, but rarely putrefies. The reason 
for this is found in the power of the lactic acid bacteria to restrain 
the growth of other species. Almost from the start, the lactic acid 
bacteria in milk grow more rapidly than the other types, and as 
they become more abundant, they prevent the other kinds from 
growing; they thus effectually restrain the growth of the putrefac- 
tive bacteria, so that milk that has begun to sour will not putrefy. 
