202 
PRINCIPLES OF BACTERIOLOGY 
Sedgwick-Tusher’s aerobioscope is the most accurate 
instrument used in studying organisms of the air. A 
fair result may be obtained by exposing a Petri dish 
containing culture media. Opening it for one to two 
minutes should be sufficient. It would be interesting to 
do this before and after sweeping a room. 
Bacteria of Milk 
Milk, having great nourishing qualities, is one of our 
chief articles of food. On account of its great food value 
it furnishes a favorable media for the growth of bac¬ 
teria. 
By careful handling, the bacteria count of milk may 
be kept down. This is accomplished by submitting the 
cows to the tuberculin test, to see that they are free from 
disease, by sanitary stables free from flies and dust, by 
clean utensils and milkers, and keeping the milk cool 
until consumed. On account of the expense, such meth¬ 
ods are not always employed, and a form of sterilization 
is used. Pasteurization is generally used to keep milk 
pure. The milk is heated in a water-bath to 160° F., for 
twenty minutes. There are several domestic pasteur¬ 
izers. They are especially useful in infant feeding. Boil¬ 
ing for ten minutes is another method for destroying the 
bacteria, but as boiled milk has a rather unpleasant odor 
and taste, it is not generally used. 
Pathogenic organisms in milk usually come from un¬ 
healthy cows, infecting it with tuberculosis, foot and 
mouth disease, septic sore throat, etc. Many cases of 
sore throat are traced directly to streptococci in milk. 
The method used to determine the number of bacteria 
in milk is similar to that used in water (see chapter 
under that heading). 
