

BACTERIA IN THE DAIRY. ye: 
reached, on the average, 30,500. From fifteen samples of milk 
obtained from the homes of people in the suburbs of Boston, the 
average was 69,000 germs; from fifty-seven samples from milkmen 
the average was 2,355,000; and from sixteen samples secured at 
groceries the average was 4,577,000 germs per cubic centimeter. 
THE INVESTIGATION HERE REPORTED. 
The milk used for the investigations, of which the details are 
given beyond, was obtained as delivered by milkmen to customers 
in Middletown. It was placed in a sterilized* flask and taken 
directly to the laboratory, where it was immediately “ plated.” 
METHOD OF CONDUCTING THE EXPERIMENTS. 
All of the apparatus was sterilized in dry heat at a temperature 
of 160° C. (310° F.) Distilled water, which had been sterilized 
by boiling on three successive days, was used for diluting. 
One cubic centimeter of the milk or cream to be tested was 
taken in a sterilized pipette and introduced into a flask contain- 
ing ninety-nine cubic centimeters of the sterilized distilled water. 
After this had been thoroughly shaken to insure the even distri- 
bution of the germs, another sterilized pipette was used to draw 
out one cubic centimeter of the diluted milk. This was inoculated 
into a test tube containing about one inch of gelatine culture 
material, which had been liquefied by placing the tube in warm 
water. The gelatine and diluted milk were thoroughly mixed 
together and then poured out on a glass plate, which rested 
on the surface of cold water. Before hardening, the gelatine 
was spread by the aid of sterilized platinum wire, into a rec- 
tangular form. The plates, after hardening, were placed in 
covered glass dishes and left at room temperature until the 
colonies of bacteria were grown. ‘The time varied from thirty- 
six hours to seven or eight days. In estimating the number of 
bacteria, from ten to twenty fields were counted in different parts 
of the plate. Multiplying the average number of colonies in a 
square centimeter by the number of square centimeters, gave the 
number of colonies on a plate. ‘This, multiplied by one hundred 
gave the number of colonies in a cubic centimeter of the original 
milk, and since each colony had grown from one bacterium, it 
gave the number of bacteria originally present in a cubic centi- 
meter of the milk. 

* That is, a flask which had been heated to sucha temperature as to kill all bacteria present 
in it. 
