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IOWA ACADEMY OF SCIENCES. 
A METHOD OF ISOLATING AND COUNTING GAS 
PRODUCING BACTERIA IN MILK. 
BY C. H. ECKLES. 
It is a well known fact that more or less gas producing 
bacteria are present in almost all ordinary milk. The 
number present varies with the season of the year and the 
treatment the milk has received. 
This class of ferments is of considerable importance on 
account of the relation it bears to cheese making. 
Practical men have long considered the development of 
gas during the process of cheese making as a serious im- 
pediment to the production of the desired quality. . This 
view has been largely sustained by scientific investigation. 
The bacteria which produce this gas mostly belong to, or are 
closely allied to the colon group. The gas is produced from 
the decomposition of the milk sugar and is generally com- 
posed of about one-third carbon dioxide and two-thirds 
explosive gas, probably hydrogen. 
During the past two years the writer has had occasion 
to determine the number of gas producing germs in a large 
number of milk samples, and during this work developed the 
following method: Agar is made up according to the usual 
methods and treated with a normal solution of sodium 
hydroxide until neutral to phenolphtalein. After filtering, 
2 per cent of lactose is added. The milk is diluted by 
adding a measured amount to a known volume of sterile 
water. A sterile pipette is used to measure a small portion 
of this diluted milk into the melted agar, which is poured 
into a Petri dish in the usual manner. After it has solidi- 
fied, a second tube of melted agar is found on top of the 
first one. This covers all the bacteria added in the first 
tube. As the growth develops, gas is produced, which 
