144 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, orare 

 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 



