140 lOAVA ACADEMY OF SCIENCES. 



These observations led to the constant use of lactose media 

 when it was desired to make a quantitative estimate or to 

 isolate the acid organism. 



A more recent study of the work done by various inves- 

 tigators led to the conclusion that much of the counting of 

 bacteria which has been done is of little value on account 

 of the kind of media used, and the lack of knowledge 

 regarding the relation it bears to the number of organisms 

 developed. It is also evident that mistakes, due to the 

 same cause, have been made in regard to the kind of 

 bacteria most common in milk. The most common mis- 

 take has been a failure to recognize that the bacterial flora 

 of milk is composed, as largely as it is, of acid- producing 

 bacteria, mostly of a single species. In order to get a 

 definite result a short series of experiments was recently 

 undertaken with the following objects in view: 



First — a. To find how the number of milk bacteria 

 developing on peptone agar compared with number grow- 

 ing on the same media with 2 per cent lactose added, h. 

 Same comparison between ordinary peptone gelatin and 2 

 per cent lactose gelatin, c. Same comparison between 

 peptone and lactose gelatin and peptone and lactose agar. 



Second. — What effect does the kind of media have on the 

 relative proportion developing, of those causing acid coag- 

 ulation; those having no effect on milk; and those coag- 

 ulating by action of an enzyme? 



