1030 BACTERIA IN AIR, SOIL, WATER, AND MILK 



lactic-acid production, however, according to Heinemann, 7 are the 

 two first-mentioned, Bacillus lactis aerogenes and Streptococcus 

 lacticus. The secret of the regularity with which some of these 

 bacteria are present in sour milk is probably found in the ability 

 of these varieties to withstand a much higher degree of acidity 

 of the culture medium than other species. In consequence, they are 

 able to persist and develop when cultural conditions are absolutely 

 unsuited to other bacteria. 



Consequent upon acidification of the milk by lactic-acid forma- 

 tion, there is coagulation of casein. Casein precipitation, however, 

 may also be due to a non-acid coagulation caused by a bacterial 

 ferment. Casein precipitated in this way may be redissolved by 

 a bacterial trypsin or casease, produced by the same or other bac- 

 teria, the milk again becoming entirely liquid, transparent, and of 

 a yellowish color. The casein precipitated by lactic-acid formation, 

 however, is never thus redissolved, because the high acidity does 

 not permit the proteolytic ferments to act. 8 



Butyric-acid fermentation in milk, a common phenomenon, is also 

 an evidence of bacterial growth. As a rule, it is produced by the 

 anaerobic bacteria, and is a process developing much more slowly 

 than other fermentations. A large number of bacteria have been 

 described which are capable of producing such changes, the chemical 

 process by which they are produced being, as yet, not entirely 

 understood. It is probable that the process takes place after hydrol- 

 ysis of the dissaccharid somewhat according to the following 

 formula : 



C H 12 Q 6 C 4 H 8 O 2 + 2 CO 2 -f- 2 H 2 . 



Special bacteria have been described in connection with this form 

 of milk fermentation, 9 most of them non-pathogenic. It is unques- 

 tionable, however, that many of the well-known pathogenic bacteria, 

 such as Bacillus aerogenes capsulatus, Bacillus cedematis maligni, 

 possess the power of similar butyric-acid formation. While less 

 commonly observed in milk, because milk is rarely kept long enough 

 to permit of the action or development of these enzymes, the butyric- 

 acid fermentation is of importance in connection with butter, where 

 it is one of the causes producing rancidity. 



7 Heinemann, Jour, of Inf. Dig., 3, 1906. 



6 Conn, Exper. Stat. Rep., 1892. 



9 Schattenfroh und Grasberger, Arch. f. Hyg., 37, 1900. 



