AGRICULTURAL BACTERIOLOGY. 



is so common as properly to be regarded as the lactic bacteria, 

 par excellence. The species, first described by Gunther, and 

 later studied by many others, sometimes under the same 

 name, and sometimes under different names, is the most 

 widely distributed lactic organism in the dairies both of this 

 country and of Europe (Fig. 22). It shows a considerable 

 variety and while the different types described by 

 different observers do not agree in every respect, 

 they are all so much alike as to be regarded 

 properly as a single species. 



2. Bacillus lactis aerogenes. This species is 

 almost as widely distributed as the first, and in 

 some places appears to be the cause of the spontaneous souring 

 of milk. Microscopically it is almost identical with the first 

 species (Fig. 23), but it differs considerably in its action on 

 milk and in other characteristics. 



There are perhaps a dozen other types of bacteria described 

 by bacteriologists as causing the normal souring of milk, and 

 among them are some which are clearly different 

 from either of the two mentioned. But these 

 two, with their varieties, must certainly be re- 

 garded as the lactic bacteria of highest importance 

 and widest distribution. A description of the long 

 list would be out of place in this work. 



One fact of the greatest importance in regard 

 to the lactic bacteria is that, so far as known, they 

 never form spores. This is of great significance, since it makes 

 it possible to destroy them by the use of moderate heat of not 

 more than 150 to 160 F., a matter of great practical 

 importance. 



The fact that milk under ordinary circumstances is sure to 

 sour and that, if examined bacteriologically when a few hours 

 old, it is found to contain lactic bacteria in overwhelming 

 numbers sometimes over 99% being B. acidi lactici would 



