222 BACTERIA 



cheese-ripening, either in the early or late stages. First, 

 there are the lactic-acid bacteria, by far the largest group, 

 and the one common feature of which is the production by 

 fermentation of lactic acid ; secondly, there are the casein- 

 digesting bacteria, present in relatively small numbers; 

 thirdly, the gas-producing bacteria, which give to cheese its 

 honeycombed appearance; lastly, an indifferent or miscel- 

 laneous group of extraneous bacteria, which were in the milk 

 at the outset of cheese-making, or are intruders from the 

 air or rennet. All these four groups may bring about a 

 variety of changes, beneficial and otherwise, in the cheese- 

 making. 



In order that the relation of bacteria to cheese may be 

 more fully understood, we may draw attention to some ex- 

 periments conducted by Professor H. L. Russell as to the 

 numbers of bacteria present during different stages of the 

 ripening, excluding those already referred to as present in 

 the rennet. It appears that there is always at first a marked 

 increase in the number of micro-organisms, which is soon 

 followed by a more gradual decline. While the casein- 

 digesting and gas-producing classes suffer a general and 

 more or less rapid decline, the lactic-acid bacteria develop 

 to an enormous extent, from which fact it would appear 

 that cheese offers ideal conditions for the development of 

 the latter. In some most interesting records Professor Rus- 

 sell has divided the ripening process into three divisions: 



1 . Period of Initial Bacterial Decline in Cheese. Wh e re the 

 green cheeses were examined immediately after removing 

 from the press, it was usually found that a diminution in 

 numbers of bacteria had taken place. This period of de- 

 cline lasts but a short time, not beyond the second day. 

 Lower temperature and expulsion of the whey would ac- 

 count for this general decline in all species of bacteria. 



2. Period of Bacterial Increase. Soon after the cheese is re- 

 moved from the press a most noteworthy change takes place 



