BACTERIA IN FOODS 223 



in green cheese. A very rapid increase of bacteria occurs, 

 confined almost exclusively to the lactic-acid group. This 

 commences in green cheese about the eighth day, and con- 

 tinues more or less for twenty days. In Cheddar cheese it 

 commences about the fifth day, reaches its maximum about 

 the twentieth day, declines rapidly to the thirtieth day, and 

 gradually for a hundred following days. During the first 

 forty days of this period the casein-digesting and gas-pro- 

 ducing organisms are present, and at first increasing, but 

 relatively to only a very slight degree. With this rapid 

 increase in organisms the curd begins to lose its elastic 

 texture, and before the maximum number of bacteria is 

 reached the curing is far advanced. Freudenreich has 

 shown that acid inhibits the growth of the casein-digesting 

 microbes and vice versd. 



3. Period of Final Bacterial Decline. The cause of this 

 decline can only be conjectured, but it is highly probable 

 that it is due to a general principle to which reference has 

 frequently been made, viz., that after a certain time the 

 further growth of any species of bacteria is prevented by its 

 own products. We may observe that the gas-producing 

 bacteria in Cheddar cheese last much longer than the pep- 

 tonising organisms, for they are still present up to eighty 

 days. Professor Russell aptly compares the bacterial vege- 

 tation of cheese with its anologue in a freshly seeded field. 

 "At first multitudes of weeds appear with the grass. These 

 are the casein-digesting organisms, while the grass is com- 

 parable to the more native lactic-acid flora. In course of 

 time, however, grass, which is the natural covering of soil, 

 * drives out ' the weeds, and in cheese a similar condition 

 occurs." In milk the lactic-acid bacteria and peptonising 

 organisms grow together; in ripening cheese the former 

 eliminate the latter. 



We have seen that the conclusion generally held respect- 

 ing these lactic-acid bacteria is that they are the main agents 



