148 BACTERIA IN MILK. 



not. The aerogenes type, therefore, grows abundantly on the sur- 

 face of culture media, like potato or agar. The second and more 

 noticeable point is the fact that it produces a fermentation of milk- 

 sugar, giving rise to a quantity of gas. When inoculated into milk 

 it causes a souring which is rapidly followed by curdling, the rapidity 

 of the curdling varying in different specimens. The curd which is 

 produced differs very much from that of the first type 

 of lactic acid bacteria. It is always more or less filled 

 with gas bubbles, and when care is taken to obtain a 

 typical curd, it appears crowded with holes, which 

 represent the bubbles of gas formed by the organism 

 (Fig. 31). The whey commonly separates in a short 

 time from the curd, and the final appearance is strik- 

 ingly different from that of the curdled milk produced by the 

 first type. 



The production of gas is the cause of the ruin of vast quantities 

 of cheese. If milk, when it is made into cheese, contains a consider- 

 able quantity of these bacteria, instead of the more common type, 

 the bacteria grow and develop gas, the cheese becomes filled with 

 the bubbles, swelling more and more, until it finally results in what 

 is known as swelled cheese (Fig. 33). At the same time that this 

 swelling occurs, the flavor of the cheese becomes unsatisfactory, so 

 that the swelled cheese may be practically worthless. These or- 

 ganisms have been the cause of the loss of enormous amounts of 

 money to cheese makers. In butter-making they are not so dis- 

 astrous, but here, too, their presence is undesirable, for they some- 

 times produce unpleasant flavors in the creams, resulting in an 

 inferior grade of butter. This type of organism, therefore, is 

 decidedly the dairyman's foe. 



Several varieties of bacteria belong to this general type of gas- 

 producing organisms. Among them is B. coli, which is very 

 similar to B. aerogenes, except that it is motile. This, an inhabitant 

 of the intestine (see page 130),' is very commonly found in milk. 



III. Bacillus Bulgarians. A third radically different type of acid 

 bacterium is one that has recently come into prominence in various 

 forms of beverages composed of soured milk. In certain parts of 





