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 
32. represent the bubbles of gas formed by the organism 
t^ig. 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 
