t60 BACTERIA IN MILK 
presently see, milk kept at 70° becomes, in a short time, almost 
completely filled with this species of bacterium at the expense of 
all the others that might originally have been there. This type of 
organism is a friend to the dairyman, unless he be a milk man who 
wants to deliver his milk sweet. 
II. Bacterium aerogenes-—This type of lactic acid organism is 
not so abundant in milk as the first, although it may be found 
more or less abundant in most samples of milk. Microscopically, 
it is practically indistinguishable from the first variety, but it 
differs in two pronounced points (Fig. 31). The first is that it 
grows luxuriantly in contact with the air or oxygen, while the 
L9%., first variety does not. The aerogenes type, 
therefore, grows abundantly on the surface of 
culture media, like potato or agar. The second 
and more noticeable point is the fact that it pro- 
Fic. 31-—Bact. duces a fermentation of milk-sugar, giving rise 
aes’ ta 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 repre- 
sent the bubbles of gas formed by the organism (Fig. 30). The 
whey commonly separates in a short time from the curd, and the 
final appearance is strikingly different from that of the curdled milk 
produced by the first type. 
The production of gas is the cause of the ruin of vast quanti- 
ties of cheese. If milk, when it is made into cheese, contains a 
considerable quantity of these bacteria, instead of the more com- 
mon 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. 32). At the same 
time that this swelling occurs, the flavor of the cheese becomes 
