TYPES OF BACTERIA FOUND IN MILK. 147 
bacterium ordinarily found in milk. For this reason, as we shall 
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. 
FIG. 31. Showing the action of different types of bacteria upon milk, a, the 
aerobic type (Bact. aerogenes); b, the anaerobic type (Boot, acidi lactici); c and d,.the 
peptonizing type, with the curd in different stages of digestion. 
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. 32). The first is that it grows luxuri- 
antly in contact with the air or oxygen, while the first variety does 
