WHAT ARK BACTERIA? 15 



them to the animal kingdom. After long discussion it has 

 been quite definitely concluded that they are closely allied to 

 yeast, and these are in turn closely related to molds. Biologists 

 agree that molds are plants, and, therefore, the conclusion to- 

 day is that bacteria are also plants, but of an extremely low 

 type. It is a matter of no special significance, excepting to the 

 scientist, whether we consider them as belonging to one or the 

 other. Their relations to the dairyman are the same whatever 

 we call them. 



^RELATIONS TO CONDITIONS 



Temperature. The bacteria behave very differently at differ- 

 ent temperatures. Their rapidity of multiplication is very de- 

 cidedly modified by the different degrees of temperatures. In 

 general, at low temperatures, multiplication is comparatively 

 slow. At freezing they do not multiply at all; but when the 

 temperature > is raised above freezing they begin to grow, and 

 their rate of multiplication increases with the increase in the 

 temperature up to a certain point, an optimum temperature. 

 If the temperature is raised higher, the rate becomes slower 

 again, until finally they stop growing. If the temperature is 

 raised still higher, they are injured; and finally by a certain 

 amount of heat they are killed. Thus there is: i. A minimum 

 temperature, the lowest at which they will all grow. 2. An 

 optimum temperature, at which they will grow best. 3. A 

 death temperature, which will kill them. 



Minimum Temperature. Some species will grow, though 

 slowly, at temperatures only just above freezing, multiplying 

 at 33, while other species will not grow at all at such tempera- 

 tures. Some will hardly grow at temperatures lower than that 

 of body heat, 98. Others again seem to require a temperature 

 much higher than this, even as high as 120 to 130. Between 

 these two limits different species show all kinds of variations. 



Optimum Temperature. This also differs very much for dif- 

 ferent species of bacteria. Some will grow at 60 to 65, 



