THE NATURE OF BACTERIA. 33 



This property of living without oxygen makes them quite unique 

 organisms, for no higher animals or plants have this power. 

 This unusual power is one of considerable significance in 

 explaining the conditions of bacterial life and action. The 

 bacteria which demand oxygen, the great majority of known 

 species, are called aerobic. Those tha,t can only live in the 

 absence of oxygen are called anaerobic. Some species that 

 can live either in the presence or absence of oxygen are called 

 facultative anaerobic. Between the two extremes of aerobic 

 and anaerobic bacteria are numerous intermediate grades. 



Relation to Temperature. Bacteria have the same rela- 

 tion to temperature as do ordinary living organisms. At 

 freezing or below it they cease to grow. As the temperature 

 rises above freezing they begin their life activities and grow 

 more rapidly as the temperature rises, up to a certain limit. 

 Above this upper limit they stop growing, and are eventually 

 killed by higher temperatures. The temperature in which the 

 maximum growth occurs is quite variable. Some grow best 

 at 70 F., many at about 95 F. and others at still higher tem- 

 peratures. The most remarkable bacteria, in this respect, are 

 certain species, recently discovered, which are unable to grow 

 at ordinary temperatures, but require a temperature above 

 125 F., and some even demanding a temperature as high as 

 140 F. before they can grow most vigorously. At tempera- 

 tures as low as 100 F. they will not grow at all. These have 

 been called thcnnophilous bacteria. \Yhat may be the signifi- 

 cance of these bacteria in the processes of nature we cannot 

 say at present. If the temperature is raised above 160 F. 

 most bacteria are quickly killed, although bacteria spores can 

 stand a much greater heat. 



Pure Cultures. In nature the different species of bacteria 



are found associated in all sorts of indefinite mixtures. Earth, 



air or water, or any other medium which supports bacterial 



life, will be found in most cases to contain numerous species 



3 



