'GENERAL PROPERTIES OF BACTERIA. 2Q 



them quite unique organisms, for no higher animals or 

 plants have this power. This unusual power is one of con- 

 siderable significance in explaining the conditions of bac- 

 terial life and action. The bacteria which demand oxygen, 

 the great majority of known species, are called aerobic. 

 Those that 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. Be- 

 tween 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, up to 

 a certain limit, grow more rapidly as the temperature rises. 

 Above this upper limit they stop growing, and are eventually 

 killed by higher temperatures. The temperature at which 

 the maximum growth occurs is quite variable. Some grow 

 best at 70 F., many at about 95 F. and others at still 

 higher temperatures. The most remarkable bacteria, in this 

 respect, are certain species, recently discovered, which are 

 unable to grow at ordinary temperatures, but require a tem- 

 perature above 125 F., and some even demanding a temper- 

 ature as high as 140 F. before they can grow most vigor- 

 ously. At temperatures as low as 100 F. they will not 

 grow at all. These have been called thermophilons bacteria. 

 What may be the significance of these bacteria in the proc- 

 esses of nature we cannot say at present. If the tempera- 

 ture 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 numer- 

 ous species side by side. It is only under some very excep- 



