36 THE FUNDAMENTAL PRINCIPLES. 



THE RELATION OF OXYGEN TO BACTERIAL LIFE. 



Bacteria are divided into aerobic and anaerobic according 

 as they require oxygen or not for their development. Some 

 thriving best in the presence of oxygen are able to develop, 

 however, without the presence of this gas ; these are called 

 facultative anaerobic. Others thriving best without oxygen 

 but able to develop in the presence of this gas are called 

 facultative aerobic. 



THE RELATION OF DEAD AND LIVING ORGANIC MATTER 

 TO BACTERIA. 



Bacteria are also divided into saprophytes and parasites, 

 according as they require for their development merely the 

 presence of decomposable organic matter or the body of a 

 living higher organism, as host, on which they live. Should 

 they also be able to live outside their host, they are called 

 facultative parasites. The disease-producing germs are always 

 parasitic. 



THE ESSENTIAL CONDITIONS OF BACTERIAL GROWTH. 



For the proper development of bacteria the following are 

 required : heat, moisture, and the presence of some decom- 

 posable organic matter, with special chemical reaction of the 

 culture-medium. 



I. Heat. 



A temperature between 10 and 40 C. is required for the 

 development of adult bacteria, but the degree varies greatly 

 according to the different species. Some of the parasites 

 thrive best at a temperature in the neighborhood of that of 

 the human body, about 36 or 37 O. ; others, the saprophytes, 

 seem to grow best at the ordinary room temperature, between 

 20 and 30 C. ; some few again seem to be able to grow 

 and multiply at a temperature near the freezing-point ; and 

 not long ago a number have been shown to develop abun- 

 dantly at a temperature above 70 C. As a general rule, 

 however, a temperature of C. and one above 60 C. are fatal 



