28 BACTERIOLOGY. 



"anaerobic" bacteria in contradistinction to another 

 group for the proper performance of whose functions 

 "\\<ren is essential, which he called "aerobic" bacteria. 

 In addition to these, there is a third group for the main- 

 tenance of whose existence the absence or presence of 

 oxygen is apparently of no moment their development 

 progresses as well with as without it ; these represent 

 the class known as "facultative" in their relation to 

 this gas. It is in this third group, the facultative, that 

 the majority of bacteria belong. Though the multipli- 

 cation of the facultative varieties is not interfered with 

 by either the presence or absence of oxygen, yet experi- 

 ments show that the products of their growth are differ- 

 ent under the varying conditions of absence or presence 

 of this gas. 



Another element which plays a most important part 

 in the biological functions of these organisms is the 

 temperature under which they exist. The extremes of 

 temperature under which bacteria are known to grow 

 range from 5.5 C. to 48 C. At the former tempera- 

 ture development is hardly appreciable, it becomes more 

 and more active until 38 C. is reached, when it is at its 

 optimum, and, as a rule, ceases with 43 C. Neither of 

 the extremes can be considered normal temperatures for 

 the growth of these organisms. The most favorable 

 temperature for the development of the majority of 

 bacteria is that of the human body, viz., 37.5 C. 



In general it may be said that for the growth and 

 development of bacteria, organic matter of a neutral or 

 slightly alkaline reaction, in the presence of moisture 

 and at a suitable temperature, is necessary. From this 

 can be formed some idea of the omnipresence in nature 

 of these minute vegetable forms. Everywhere that 

 these conditions exist, bacteria can be found. 



