54 BACTERIOLOGY 



it from the air as free oxygen, while the anaerobic species, 

 not possessed of this ability, obtain it through the decom- 

 position of more or less stable oxygen-containing com- 

 pounds. 



Though the multiplication of the facultative varieties is 

 not interfered with by either the presence or absence of free 

 oxygen, yet experiments demonstrate that the products of 

 their growth are different under the varying conditions of 

 absence or presence of this gas. For example : in the case 

 of certain of the chromogenic forms the presence or absence 

 of oxygen has a very decided effect upon the production of 

 the pigments by which they are characterized. 



Note. — Observe the difference between the intensity of 

 color produced upoii the surface of the medium and that 

 along the track of the needle in stab-cultures of bacillus 

 prodigiosus and of spirillum rubrum. In the former the red 

 color is apparently a product dependent upon the presence 

 of oxygen, while in the latter the greatest intensity of color 

 occurs at the point furthest removed from the action of 

 oxygen. 



Influence of Temperature upon the Growth. — Another 

 factor which plays a highly important part in the biological 

 functions of these organisms is the temperature under which 

 they exist. The extremes of temperature between which 

 the majority of bacteria are known to grow range from 5.5° 

 to 43° C. At the former temperature 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 

 at 43° C; though species exist that multiply at as high a 

 temperature as 70° C. and others at as low as 0° C. The 



