34 BACTERIOLOGY. 



fered with by either the presence or absence of 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 chromo- 

 genic 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 upon the surface of the medium and 

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

 the bacilhis prodigiosus and of the spirillum rubrum. 

 With 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 

 farthest removed from the action of oxygen. 



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 most bacteria are known to 

 grow range from 5.5° C. to 48° C. At the former 

 temperature development is hardly appreciable ; it be- 

 comes more and more active until 38° C. is reached, 

 when it is at its optimum, and, as a rule, ceases with 

 43° C ; though it is said that species exist that will 

 multiply at as high a temperature as 60° C. and others 

 as low as 0° C. The most favorable temperature for 

 the development of pathogenic bacteria is that of the 

 human body, viz., 37.5° C. There are a number of 

 bacteria commonly present in water, the so-called 

 normal water bacteria, that grow best at about 20° C. 



