CONDITIONS INFLUENCING GROWTH OF BACTERIA. 29 



otherwise inexplicable contamination of cultures in the labora- 

 tory. 



Conditions influencing the growth of bacteria : It 

 is evident from the foregoing that certain conditions are 

 necessary for the development of bacteria and the manifesta- 

 tion of their presence. We will consider these requirements 

 separately. 



Oxygen : Most bacteria require oxygen. Some will develop 

 only when there is not even a trace of oxygen present ; while 

 others can, in a measure, accommodate themselves to sur- 

 rounding conditions (facultative). In accordance with their 

 affinity for oxygen, bacteria are divided into : aerobes, those 

 which require oxygen ; and anaerobes, those which do not 

 require oxygen ; further, there are the facultative and obliga- 

 tive aerobes and anaerobes. The following examples will 

 serve to illustrate : 



OBLJGATIVE AEROBE Bacillus subtilis. 

 OBLIGATIVE ANAEROBE Bacillus telani. 

 FACULTATIVE ANAEROBE Bacillus typhosus. 



Light : Most bacteria are not influenced by ordinary light ; 

 but the direct rays of the sun or reflected light either kill 

 bacteria or retard their development. The same is true of 

 the electric arc light. A blue light materially interferes with 

 the life-processes of bacteria. The virulence of pathogenic 

 bacteria is reduced if they are grown in the light. Some of 

 the color-producing bacteria, however, will not produce their 

 pigment unless the culture is exposed to the light. Some 

 cultures, on the other hand, must be kept in the dark. 



Electricity : The electric current checks bacterial develop- 

 ment. The effect of the Roentgen ray on bacteria is still a 

 matter of speculation ; and further study and experimentation 

 are necessary before any positive statements can be made as 

 to its influence on bacterial activity. The Roentgen ray and 

 direct sunlight are being used in the treatment of pulmonary 

 tuberculosis, but the results are still sub judicce. The use of 

 the Roentgen ray in cancer is attended apparently by very 

 good results ; but it is still an open question whether or not 

 this disease is of bacterial origin. 



Water : All bacteria require a certain amount of moisture. 



