112 GENETIC VARIATIONS 



that occurred could not be accounted for as due to contamina- 

 tion with another bacterial species. 



The result of these intensive studies has been to show that 

 genetic variations of many types do indeed occur in bacteria, 

 and particularly that such are brought about by changed 

 environmental conditions. They show that many well known 

 changes in bacterial cultures are due, not to contamination, 

 but to inherited changes produced under the action of the 

 environment.^ Some of the commoner types of change are the 

 following: 



Attenuation, or decrease in virulence: Disease-producing 

 bacteria are kept for a long time under unfavorable condi- 

 tions, and these cause them gradually to lose their power to 

 produce disease. This decreased virulence is hereditary. 



Increase of virulence: A change, the reverse of that just 

 mentioned, is brought about in many disease-producing bac- 

 teria by subjecting them to appropriate conditions. By inocu- 

 lating them successively into animals of higher and higher 

 resistance, they are caused to acquire by degrees a higher 

 virulence, and this acquired virulence is inherited. 



Another type of inherited change induced by environmen- 

 tal conditions consists in alterations in the secretions or excre- 

 tions of the bacteria. Bacteria that secrete mucous coverings 

 are caused to secrete less mucus, or none at all, and the 

 changed condition is inherited. Some bacteria secrete colored 

 materials; by subjection to certain conditions, they are caused 

 to cease producing the colors; and the change is inherited. 



Most important perhaps are the acquirement of adaptive 

 changes in metabolism. There are certain kinds of bacteria 

 that can split up particular kinds of sugar, such as lactose, 

 getting nutriment from them. Certain other related races can- 

 not. If cultivated in the presence of this kind of sugar, these 



