RELATION OF BACTERIA TO DISEASE. 257 



plate method, or by its modifications. The isolated colony 

 which develops on a plate, is derived from a single cell 

 and is, therefore, a pure culture. Transplantations frorr^ 

 such a colony, when properly made into tubes of gelatin, 

 agar, or bouillon, in turn give rise to pure cultures. Sub- 

 sequent transplantations, from tube to tube, can be made as 

 often as may be desired, or as may be necessary. Each 

 growth thus obtained is called a generation. 



In many cases, as in tuberculosis, anthrax, and hog chol- 

 era, the organisms have thus been carried through several 

 hundred consecutive generations without impairment of 

 pathogenic properties. In other instances, as in glanders, 

 the organism does not find in our artificial media the condi- 

 tions favorable for its growth, and, as a result, it under- 

 goes a physiological alteration so that the cultures become 

 less and less active, till finally they cease to have any effect 

 on animals. This change in the physiological properties of 

 an organism known as attenuation is frequently accom- 

 panied by a corresponding decrease in the vitality of the 

 growth, so that, when the virulence is wholly lost, the cul- 

 ture soon dies out. Sometimes, however, the organism 

 adapts itself to the artificial media, and continues to grow, 

 although with diminished pathogenic properties. The 

 phenomenon of attenuation, it should be understood, is 

 not confined to the pathogenic bacteria. Corresponding 

 changes in the activities of many non-pathogenic bacteria 

 are constantly being met with. Thus, a phosphorescing 

 bacillus may emit a strong light when first obtained, but in 

 a short time it may lose this power. The B. ruber 

 of Kiel usually give an intense red growth on potato, but 

 it is not uncommon to meet with it as a colorless growth. 

 Again, a given -organism may rapidly coagulate milk, but 

 after it has been cultivated on artificial media it may lose, 

 more or less completely, the power of altering the compo- 

 sition of milk. These illustrations teach, that when bac- 



17 



