136 TEXT-BOOK OP BACTERIOLOGY. 



words, we have not yet succeeded in producing an enduring in- 

 crease of the natural virulence of bacteria, in changing toxic into 

 infectious kinds, or in enabling the latter to infect animals which 

 were proof against them, or to display a quicker or stronger action 

 than was previously observed in them. 



On the contrary, the opposite phenomenon, an enduring de- 

 crease — nay, even the complete irrecoverable loss — of virulence has 

 been observed in very many cases, and in the most different species 

 of pathogenic bacteria. In the year 1880 Pasteur surprised the 

 scientific world by the discovery that under certain circumstances 

 the micro-organisms of chicken cholera lose their poisonous power, 

 to a greater or less extent, without showing any change in their 

 appearance, way of growth, etc. 



Toussaint and Pasteur found that the anthrax bacilli could be 

 deprived of their virulence in like manner, and the same fact has 

 since been proved with regard to the bacilli of swine erysipelas and 

 the symptomatic anthrax, the pneumonia bacteria of A. Fraenkel, 

 and some others. 



This d iminution of virulence is the result of two essentially dif- 

 ferent causes. The one, which we may call the natural cause, has 

 recently been more fully elucidated, particularly by Fliigge. It 

 is a gradual diminution of infectious power in bacteria which we 

 compel to vegetate for a long time separated from their natural 

 conditions of growth, on our artificial food media, and under the at- 

 mospheric conditions of our laboratories. By a gradual adaptation 

 to the altered, saprophytic way of life, or by a progressive selection 

 of such cells as are naturally more capable of adopting this altered 

 way of life, the original capacity for development within a foreign 

 body diminishes more and more. As an outward sign of the change 

 which has taken place, we observe that the culture now shows a 

 much more luxuriant and rapid growth on the lifeless food-medium 

 than was at first the case, when the conditions were yet new and 

 strange. Not all the pathogenic bacteria possess this power of 

 " cutting their coat according to their cloth " and adapting them- 

 selves to outward circumstances. Some cling with marked tenacity 

 to their proper character, which they do not leave even when com- 

 pelled to exist for whole years outside the body. Others, as the 

 bacilli of glanders, the streptococci of erysipelas, Fraenkel's pneu- 

 monia bacteria, the diphtheria bacilli, etc., lose their virulence very 

 quickly. 



A similar phenomenon may also be occasionally observed in the 

 case of the saprophytic bacteria. Hueppe and his pupils- have 

 shown, for insta,nce, that the sour-milk bacillus and the blue-milk 



