36 BACTERIA 



the other hand, and in conditions where organic matter is 

 greatly predominating, we may expect to see further light 

 on the subject of antagonism. 



Attenuation of Virulence or Function. It was pointed out 

 by some of the pioneer bacteriologists that the function of 

 bacteria suffered under certain circumstances a marked 

 diminution in power. Later workers found that such a 

 change might be artificially produced. Pasteur introduced 

 the first method, which was the simple one of allowing 

 cultures to grow old before sub-culturing. Obviously a pure 

 culture cannot last for ever. To maintain the species in 

 characteristic condition it is necessary frequently to sub- 

 culture upon fresh media. If this simple operation be 

 postponed as long as possible consistent with vitality, and 

 then performed, it will be found that the sub-culture is 

 attenuated, i. e., weakened. Another mode is to raise the 

 pure culture to a temperature approaching its thermal death 

 point. A third way of securing the same end is to place it 

 under disadvantageous external circumstances, for example 

 a too alkaline or too acid medium. A fourth, but rarely 

 necessary, method is to pass it through the tissues of an 

 insusceptible animal. Thus we see that, whilst the favour- 

 able conditions which we have considered afford full scope 

 for the growth and performance of functions of bacteria, 

 we are able by a partial withdrawal of these, short of that 

 ending fatally, to modify the character and strength of 

 bacteria. In future chapters we shall have opportunity of 

 observing what can be done in this direction. 



