RESISTANCE OF BACTERIA 191 



first flask is distributed into 20 tubes, 10 cc. to each. In all of these 

 dissolution takes place normally, being permanent in 19, showing a 

 secondary culture in 1 . The second flask is portioned out the next day, 

 that is, after dissolution is completed, 10 cc. being placed in each of 

 20 tubes. None of these become turbid. When this second part of 

 the experiment is repeated, 18 remain clear, and 2 tubes yield secondary 

 cultures. 



Each flask of suspension contained 50,000 milhon bacilh, and the 

 above experiments show that of this number but one or two were capable 

 of acquiring an immunity to the very active bacteriophage. It is these 

 "immune" bacilli which give rise to organisms that enjoy the same 

 degree of resistance. 



Secondary cultures, then, have their origin in the operation of the 

 phenomenon of natural selection, whereby some bacilli show a greater 

 aptitude than others to the acquisition of a resistance to the bac- 

 teriophage. 



The phenomenon of secondary culture formation is governed by the 

 individual properties of the bacterium and bacteriophage. Against a 

 single strain of bacterium the less virulent the bacteriophage the greater 

 will be the proportion of secondary cultures, or, in other words, the 

 greater is the number of bacilli in the suspension capable of acquiring a 

 resistance. 



Gratia^^" has suggested that resistance to the action of the bacterio- 

 phage may not consist in the acquisition of resistance by the bacterium 

 but in a selection of those bacteria naturally endowed with this property 

 prior to the action of the bacteriophage. That a selection takes place 

 is precisely what I had shown previously^^^ by means of an experiment 

 which has been presented in this present section. But it operates not 

 through a selection of the bacteria naturally endowed with a resistance, 

 but through a selection of those susceptible bacteria which are the more 

 apt at acquiring resistance. This experiment shows definitely that 

 what takes place is really an acquisition, in the strictest sense of the 

 word, of resistance to the action of the bacteriophage. This is empha- 

 sized by the fact that a resistant bacterium gradually loses resistance in 

 the absence of virulent bacteriophage corpuscles. If resistance can be 

 lost, obviously it is only because it has been acquired. We will consider 

 further this phenomenon in a later section. 



3. VARIABILITY IN ACQUIRED RESISTANCE 



Appelmans and Wagemans^*' have published some experiments 

 designed to show that bacteria may become resistant to one race of the 



