38 THE BACTERIOPHAGE 



In this experiment the lysis was almost entirely accomplished 

 within a space of fifteen minutes, that is, during the period of 

 time between two and three-quarters and three hours after the 

 inoculation. 



A single ultramicroscopic bacteriophage is, therefore, adequate 

 to provoke lysis. If successive dilutions of a culture of the bac- 

 teriophage are prepared, one drop of the culture into a tube of 

 sterile bouillon, one drop of this first dilution into a second tube, 

 a drop of the second into a third, and so on, and if into each of 

 these dilutions a fixed quantity of a concentrated Shiga culture 

 is introduced, lysis is secured in all tubes which have received at 

 \ least one ultramicrobe. This is usually the first four tubes of the 



series. The remaining tubes will show a normal growth of the 

 Shiga bacillus. Since we have been able to make counts of the 

 bacteriophage and recognize the rapidity with which even a single 

 ultramicrobe can proliferate and bring about lysis, these observa- 

 tions are self-explanatory. Without this means of investigation 

 one woulpl be liable to commit a serious error and to conclude 

 that the sterile bouillon of the first four tubes had contained a 

 culture of the bacteriophage other than that introduced in pre- 

 paring the dilutions. Incidentally, this error has been committed 

 by certain authors. In reality, while there has been a dilution, 

 the diluted culture was active just so long as there was to be 

 found a single ultramicrobe. 



Recognizing the number of ultramicrobes in a lysed suspension, 

 which has, in effect, become a culture of the bacteriophage, and 

 the value of the dilution, it can be mathematically determined 

 whether a bacterial suspension inoculated with such a dilution 

 will undergo lysis or not. This test has been performed by ex- 

 periment more than a hundred times with very diverse strains 

 of the bacteriophage. 



THE BACTERIOPHAGE: AN OBLIGATORY PARASITE 



Whatever may be the medium employed, in the absence of a 

 bacterium for which the bacteriophage is active, multiplication 

 of the ultramicrobes never takes place. And this remains true 

 even if inoculated into a medium containing, instead of living 

 bacteria, organisms that have been killed by any procedure what- 



