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 would 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- 



