12G THE BACTERIOPHAGE 



which did not contain masses of bacteria have been included, the actual 

 index is therefore somewhat higher. 9 



The opsonic action of a culture of the bacteriophage manifests 

 itself with such rapidity that it is improbable that the opsonic 

 power can be exercised directly by the ultramicrobes. We have 

 seen, in fact, that the bacteria are parasitized only after an ap- 

 preciable lapse of time, — ten to twenty minutes. 



Experiment XXXV. Leucocyte suspension, Shiga suspension, and 

 anti-Shiga bacteriophage culture are mixed in equal parts. After various 

 periods of incubation drops of the mixture are examined showing: 



TIME INTERVAL 



Immediately after mixing. 



After 1\ minutes 



After 5 minutes 



After 1\ minutes 



After 10 minutes 



After ten minutes some of the leucocytes are so completely filled with 

 bacilli that counting is impossible. The figure given is a minimum based 

 only on leucocytes in which masses of bacteria were not present to interfere 

 with enumeration. 



The opsonic power must be exercised, not by the ultramicrobes, 

 but by the lysin contained in the culture, as the following ex- 

 periment proves. 



Experiment XXXVI. Two milligrams of the alcohol precipitate (of 

 which we have spoken above), still moist, are dissolved in 10 cc. of 0.8 

 per cent saline. A mixture is made of equal parts of this solution, sus- 

 pension of Shiga bacilli, and leucocytic suspension. After 15 minutes at 

 38°C. microscopic examination of stained preparations shows that 100 

 leucocytes have taken up 536 bacilli (as certain leucocytes contain masses 

 rendering a count impossible the figure is a minimum). The index is 14.9. 



The opsonic power of cultures of the bacteriophage is, there- 

 fore, due to the lysin secreted by the ultramicrobes, to the lysin 

 which remains in the culture once the bacteria have been dis- 



9 It will be recognized that the opsonic indices obtained with sera are 

 far below these secured with cultures of the bacteriophage. With the 

 former an index of 2 is exceptional. 



