80 THE BACTERIOPHAGE AND ITS BEHAVIOR 



10~^ may be covered by a layer of culture eroded by confluent plaques. 

 The agar slant prepared from the 10~^ dilution will present a layer of 

 staphylococcus growth spotted by about 100 plaques, and the tube 

 corresponding to 10~^ will give a culture where only about 8 plaques can 

 be found. 



The sole difference appearing in the results of these experiments is 

 that the different bacteriophage filtrates each act up to a different dilu- 

 tion. Aside from this difference in "strength" the aspect of the phe- 

 nomenon is always the same. A further fact revealed by these experi- 

 ments is, and this is of extreme importance, that the number of plaques 

 formed bears, in every experiment, a strict relationship to the quantity 

 of bacteriophage liquid. When there are, for example, 20 plaques on 

 the agar slant corresponding to a given dilution there will be, practically, 

 one-tenth as many plaques on the agar tube corresponding to the dilu- 

 tion which is ten times greater. 



As we know that the bacteriophage principle reproduces in the course 

 of its action, since the phenomenon is indefinitely reproducible in series, 

 let us take a suspension of dysentery bacilli and inoculate it with an 

 extremely minute quantity of the bacteriophage liquid, for example, 

 with 1 cc. of a 10"'^ dilution of a bacteriophage filtrate. Place this 

 bacterium-bacteriophage mixture in the incubator at 37°C. and from 

 hour to hour throughout the incubation spread one drop upon an agar 

 slant or an agar plate. As the bacteriophage begins to multiply in the 

 course of its action the plaques should increase in number, and in fact, 

 after these agar subcultures have been incubated for 24 hours, we will 

 find the following: 



The agar tube planted from the mixture, after incubation for but one 

 hour, shows no plaques; it is covered by a normal growth of dysentery 

 bacilh. The agar tube prepared one hour later, that is to say, after the 

 trace of bacteriophage has been in contact with the bacilli for two hours, 

 will show about a dozen plaques. The agar tube seeded after the action 

 has progressed for 3 hours is covered by a bacillary growth through 

 which about 100 plaques are scattered. The appearance of this agar 

 tube is then approximately the same as that which had received a drop 

 of the 10~^ dilution of the preceding experiment. But in the first 

 experiment the suspension was spread upon agar immediately after the 

 inoculation of 10~^ cc. of bacteriophage liquid, while here the 10~'^ 

 cc. of hquid bacteriophage added to the suspension has acted during 3 

 hours and has, therefore, had time to multiply. 



The agar tube planted after the suspension had remained for 4 hours 



