70 THE BACTERIOPHAGE 



A suspension of B. typhosus containing 250,000,000 bacilli per cubic centi- 

 meter is prepared and distributed into four sterile tubes, 10 cc. to the tube. 

 Each of these tubes is then inoculated with 0.0001 cc. of a filtrate; the first 

 with a filtrate prepared on the first day, tube 2 with that prepared on the 

 second day, etc. After shaking, 0. 1 cc. is taken from each tube and plated, 

 as usual, on an agar plate, taking care that the agar layer in all the plates 

 is of the same depth simply to make all the conditions comparable. After 

 incubation the following results are obtained : 



1. The filtrate prepared from the stool taken on the first day of the 

 appearance of the bacteriophage shows 16 very small plaques, pin-point 

 in size. 



2. The filtrate derived from the stool of the second day after the 

 appearance of the bacteriophage shows 31 plaques, having a diameter 

 of less than 1 mm. each. 



3. The filtrate made from the stool of the third day gives 52 plaques, 

 with diameters of about 2 mm. 



4. The filtrate prepared from the stool of the fourteenth day shows 

 42 plaques, each with a diameter of less than 1 mm. 



Strain 3 is by far the most virulent, a conclusion that is supported by 

 the fact that in its isolation it induced a total lysis of the bouillon culture 

 of the typhoid bacillus. Strains 2 and 4 are much less virulent. Only by 

 a dozen passages was it possible to effect an enhancement in virulence 

 sufficient to give the same result. And at that time, when plated on agar 

 the plaques had a diameter of 2 mm. 



These experiments demonstrate very well that with equal 

 virulence the plaques on the surface of a medium are approxi- 

 mately equal in size. The process of measuring virulence by 

 counting the plaques and thus determining the rate of multiplica- 

 tion is certainly more exact than is observation of lysis in a fluid 

 medium. It is, unfortunately, too complicated to be applied in 

 routine practice when a large number of strains must be examined, 

 as is the case when working with patients. 



RESISTANCE OF THE BACTERIUM 



In the first of the two experiments just cited (Experiment XV, 

 A) we have seen that the bacterium was successful in developing 

 in spite of the presence of the bacteriophage. The virulence of 

 the bacteriophage, then, although constituting a most important 

 factor in the phenomenon is not the only consideration. The 

 bacterium is capable of resistance. 



