368 



GROWTH OF BACTERIOPHAGE 



This method therefore, does not give the number of phage particles but the 

 number of loci within the solution at which one or more phage particles exist. 

 These loci will hereafter be called "infective centers." The linear relationship 

 between phage concentration and plaque count (Fig. 1) does not prove that the 

 number of plaques is equal to the number of infective centers, but only that it is 

 proportional to this number. We shall call the fraction of infective centers which 



2 4 6 18 32 64 128 



Eelative concentration of phage 



256 



Fig. 1. Proportionality of the phage concentration to the plaque count. 

 Successive twofold dilutions of a phage preparation were plated in duplicate 

 on nutrient agar; 0.1 cc. on each plate. The plaque counts from two such series 

 of dilutions are plotted against the relative phage concentration, both on a loga- 

 rithmic scale. 



produces plaques the "efficiency of plating." With the concentrations of phage 

 and bacteria which we have used this coefficient is essentially the fraction of 

 infected bacteria in the suspension spread on the plate, which goes through to lysis 

 under our cultural conditions on the agar medium. After plating, the phage 

 particles released by this lysis infect the surrounding bacteria, increasing only the 

 size, and not the number of plaques. 



40 



