478 BACTERIOPHAGES 



of infection can be calculated by use of the Poisson formula. 

 With high multiplicities of infection essentially all the bacteria 

 are infected, and the number of infected bacteria as determined 

 by plaque count are the same as the bacterial assay before 

 infection. 



If it is desirable to avoid use of .serum in the one-step growth 

 experiment, e.g., in a nutrition study, the adsorption tube can 

 be diluted 1 :100 in the growth medium to stop adsorption and 

 F.G.T. and S.G.T. prepared from this dilution. In this in- 

 stance the number of infected bacteria can be determined by a 

 separate dilution into antiserum followed by plaque assay. 

 This experiment differs from the foregoing in that all plates 

 from F.G.T. and S.G.T. will have some plaques owing to input 

 phage which never became adsorbed, in addition to the plaques 

 caused by infected bacteria. That is, the plates made during 

 the latent period will record total infective centers, or free phage 

 plus infected bacteria. In most instances it is not possible to 

 distinguish on the plate the plaques caused by infected bacteria 

 from the plaques caused by free phage. However, the plates 

 made from dilutions of the serum tube will give the number of 

 infected bacteria, and the difference between the total infective 

 centers and the infected bacteria will give the free phage. 



The free phage can also be determined directly by centi^f- 

 ugation of a sample of the dilution tube, followed by assay of 

 the supernatant fluid for free phage (described on p. 467). In 

 the case of multiplicities of infection of 0.1 or less, the sum of the 

 free phage and the infected bacteria, i.e., total infective centers, 

 will equal the phage input, since the proportion of multiply 

 infected bacteria is small. However, with higher multiplicities 

 of infection, the proportion of multiply infected bacteria in- 

 creases and the total of infective centers becomes considerably 

 less than the phage input. 



This point is illustrated in the following example in which 

 the results of plaque counts are recalculated in terms of con- 

 centrations in the adsorption tube. 



