M. DELBRUCK 651 



3. PIB between 1 and 200 



For these intermediate cases observations on agar plates showed not an 

 intermediate type of lysis but a gradual shift of the fractions of bacteria 

 that are lysed by fading or by swelling respectively. Table II illustrates 

 this point. 



There is no ambiguity regarding the type of lysis a particular bacterium 

 has undergone, except in rare cases when the fading has proceeded too far 

 by the time of the next inspection, so that the form cannot be ascertained 

 any more. 



Bronfenbrenner, Muckenfuss, and Hetler in 1927 (12) and Bayne-Jones and Sand- 

 holzer in 1933 (13) have published very interesting photomicrographic moving pictures 

 of lysing bacteria. They describe essentially the same morphological types of lysis 

 which we find. In their experiments, however, the conditions of infection were not 

 systematically varied and the ratio phage/bacteria was in no case determined. The 

 significance of variations in the lytic process was therefore not recognized. 



One Step Growth Curves 



Phage growth curves with these strains of phage and bacteria show the 

 same general features as those described by Ellis and Delbriick (1) for the 

 strains Bi and Pi. If phage is added at time zero to an excess of bacteria 

 the plaque count stays constant for 17 minutes, then rises, at first sharply 

 and then more gently (on the logarithmic plot!) until about 30 minutes. 

 At that time the first step is nearly completed. If the growth mixture has 

 not been diluted before the beginning of this first rise another sharp rise 

 begins at about 34 minutes (Fig. 3). If reinfection has been prevented by 

 extreme dilution there is very little further increase in plaque count (see 

 Fig. 2 of the preceding paper). 



It was decided to study in more detail this first step. The condition of 

 extreme dilution under which the one step growth curve has to be measured 

 facilitates an accurate analysis, because the samples which have to be 

 plated at definite intervals need only be mixed with bacteria and plated, 

 without further dilution. The time to which the assay is to be referred can 

 then be defined within a fraction of a minute. 



Fig. 4 shows the values obtained from three such growth curves. It 

 should be noted that since the plaque count is plotted directly (instead of 

 logarithmically as in most plots of this kind) the sampling error (which is 

 proportional to the measured value) is more conspicuous near the upper 

 end of the growth curve. The actual percentage deviations are experi- 

 mentally larger near the beginning of the rise, because here the plaque 



65 



