ANTIGENIC PROPERTIES 103 



or to aggregation of phage particles. Frequency distribution 

 curves for plaque sizes of phage CI 6 before and after treatment 

 with antiserum are given by Burnet, Keogh, and Lush (1937). 

 The distribution for the incompletely neutrahzed preparation 

 was bimodal with one maximum at less than Vg the peak plaque 

 diameter of the unimodal curve for normal phage particles. 

 Surprisingly, Delbriick (1945b) failed to find any difference 

 between normal T2 phage and serum-survivors at the 1 per 

 cent level with respect to adsorption, latent period, or burst 

 size (cf. Adams and Wassermann, 1956). The reason for the 

 discrepant observations wdth these closely related phages is 

 not obvious. 



Gough and Burnet (1934) extracted from suspensions of the 

 host bacterium a phage-inhibiting agent (PIA) which was able 

 to inactivate phage particles in m.uch the same manner as anti- 

 phage sera. They regarded the extract as a solution of the 

 bacterial receptors for phage adsorption. Burnet and Freeman 

 (1937) investigated the effect of PIA on phage preparations 

 which had been inactivated to the extent of 90 per cent with 

 dilute antiphage sera. The survivors of serum inactivation 

 proved to be very resistant to the inhibiting action of PIA 

 whereas the untreated phage population was 99 per cent in- 

 activated by PIA. This is further evidence that phage anti- 

 body can modify the properties of phage particles without in- 

 activating them. It also suggests that the isolated PIA is not 

 identical to the receptor substance on the bacterial surface, 

 since the survivors of the serum action are still able to infect the 

 host cell although they are resistant to the action of PIA. 



Another very striking property of partially neutralized phage 

 preparations was demonstrated by Hershey, Kalmanson, and 

 Bronfenbrenner (1944) using a variant of phage T2. The var- 

 iant adsorbed very poorly to bacteria in a salt-free medium 

 and had a relative efficiency of plating of 10~^. In a medium 

 containing 0.1 N' NaCl the phage adsorbed well and the effi- 

 ciency of plating was about half of that under optimal condi- 

 tions. The phage was treated with very dilute antiphage 



