154 BACTERIOPHAGES 



kinctically, and it was found lliat j)hagc inaclivalion by this bac- 

 terial extract was remarkal)ly similar to phage neutralization by 

 antiphage serum. The inactivation rate was initially first order 

 and later decreased. The fraction of the phage inactivated was 

 independent of the initial phage concentration. The rate was 

 directly proportional to the PIA concentration and had a tem- 

 perature optimum at about 37 ° C. The phage particles which 

 survived incubation with PIA produced plaques which were 

 smaller than normal and widely variable in size suggesting a de- 

 creased rate of adsorption to the host cell. 



Burnet concluded that PIA is intimately associated with the 

 somatic bacterial antigen, and that this substance blocks phage 

 infection by combining with receptor sites on the phage particle 

 which in the normal course of events make contact with the 

 bacterial surface. 



Further interesting experiments were reported by Burnet and 

 Freeman (1937) using phage H (related to T2) and PIA from a 

 Flexner Y strain of shigella. A variant of phage H was isolated 

 which adsorbed at a slower rate to its host cell. The variant 

 also differed from its parent in a number of other characteris- 

 tics : larger plaque size, higher titer stocks, greater susceptibility 

 to heat inactivation, and more rapid inactivation by antiphage 

 serum. The variant was also extremely resistant to a prepara- 

 tion of PIA which rapidly inactivated the parent phage stock. 

 A similar variant was isolated from stocks of phage CI 6. 



Successive interaction of antiphage serum and PIA with phage 

 H was also examined. Phage which had been treated with 

 dilute antiphage serum to a survival of 50 per cent proved to be a 

 great deal more resistant to inactivation by PIA than was un- 

 treated phage. In contrast treatment of phage with PIA had no 

 effect on its subsequent neutralization with antiphage serum. 

 These facts suggest that combination of phage with PIA is dif- 

 ferent from combination of phage with antiphage serum or with 

 host cells. 



Phage H which had been inactivated to better than 95 per 

 cent by PIA was still able to adsorb to the host cell as judged by 



