ADSORPTION OF PHAGE TO HOST CELL 153 



activity was dependent in many cases on the intact structure of 

 the bacterial antigen, and could be destroyed by relatively mild 

 reagents. 



Apparently the first demonstration of the inactivation of 

 bacteriophages by bacterial extracts was made by Levine and 

 Frisch (1934). Saline extracts of salmonella and shigella 

 species were found to protect the homologous organisms from 

 attack by phage. The active substance could be precipitated 

 by alcohol and partially purified . These results were confirmed 

 and extended by Burnet (1934b) who referred to the active sub- 

 stance as "phage-inhibiting agent" (PIA). The PIA was pre- 

 pared by autolyzing the bacteria for two days at 55 ° C, after 

 which bacterial debris was removed by centrifugation and the 

 solution filtered through gradacol membranes of one micron 

 pore size. Filtration through Seitz or Berkefeld filters resulted 

 in loss of activity. The activity was measured by incubating 

 appropriate dilutions of PIA with diluted phage preparations and 

 determining the surviving phage by plaque count assays. Ex- 

 tracts were prepared from five variant strains of Flexner Y dysen- 

 tery bacilli, and tested for inhibiting activity against eight differ- 

 ent phages. In general it was found that phages attacking a 

 given bacterial strain were inhibited by extracts from that strain, 

 while these extracts had no effect on phages to which the bac- 

 terial strain was resistant. Occasional extracts that failed to 

 inactivate the expected phages are not surprising in view of the 

 lability of many receptor substances. 



The active extracts invariably gave precipitates when mixed 

 with homologous antibacterial serum, the amount of precipitate 

 being proportional to the phage inhibiting activity. After re- 

 moval of the specific precipitate, the supernatant fluid was devoid 

 of phage-inhibiting activity. The neutralization of PIA by anti- 

 bacterial sera is parallel to the agglutination by these sera of the 

 bacteria from which the extracts were prepared. An antiserum 

 which agglutinated several bacterial strains also neutrahzed the 

 PIA extracted from these strains. 



The action of the most potent preparation of PIA was studied 



