PHAGE TAXONOMY 423 



workers, perhaps most thoroughly by Muckenfuss (1928) who 

 suggested the use of ncutraUzing antisera for the identification 

 and classification of bacteriophages. 



Careful investigation by numerous individuals of the proper- 

 ties of pure strains of bacteriophages indicated that there were 

 great differences from one strain to another even of phages at- 

 tacking a single bacterial strain. However, the properties of 

 any one phage strain were remarkably constant through re- 

 peated subculture and were not subject to capricious alteration. 

 The existence of "adaptive" modifications in the properties of a 

 phage strain were early recognized but these usually involved 

 change in only one property at a time. The possibility of a sys- 

 tematic classification of the phages became evident as soon as 

 large numbers of phage strains attacking a single group of bac- 

 teria were investigated. Several such schemes for phage classi- 

 fication were proposed nearly simultaneously for different groups 

 of phages. 



A classification for the cholera phages was proposed by Ashe- 

 shov, Asheshov, Khan, and Lahiri (1933) based primarily on 

 serological specificity but including also plaque morphology and 

 the resistance patterns of phage-resistant host cells. The coli- 

 dysentery phages were classified into eleven distinct serological 

 groups by Burnet (1933d) who also demonstrated that this group- 

 ing was correlated with plaque morphology, with sensitivity to 

 inactivation by urea and by photodynamic action, and with in- 

 hibition of phage growth by citrate. These same criteria were 

 later applied by Burnet and Lush (1935) to the classification of 

 phages attacking strains of Staphylococcus. Sertic and Boulgakov 

 (1935) classified 75 strains of typhoid phage into 14 antigenic 

 types and correlated these types with plaque morphology, 

 particle size by ultrafiltration, heat resistance, and virulence. 

 The property of virulence was measured by the length of time it 

 took for a phage infected culture of bacteria to lyse under stand- 

 ard conditions and is roughly proportional to the latent period. 

 Delbriick (1946b) classified seven strains of coliphage into four 

 serological groups and demonstrated that this classification was 



