422 BACTERIOPHAGES 



The discovery of the phenomenon of transduction (Zinder and 

 Lederberg, 1952) furnished a means for testing the genetic com- 

 patibiHty of strains within the salmonella group. It now seems 

 probable that the 150 "species" really should be considered as 

 150 serological or biochemical varieties within a single salmonella 

 species. 



The next higher category, the genus, should include a group of 

 species of presumably common phylogenetic origin, separated by 

 a decided gap from other similar groups (Mayr, Lindsley, and 

 Usinger, 1953). The genus is a category more difficult to define 

 than the species and is a more artificial concept. 



In the Linnaean classification each organism has a two-word 

 name, the first specifying the genus and the second the species to 

 which the organism has been assigned. Such a binomial nomen- 

 clature has been appHed to bacteriophages by Holmes (Bergey's 

 Manual, cited above), who lists 46 species of phage within the 

 genus Phagus. The use of such a nomenclature suggests that 

 adequate criteria are available to characterize the species cate- 

 gory and the genus category. Such criteria are not available 

 and furthermore it would be virtually impossible to allocate 

 a newly isolated bacteriophage to one of these species on the 

 basis of the published characteristics. Identification of a bac- 

 teriophage would require direct comparison with type cultures, 

 yet for many of the species listed by Holmes such type cultures 

 do not exist. At the present stage of development neither the 

 classification nor the nomenclature of Holmes is of any value. 



2. Various Proposals for Phage Classification 



D'Herelle (1926) proposed that there was only one species of 

 bacteriophage, Protobios bacteriophagus, with extreme powers of 

 adaptive variation. He felt that antigenic specificity and resist- 

 ance to destructive agents could vary as the phage became 

 adapted to growth in different hosts. Yet Bruynoghe and Appel- 

 mans (1922) had previously shown that two strains of typhoid 

 phage were antigenically distinct from each other. The anti- 

 genic specificity of phages was repeatedly confirmed by various 



