424 BACTERIOPHAGES 



correlated with plaque size, morphology in the electron micro- 

 scope, minimum latent period, and rate of adsorption to the host 

 cell. Other classifications for limited groups of phages have 

 been published by McKie (1934) for temperate coli-dysentery 

 phages, by Rountree (1949b) for staphylococcal phages, by 

 Friedman and Cowles (1953) for megaterium phages, by Wil- 

 kowske. Nelson, and Parmelee (1954) for lactic acid streptococcal 

 phages, and by Evans (1934) and Evans and Sockrider (1942) 

 for hemolytic streptococcal phages. The classification of some 

 enteric phages has been described by Adams (1952) and by 

 Adams and Wade (1954, 1955). 



3. Taxonomic Criteria at the Species Level 



As a result of numerous studies with various groups of phages in 

 different laboratories a number of useful taxonomic criteria for 

 bacteriophages have been proposed and tested. These were 

 summarized and discussed by Adams (1953b). Since then cer- 

 tain of these criteria have been further tested with groups of 

 coliphages and we are in a better position to discuss critically 

 their taxonomic utility. 



a. Serological Relationship 



The serological criterion is placed first because it is experi- 

 mentally simple and because very extensive application has 

 shown it to be the most useful single test of phylogenetic relation- 

 ship. The neutralization of the infectivity of one phage strain 

 by the antiserum to a second phage strain indicates a close biolog- 

 ical relationship between the two strains providing certain 

 sources of error are eliminated. The most obvious of these is that 

 the antiserum under test may contain unsuspected antibodies 

 against some unrelated phage, through natural immunization of 

 the animal with phage in its own intestine or through contamina- 

 tion of the immunizing phage with heterologous phage antigen 

 through unclean apparatus, or from a lysogenic host bacterium. 

 To be definitive the serological relationship should be inde- 



