PHAGE TAXONOMY 425 



pendent of the host bacterium on which the immunizing phage is 

 grown. Also the cross-reacting antibody should be removed 

 from the serum by absorption with homologous phage at the 

 same rate as is removal of homologous antibody. 



In contrast to the results of a positive reaction, the failure of 

 an antiphage serum to neutralize a heterologous phage does not 

 preclude a close relationship. In the case of two phages for 

 Serratia marcescens it was not possible to detect directly a serolog- 

 ical relationship, yet each of the two phages was clearly related 

 serologically to other phages of the same group (Adams and 

 Wade, 1954). This serological group which includes phages 

 T3, T7, and D44 is markedly diverse antigenically, suggesting 

 that antigenic specificity in bacteriophages is subject to modifica- 

 tion by mutation. Genetic control of serological specificity is 

 also suggested by hybridization experiments in the T5 species 

 (Fodor and Adams, 1955) and in the T2 species (Streisinger, 

 1956b). There is no evidence so far suggesting large scale 

 changes in serological specificity as a result of single gene muta- 

 tions. One may conclude that serological relatedness has in- 

 variably been correlated with close relationship in other charac- 

 teristics as well. Absence of serological relationship, however, 

 does not preclude close relationship in other characteristics. 



The structural diflferentiation of phage particles into head and 

 tail portions is correlated with antigenic diflferentiation into 

 serologically unrelated head antigens and tail antigens. The 

 anti-tail antibodies are involved in neutralization, whereas the 

 anti-head antibodies are detected by agglutination and comple- 

 ment fixation reactions. Serological cross reactions involving 

 the head antigens have been demonstrated with the related 

 phages T2, T4, and T6 by Lanni and Lanni (1953), and with the 

 related phage pair T5 and PB by Fodor and Adams (1955). 

 Rountree (1952) has reported cross reactions by the complement 

 fixation test among staphylococcal phages which failed to cross 

 react in the neutralization test. These experiments suggest the 

 possible taxonomic value of the phage head antigens but not 

 enough work has been done as yet to warrant conclusions. The 



