270 G. S. STENT 



exclusion of the superinfecting DNA from transfer or genetic contribution) 

 can be prevented by conditions known to inhibit the action of this enzyme 

 (French et at., 1952). It is not unlikely that superinfection breakdown is 

 related to the general increase in DNAase activity which occurs in T-even 

 infected bacteria (Pardee and Williams, 1952, 1953) and which represents the 

 phage-induced destruction of a bacterial ribonucleic acid inhibitory to the 

 action of an endogenous bacterial DNAase (Kozloff, 1953). The significance 

 and possible role of superinfection breakdown and increased DNAase activity 

 for vegetative bacteriophage multiphcation still remain to be elucidated, 

 however. 



VI. Comparative Bacterial Virology 



Before closing this account of the biological and chemical sequence of phage 

 multiplication, it must once more be recalled to the reader that practically 

 all of the experimental evidence cited in the preceding sections has concerned 

 only one restricted, and from many points of view, rather peculiar group of 

 bacterial viruses, the T-even strains active on E. coli. Surely, it is presumptuous 

 to have spoken so frequently of "the bacteriophage" when, in fact, only the 

 T-even group could have been properly meant, and when the applicabihty to 

 bacterial viruses in general of this or that inference may be either unsupported 

 by any direct evidence or, worse yet, even be contradicted by experience. The 

 fact is, however, that with no other type of bacteriophages has there been 

 carried out an amount of chemical and biological work comparable to the effort 

 already expended on the T-even strains. Any attempt to present an integral 

 view of phage multiplication must, therefore either be bUthely one-sided in 

 favor of the T-even strains or else be saturninely interspersed with obscuran- 

 tist reminders of the existence of other less-known systems. Nevertheless, we 

 shaU glean the ground of the preceding sections for a brief appreciation of 

 similarities and differences between the T-even strains and those other phage 

 types on which some relevant information is available. 



First of aU, the essential features of the one-step growth curve appear to be 

 the same for aU bacteriophages which have been examined from this point of 

 view, i.e., a latent period of constant titer, followed by hberation of a burst of 

 progeny particles after lysis of the host cell. Likewise, the eclipse phenomenon 

 has been observed with all those bacterial viruses where it has been sought. 

 The total disappearance of infectivity from an infected cell at early stages of 

 the latent period, furthermore, is also encountered with those animal viruses 

 for which techniques of study comparable to those of bacteriophage experi- 

 mentation are available (Dulbecco, 1955). In fact, the existence of a non- 

 infective eclipse phase during their life cycle is perhaps that attribute of 

 viruses which can serve most readily to distinguish them from other obligate 

 intracellular parasites (Luria, 1950). 



