34 Perspectives in Microbiology 



bacterial nucleus, the intercellular transport of the frag- 

 ment, and its injection into the new host bacterium. 



Insofar as we elect to regard hereditary viruses as part 

 of the genotype, symbiotic infection is also a species of 

 recombination. Lysogenization in particular can be analo- 

 gized to transduction or even fertilization (15), especially 

 since, at least in E. coli K-12, the prophage is incorporated 

 as if it were a typical genetic factor (25). In addition, traits 

 usually attributed to the "bacterium itself," whatever this 

 means, may be dramatically converted by lysogenization 

 per se, as in the toxigenic variation of diphtheria bacilli 

 (5, 17) and the change in lysotype (1) or somatic antigen 

 from group E-1 to E-2 (21, 47) in Salmonella. 



It is less urgent to distinguish whether lysogeny is a 

 transduction, which is mostly semantic manipulation, than 

 to describe the role of the phage particle. This may be con- 

 sidered as a miniature bacterium, with a skin and a "nu- 

 cleus." The phage nucleus itself is the agent of genetic 

 conversion in lysogeny; it behaves rather as if it were a 

 special segment of a bacterial chromosome, but as with any 

 virus we cannot say whether this evolved by gradual para- 

 sitic degeneration or by abrupt mutation. 



In addition to the phage nucleus, the skin may enclose 

 other fragments, the residue of the lysed bacterium. The 

 relationship of these fragments to the prophage in Sal- 

 monella is obscure. The simplest picture is that they are 

 adventitiously included, together with the phage nucleus, 

 during the maturation of the phage particle. However, it 

 has not yet been possible to study the localization of pro- 

 phage in Salmonella by the methods employed for E. coli 

 K-12, and it cannot, therefore, be excluded that the frag- 

 ments transduced by any given phage particle .are related 

 to a less rigidly predetermined reproductive site of the 

 nucleus of that particle. In any event, every genetic factor 

 so far tested in Salmonella is subject to transduction, al- 

 though the quantitative efficiency may vary by as much as 



