INTRODUCTION V 



of the phage. D'Herelle (1930) referred to this phenomenon as 

 "symbiose bacterie-bacteriophage," a name that reflects his 

 interpretation of lysogeny. He assumed that the bacterial 

 population was composed of individuals covering a considerable 

 spectrum of susceptibility to phage action, while the phage 

 population covered a considerable range of virulence. The most 

 virulent phage particles would multiply at the expense of the 

 most susceptible bacteria and in this way bacteria and phage 

 particles would be perpetuated in mixed culture. D'Herelle re- 

 fused to recognize any other explanation for lysogenic strains of 

 bacteria. An essentially similar idea was proposed by Del- 

 bruck (1946b) for a phenomenon he called "pseudolysogenesis," 

 in which the contaminating phage reproduced at the expense of 

 phage-susceptible bacteria produced by mutation during growth 

 of a phage-resistant culture. Pseudolysogenic bacterial strains 

 have been termed "carrier strains" by Lwoff (1953). 



In contrast to pseudolysogenic or carrier strains are those 

 bacterial strains that exhibit true lysogeny. In such strains the 

 prophage multiplies as an integral part of the genetic apparatus 

 of each bacterial cell. During bacterial growth the conversion 

 of prophage to vegetative phage occurs spontaneously with a 

 small but characteristic frequency, for example in one cell per 

 million cell generations, as evidenced by the lysis of an occasional 

 cell to liberate infective phage particles. In some but not all 

 lysogenic cultures the natural frequency of productive lysis may 

 be greatly increased by exposure to ultraviolet light, X-rays, 

 nitrogen mustard, peroxides, and certain other agents. The 

 biochemical mechanism of such "induction" is unknown. 



It has been repeatedly demonstrated that each bacterial cell 

 in a lysogenic culture is capable of transmitting the potentiality 

 of phage production to its progeny in the absence of contact with 

 exogenous phage. With inducible strains of bacteria, for in- 

 stance, it is possible with suitable doses of ultraviolet light to 

 cause at least 90 per cent of the cells to lyse and liberate viable 

 phage particles (LwofF, Siminovitch, and Kjeldgaard, 1950). 

 In the classic experiment of den Dooren de Jong (1936), spores 



