Introduction xix 



ance of any mature progeny, Luria and Latarjet (113) irradiated phage- 

 infected bacteria with UV at various stages of intracellular phage growth. 

 They reasoned that if the UV sensitivity of the vegetative phage is equal to 

 that of the free, extracellular virus, then the result of this irradiation experi- 

 ment ought to be a family of multiple-hit survival curves from which the instan- 

 taneous number of vegetative phages present at the time of irradiation could 

 be inferred. The outcome of Luria and Latarjet's experiment was contrary to 

 their expectation, however; instead of the anticipated multiple-hit survival 

 curves, a family of straight lines of ever decreasing slope was observed, indica- 

 tive of the fact that the intrinsic UV sensitivity of the vegetative phage is much 

 less than that of the free virus. This is also evident from Benzer's (11) im- 

 proved experimental design of the Luria-Latarjet technique, presented here as 

 Benzer's third paper. The meaning of the great reduction in UV sensitivity 

 of the vegetative phage has not yet found an entirely satisfactory explanation. 

 On the one hand, as is evident from Benzer's report, some phages do not mani- 

 fest this effect, so that UV irradiation of bacteria infected with such phages 

 actually gives rise to the family of multiple-hit curves anticipated by Luria 

 and Latarjet. On the other hand, the vegetative phage is also much more 

 resistant to inactivation by decay of incorporated P'^'- atoms than the extra- 

 cellular P^'-labeled virus (138). It seems likely, however, that the reduction in 

 radiosensitivity of the vegetative phage reflects some important aspect of the 

 function and replication of the viral DNA, and some of the possible interpre- 

 tations have been discussed in several reviews (94, 136, 139, 142). In any 

 caise, the method of Luria and Latarjet has found a number of valuable 

 applications in the study of intracellular virus growth, not only with bacterio- 

 phages but also with plant and animal viruses (134, 135, 51, 128). 



Within a few years of the discovery of the bacteriophage, lysogenic 

 bacterial strains were found which appear to "carry" bacteriophages, in the 

 sense that phage particles are always present in the culture fluid of such 

 strains (21, 60). It was soon realized that this association of phage and 

 bacteria cannot be of a casual nature, since it is impossible to permanently 

 free lysogenic strains from the phage they carry by methods which ought to 

 kill or remove the virus particles, such as heating, anti-phage serum neutraliza- 

 tion, or single-colony purification (8, 18, 121). The nature and significance of 

 lysogeny then remained a subject of intense controversy for about 30 years, 

 some workers denying the existence of "true" lysogeny and others claiming 

 that lysogeny disproves the whole notion that bacteriophagy involves an infec- 

 tion of bacteria by virus particles. Nevertheless, a few bacteriologists, such as 

 Burnet and McKie (35), and the elder Wollman (156), already envisaged 

 that lysogeny represents an innate capacity of bacterial cells for phage produc- 

 tion. In order to establish firmly some of the basic but controversial facts of 

 lysogeny, Lwoff began a study of this phenomenon after the Second World 

 War and in 1950 published the paper (116) presented here. In this work 

 Lwoff and Gutmann demonstrate unequivocally that each bacterium of a lyso- 



