LYSOGENY 321 



original cell. It is, however, perpetuated, not in a form similar to that of the 

 infecting particle, but in a noninfectious form, the 'prophage, which is inte- 

 grated with the genetic apparatus of the host cell. 



II. Historical Development of Lysogeny 

 A. Discovery and Criteria of Lysogeny 



Soon after the discovery of bacteriophage by Twort and d'Herelle, it was 

 observed by several workers, in particular by Bordet and Ciuca (1921) and 

 by Gildemeister (1921), that strains could be isolated from natural sources 

 which would produce bacteriophages active on bacteria of other strains. 

 It was later realized that such phage-producing strains could correspond to 

 one or the other of two different situations. In one case, the equilibrium 

 between bacteria and phage is at the population level. In such carrier 

 strains, phage production can be ultimately ascribed to a population equili- 

 brium between resistant and susceptible ceUs, the latter being constantly 

 reinfected by external phage particles. Bacteriophage-free cultures can be 

 easily obtained by serial colony isolations or by treatments which will in- 

 activate free phage or prevent its adsorption onto the host cells. 



Completely different is the case of the lysogenic strains, in which the asso- 

 ciation between the phage and the bacterium is at the intrabacterial level. 

 As shown by Bail (1925) and by Bordet (1925), after numerous isolations of 

 single colonies, each colony will stiU give rise to a phage-containing culture. 

 When lysogenic bacteria are grown under conditions which ])revent reinfec- 

 tion with external phage particles, as for example, in a medium containing 

 specific antiphage serum (MacKinley, 1925), each bacterium is able to give 

 rise to a lysogenic clone. It was thus clear that the ability of lysogenic 

 bacteria to produce phage is independent of the presence of free phage in the 

 surrounding medium and is a property of each single ceU of the culture. 



Another important finding of Bail (1925) and of Bordet (1925) was that 

 when sensitive bacteria are exposed to the phage produced by lysogenic 

 strains, some of the bacteria survive and give rise to lysogenic clones. More- 

 over, the phage produced by such "artificial" lysogenic strains is in all 

 respects identical with the phage used for the initial infection. 



B. Concept of Prophage 



Each bacterium of a lysogenic strain, whether isolated from nature or 



"artificially" prepared by lysogenization, is therefore able to perpetuate the 



ability to produce phage. This ability appears, as written by Bordet in 1925, 



to be "inscribed in the heredity texture" of the bacterium. But under what 



vol. II — 21 



