322 F. JACOB AND E. L. WOLLMAN 



form is the ability to produce phage perpetuated in lysogenic bacteria? It 

 was shown by den Dooren de Jong (1931) that treatments which would 

 inactivate free phage of lysogenic Bacillus megatherium, such as heating at 

 100°C. for 20 minutes, would leave unimpaired the lysogenic character of 

 cultures obtained from spores which had been submitted to the same treat- 

 ment. Moreover, when lysogenic bacteria are disrupted by various means, 

 such as lysis with unrelated phages (Burnet and McKie, 1929) or lysozyme 

 (WoUman and AVollman, 1936; Gratia, 1936), no infectious particles can be 

 found. Phage, therefore, is not perpetuated in lysogenic bacteria in the form 

 of infectious particles, but in a noninfectious form. 



An important question raised by the existence of lysogenic bacteria is that 

 of the mechanism by which infectious phage particles are produced from the 

 noninfectious "anlage" (Burnet and McKie, 1929) that they perpetuate. 



It is generally observed that in growing cultures of lysogenic bacteria, the 

 bacteriophage titer runs parallel with the number of bacteria of the culture. 

 Phage production could result equally from a continuous secretion in the 

 course of bacterial growth or from the discontinuous lysis of a small but 

 constant fraction of the bacterial population. The first hypothesis was 

 generally favored — in particular, by Northrop (1939), who, by compar- 

 ing the kinetics of the production of enzymes and phage, concluded that 

 phage, Hke enzymes, was secreted by lysogenic bacteria in the course of their 

 multiplication. 



After an eclipse period of about ten years, these problems were reinvesti- 

 gated by Lwoff. Multiplication of lysogenic B. megatherium and the appear- 

 ance of bacteriophage were followed, not in mass cultures, but under the 

 microscope, in pedigrees of bacteria isolated with a micromanipulator. It 

 was thus observed by Lwoff and Gutmann (1950) that lysogenic B. mega- 

 therium may undergo up to 19 divisions without any release of phage into 

 the surrounding medium. Thus, multipHcation of lysogenic bacteria can 

 proceed without phage production. In other clones, however, by its sudden 

 disappearance, the lysis of a cell could occasionally be observed. Then, and 

 then only, the presence of phage (about a hundred particles per lysed cell) 

 was detected in the microdrop. Therefore, the production of phage by lyso- 

 genic bacteria is a discontinuous process; it is the consequence of the lysis 

 of a small fraction of the lysogenic population. In the case of lysogenic bac- 

 teria, as in the case of sensitive bacteria infected with phage, the production 

 of infectious particles is a lethal process, each producing cell Uberating a 

 fuU burst of infectious particles. Lysogenic bacteria can survive and grow 

 only when they do not produce phage. Their capacity of producing phage is, 

 and can only be, a potential character. To the noninfectious form of bacterio- 

 phage which is the bearer of this potentiahty and which can develop eventu- 

 ally into phage, Lwoff and Gutmann have given the name of prophage. The 



