140 BENZER ET AL. 



from phage by means which inactivate free phage or prevent adsorp- 

 tion to the host cell. 



Among strains which comply with this definition and which 

 have been most carefully studied are the strain of E. coli of Lisbonne 

 and Carrere (Bordet and Renaux, 1928) and the B. megatherium 899 

 (Den Dooren de Jong, 1931; Wollman and Wollman, 1938; Lwoff, 

 1950)- 



For the case of B. megatherium 899, the following facts have been 

 found: 



( 1 ) When equal aliquots of a carefully washed suspension of the 

 lysogenic strain are plated for plaque formation on the indicator strain, 

 on the one hand, and for colony count, on the other hand, an equal 

 number of plaques and colonies is found. 



(2) The progeny from each spore (even when heated to temper- 

 atures which destroy free phage) and from each bacterium is a true 

 lysogenic strain. 



(3) A culture remains lysogenic even after serial transfers in 

 medium containing anti-phage serum. 



(4) Even though calcium ions are required for the propagation 

 of the megatherium phage on the sensitive strain, serial transfers of 

 the lysogenic strain in calcium- free medium leave its lysogenic function 

 unimpaired. 



(5) Isolated bacteria cultivated under the microscope can undergo 

 up to 19 divisions without release of phage to the mediimi, and the 

 progeny is still lysogenic. Lysis of isolated bacteria can take place 

 without production of free phage, but appearance of phage in the 

 medium is always preceded by the lysis of a cell. 



(6) If a washed suspension of lysogenic bacteria is artificially 

 lysed by means which are knov^oi not to affect free phage (lysozyme), 

 no phage is released to the medium. 



These findings show that virus reproduction in true lysogenic 

 strains differs conspicuously from that observed with T-phages and 

 their E. coli host, as described in previous sections. Each single bac- 

 terial cell appears to be capable of carrying the potentiality for repro- 

 duction of the virus without further intervention of external phage 

 while at the same time undergoing the processes of assimilation and 

 multiplication. The fact that there may be lysis, either spontaneous or 

 induced, in which no infective particles are released to the medium. 



