SYLLABUS 139 



ing from a single colony, can form plaques when plated on one of 

 several bacterial strains, the culture is said to be lysogenic. The strains 

 sensitive to the action of the phage carried by the lysogenic bacteria 

 are called indicator strains, and can be used to prepare stocks of this 

 phage. 



When s^^stematic searches of this kind have been carried out, a 

 large proportion of the bacterial strains examined, whether freshly 

 isolated or taken from laboratory collections, were found to be lyso- 

 genic. It is furthermore impossible to ascertain whether a strain which 

 did not give plaques on the indicator strains employed would not have 

 proved to be lysogenic if a suitable indicator strain had been available. 

 Such findings have been made with a number of genera (sporulated 

 soil bacteria, salmonellae, pseudomonas, vibrio, corynebacteriae, sta- 

 phylococci, etc.). Lysogenic strains, in fact, seem to be the main reser- 

 voir of bacteriophages in nature. 



The above definition, although widely accepted, is too ambiguous 

 to permit an understanding of the underlying factors involved. The 

 equilibrium between the development of bacteria and the reproduction 

 of phage can, in effect, be one of two types of phenomena of obviously 

 different significance. On the one hand, it can be a population equili- 

 brium involving external phage and the whole bacterial population. 

 Such strains may be called carrier strains. On the other hand, the 

 equilibrium may exist at the intra-cellular level. The term "true^^ 

 lysogenic strain should be reserved for this second case. 



Carrier Strains. The carrier strains do not represent a phage-host 

 relation of fundamental difference from those discussed extensively in 

 previous sections. Such strains can always be freed from phage, and 

 thus lose their lysogenicity, by methods which inactivate free phage or 

 prevent its adsorption to the host cell. The apparent lysogenicity can 

 be recognized to be due to either one of two principal reasons: 



(1) The bacterial cells are sensitive to the action of the phage, 

 but in a culture, uninfected cells outgrow virus reproduction. This may 

 be due to such factors as poor adsorbability, long latent period, low 

 burst size, etc. 



(2) The great majority of the bacterial cells are resistant to the 

 action of the phage but may adsorb it reversibly, i. e., "carry" it. Mul- 

 tiplication of the phage occurs by lysis of sensitive mutants arising in 

 the culture. 



"True" Lysogenic Strains. True lysogenic strains cannot be freed 



