328 THE BACTERIOPHAaH 



Because of these difficulties in accepting the virus hypothesis, various alterna - 

 tive theories have been propounded. 



Kabeshima (1920) suggested that the phage was a catalyst, activating a pro-ferment 

 present in the bacteria themselves. On this view the liberation of the ferment in an active 

 form would, of course, have to be regarded as an essential consequence of the lysis of the 

 bacterial cells, the process, when once set going, being self-reproducing. 



Bordet (see Bordet and Ciuca 1920, Bordet 1923, 1925) sought to reconcile the con 

 ception of an inanimate phage with its reproduction in an unhmited series in a rather 

 different way. He suggested that the phenomenon was a true autolysis, the active agent 

 being produced exclusively from the bacteria themselves. Tlie origin of the autolysis he 

 traced to a disturbance of the normal equilibrium between the assimilative and metabolic 

 activities of the bacterial ceU, adding the supposition that the substances set free during 

 the autolysis of the cells initially affected were able to act in some way upon susceptible 

 but hitherto unaffected cells, and to initiate in them the same series of autolytic changes. 

 Once started, the process would thus be transmissible in series, provided that susceptible 

 bacteria were present, and that these bacteria were metabolically active. 



Northrop (1939) emphasized the greater likeness of many of the characteristics of 

 phage production and action to the production and action of enzymes, and in support 

 of his thesis demonstrated the similarity in the production of an extracellular gelatinase 

 and of phage from a lysogenic strain of B. megatherium. 



Another hypothesis, advanced by Hadley (1927, 1928), and since put forward in a 

 rather different form by WoUman (1925, 1927, 1928, 1929, 1934o, b, 1935) [see also WoUman 

 and Wollman, 1932, and Lwoff, 1936], assumes the phenomenon to be purely bacterial 

 in origin, but relates it to the genetic, not to the metabolic activities of the bacterial ceU. 

 The phage, on this view, would be regarded as analogous to some gene-carrying con- 

 stituent of the bacterial ceU, or some filtrable phase in a complex life-cycle. It must, 

 of course, be assumed that the addition of this cellular component, or bacterial phase, 

 to a young culture of susceptible organisms, so alters their genetic behaviour that they 

 undergo lysis diiring the process of multiphcation, and, in so doing, reproduce the active 

 agent in large amount. As further evidence for their view, Wollman and WoUman (1936, 

 1938) maintain that each living cell of a phage-carrying strain of B. megatherium hberates 

 one, and only one particle of phage ; if the phage were an externally infecting virus, 

 it is argued, the number present in a carrier cell shovild vary from ceU to cell. Gratia 

 (1936) and Flu (1938«), working with the same strain of bacterium, have both been unable 

 to eUcit the one-to-one ratio described by the WoUmans ; both find the number of particles 

 hberated can exceed the number of bacterial cells (see also Lewis and Worley 1936). 



There are, we think, adequate reasons for accepting the virus hypothesis, at 

 least as the most probable explanation of all the recorded facts. In setting out 

 the evidence that seems to us to justify this conclusion, it will be convenient to 

 discuss, seriatim, certain aspects of the nature arid behaviour of the phage ; but 

 before doing so one point may be made clear, since it will be involved in our con- 

 sideration of each other question in turn. 



D'Herelle, for reasons that are not easy to appreciate, has upheld the view 

 that the phage is a single living organism, Protobios bacteriophagtim, which may 

 adapt itself to live at the expense of a wide variety of different bacteria. The very 

 extensive evidence that is now available is quite incompatible with this view. 

 There is not one phage, but an enormous number of different phages ; and in order 

 to obtain constant and reproducible results it is as necessary to work with pure 

 strains of phage as with pure cultures of bacteria. Neglect of this precaution has 

 rendered many recorded studies of very doubtful value. 



Pure strains of phage may be obtained in two ways. Since a given strain of 



