BACTERIOPHAGE MUTATION 37 



Secondly, the production of the resistant lysogenic strain SF/(j provides a 

 clear-cut example of the direct positive induction of change in bacterial character 

 by the action of a bacteriophage. In this instance the alternative of selection by 

 phage from pre-existent variants in the population submitted to lysis is definitely 

 excluded. The rapidity with which the change is induced is noteworthy. Within 

 an hour of contact with phage C the surface of the bacterium has changed so that 

 it no longer adsorbs either phage C or C and becomes insusceptible to their action. 

 This changed character is then transmitted indefinitely to its descendants. It is 

 not possible to say whether this surface change results from an altered genetic 

 constitution of the bacterium or is directly induced by the associated phage at each 

 generation. According to Wollmann's hypothesis the distinction between the two 

 alternatives would disappear, the phage being regarded as a gene re-introduced 

 into the genetic make-up of the organism. 



Finally, we have the demonstration of a definite mutation of a bacteriophage 

 occurring in the course of lysogenesis. Most accounts of bacteriophage mutations 

 are open to the same objections as can be raised against the claims to have produced 

 phage from normal bacterial cultures. In the present instance the mutation ap- 

 pears to affect only one aspect of the phage — that which determines the relative 

 proportion of effective contacts which give rise to lysis and resistance respectively 

 — the other recognizable characteristics remaining unchanged. Change in the 

 character of a phage incorporated in a lysogenic strain has recently been described 

 by d'Herelle and Rakieten (1934) and, although there is rarely any evidence of 

 increase in vigour in these cases, the C to C change cannot be regarded as unique. 



SUMMARY. 



A bacteriophage (C) acting on the non-pathogenic white coccus SF is charac- 

 terized by the unusual frequency with which it provokes the appearance of resist- 

 ant forms. Each plaque shows a central growth of resistant culture and suitable 

 experiments indicate that under the usual conditions of growth 10 or 20 p.c. of 

 effective contacts between phage and susceptible bacterium result in the appear- 

 ance of resistant lysogenic variants, the remainder initiating lysis in classical 

 fashion. The resistant culture shows no gross evidence of lytic action on agar, but 

 in broth culture liberates considerable amounts of phage. In young cultures this 

 phage is of the original type, but as the culture ages a variant phage C makes its 

 appearance. This produces plaques of ordinary appearance without the central 

 resistant growth, and gives only a small number of predominantly non-lysogenic 

 resistant colonies after lysis of agar cultures. In all other respects, serological 

 character, for example, the variant phage is indistinguishable from the parent 

 phage C. Certain aspects of these phenomena are briefly discussed. 



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