644 GROWTH OF PHAGE AND LYSIS 



overstepped. A considerable loss in total phage parallels the clearing of the 

 culture. If sufficient phage is added so that the ratio phage/bacteria is 

 greater than the threshold value lysis begins almost at once. This experi- 

 ment was later repeated and confirmed by Northrop (4, 5) with purified 

 concentrates of phage. 



Recently Northrop (5) found with a susceptible megatherium strain and 

 homologous phage that the bulk of the phage was liberated before the cul- 

 ture began to clear. He found further with a lysogenic strain which never 

 showed clearing but produced phage lysing the sensitive strain, that the 

 yield in phage from this lysogenic strain was large compared to the number 

 of bacteria present in the culture. 



All these results indicate that in these strains lysis, if it occurs at all, is 

 brought about by a mass attack of the phage on the bacteria after the phage 

 have grown and been liberated into solution. 



We have now studied in more detail the relation between phage growth 

 and lysis in a new sensitive strain of B. coli and homologous phage and have 

 obtained results which may offer a basis for reconciling the two diverging 

 lines of interpretation. 



We have found in this strain two entirely difTerent types of lysis, which 

 we designate as "Lysis from within" and "Lysis from without." 



Lysis from without is brought about almost instantly by adsorption of 

 phage at a threshold limit, which is equal to the adsorption capacity of that 

 bacterium. No phage are liberated in this case, on the contrary, the ad- 

 sorbed phage are lost. The phage attack the cell wall in such a way as to 

 permit swelling of the cell, and its deformation into a spherical body. 



Lysis from within is brought about by adsorption of one (or few) phage 

 particle(s). Under favorable conditions this one phage particle multiplies 

 during a latent period within the bacterium up to a threshold value (which 

 is equal to the adsorption capacity). When the threshold value is reached, 

 and not before, the phage is liberated by a sudden destruction of the proto- 

 plasmic membrane, which permits a rapid exudation of the cell contents 

 without deformation of the cell wall. 



It would seem that the results of other observers may be explained by 

 postulating that 



{a) In the case of Staphylococcus aureus the observable clearing is caused 

 by lysis from without. Lysis from within either does not exist here and is 

 replaced by continuous phage secretion ; or it exists but leads only to a slow 

 equalization of the refractive indices of the cell interior and the milieu. 

 The decrease of total phage during lysis is caused by the adsorption of 

 phage in the process of lysis from without. 



58 



