THE MECHANISM OF PHAGE LYSIS 333 



observed after treatment of the phage at 50° C. with various anions (Ohashi 1938). The 

 inhibition of lysis, by which inactivation is measured, may be due to an action, not on 

 the phage particle itself, but on the sensitive bacterium. For instance, Gest (1943) has 

 shown that with high concentrations of univalent cations, hke Na, and moderate con- 

 centrations (01 molar) of Mg ions, plate lysis may be completely masked by overgrowth 

 of resistant bacteria, which are apparently produced in large numbers in the presence 

 of the cations. Some of the larger phages are rapidly inactivated by a strong iirea solution, 

 a reagent that acts similarly on many bacteria and viruses (Burnet 19336, 1934). 



It has been shown by several workers that some, but not aU, phages are unable to 

 produce lysis in bacterial cultures growing in a medium from which the calcium ions have 

 been removed by previous treatment with citrate (Stassano and de Beaufort 1925, Bordet 

 1926, Asheshov 1926, Burnet 19336). This fact allows a useful differentiation of phages 

 into " citrate-sensitive " and " citrate -resistant " strains ; but it should be noted that 

 an initially citrate -sensitive phage may often be trained to produce lysis in a citrate- 

 containing medium. 



The Mechanism of Phage Lysis. 



In considering the mechanism of phage lysis we may first deal with the pheno- 

 menon in a general sense, leaving the problem of specificity for later consideration. 



D'Herelle's conception of the lytic process, based of course on the view that 

 the phage is a parasitic virus, is simple and straightforward. He believes that a 

 phage particle enters a growing bacterial cell, multiplies within it, and causes its 

 more or less explosive disintegration when the limit of distension has been reached. 

 This limit appears to vary over a considerable range ; but disruption usually occurs 

 when the number of particles in the cell have reached some figure between 6 and 60. 

 In support of this view, d'Herelle states that the addition of very small amounts of 

 phage to a growing culture is followed by a step-like increase in phage titre during 

 the earlier stages of growth, successive sudden increases occurring at intervals of 

 20 to 30 nainutes. After a few such jumps in titre the phage concentration rises 

 logarithmically until lysis occurs. This change from a discontinuous to a continuous 

 rise would, of course, be expected ; since the successive increases would soon get 

 out of step, and their combined effect would give a steady rise in titre. 



Burnet (1929c) has recorded observations that are in entire accord with d'Herelle's 

 view. To a number of small tubes, each containing a young, actively growing culture 

 of a sensitive bacterium, he added a phage filtrate so highly dUuted that, on the average, 

 each tube received a single particle. At short intervals thereafter the whole contents of 

 one of the tubes was spread on the surface of an agar plate, and the resulting plaques were 

 counted. The results showed that there was no detectable increase during the first 20 

 minutes or so. After this time there was a sharp and sudden rise. Thus, a series of tubes 

 plated at one-minute intervals, over the appropriate time range, gave 1, 0, 1, 2, 0, 1, 80, 

 0, 1, 120, 1, 230, and 100 plaques. The absence of values intermediate between 1 and 

 80, makes it clear that the free phage in the culture was not increasing by twofold steps, 

 as would occur during the early generations of a single bacterium multiplying by binary 

 fission ; and the only obvious explanation of such findings is that the phage particles are 

 dividing in, or on, a bacterium, and are suddenly liberated into the surrounding fluid when 

 that bacterium disrupts. These observations have been fuUy confirmed by EUis and 

 Delbriick (1939) and Hershey and Bronfenbrermer (1943). 



The process of phage lysis has also been studied quantitatively by observations carried 

 out on a lysing culture as a whole (see, for instance, Lepper 1923). Krueger and Northrop 

 (1931) determined the rates of growth of staphylococci and staphylococcal phage in a 

 lytic mixture. After a short lag period, the concentration of bacteria and phage both 

 increased steadily up to the moment of visible lysis. The rate of increase of phage was 

 consistently greater than that of the bacteria. Chfton and Morrow (1936) demonstrated 



