244 G. S. STENT 



period and reach their minimum value when the final plateau of progeny 

 infectivity has been attained, i.e., when all the bacteria which are going to 

 lyse have lysed (Delbriick, 1940; Doermann, 1952). 



The mechanism by which the phage-infected bacterial cell is lysed has been 

 the object of numerous investigations since the first days of bacteriophage 

 research. Nevertheless, the nature of the lytic reaction still does not appear to 

 be entirely understood. Some of the early observations were bedeviled by the 

 circumstance that there exists not one but two completely different processes 

 by which bacteriophages can lyse susceptible bacterial cells (Delbriick, 1940). 

 One of these, lysis-from-without, represents an immediate dissolution of 

 bacteria, often encountered when the multiplicity of infection is much 

 greater than one (Bail and Matsumoto, 1923; Meuli, 1923; Krueger and 

 Northrop, 1930). Loss of the input phages, rather than their multiplication, is 

 connected with this form of lysis, which can proceed in, or is even favoured by, 

 the absence of an active metabolism of the host cell (Heagy, 1950). It seems 

 very likely that lysis-from-without is a consequence of the digestion of parts 

 of the bacterial cell wall by the attachment organs of the adsorbed phage 

 particles (Weidel, 1951; Puck, 1953; Barrington and Kozloff, 1956; cf. 

 Chapter 2). Since in the course of intracellular phage growth, bacteria become 

 progressively more resistant to lysis-from-without induced by subsequent 

 superinfection with a high multiplicity of secondary phage particles (Visconti, 

 1953), it can only be the second of the lytic processes, or lysis-from-ivitJiin, 

 that is really the form of lysis properly connected with intracellular phage 

 multiplication and hence of concern to us here. Direct microscopic observations 

 of the lysis of phage-infected, rod-shaped bacteria were already carried out 

 by d'Herelle (1921), and later by daCosta Cruz (1926), who noticed that the 

 cells assume a swollen or spherical form shortly before their dissolution. Use 

 of microcinematography showed that the dissolution of the bacteria and the 

 release of their contents resembles more an explosion, as if the final act of 

 cell lysis corresponds to a bursting of the cell envelope by internal pressure 

 (Bronfenbrenner, 1928; Bayne-Jones and Sandholzer, 1933). Although it may 

 be the internal pressure which delivers the coup de grace to the infected cell, 

 it is probable that intracellular phage multiplication first engenders a weaken- 

 ing of the structural members of the bacterium, so that the rod-shaped cell 

 loses its rigidity and, shortly before its end, assumes the spherical shape of 

 least surface-to-volume ratio, before the tensile strength of the envelope 

 falls below the stress exerted by the osmotic forces. 



The weakening of the bacterial structures connected with lysis-from- 

 within has generally been thought to be the work of intrabacterial enzymes, 

 although opinions have differed as to whether the lytic enzymes are the 

 "endoferments" responsible also for the autolysis of uninfected bacteria 

 (Bronfenbrenner, 1928; Wollman and Wollman, 1933), or whether they 



