336 ■ THE BACTERIOPHAGE 



Apart from these quantitative data, it is, of course, possible to observe the 

 occurrence of phage lysis under the microscope, and this has in fact been done (see 

 d'Herelle 1921, 1926, Twort 1922, Wollman 1925, Burnet 1925, v. Preisz 1925, 

 Mannrnger 1926, da Costa Cruz 1926, Bronfenbrenner et al. 1927, Bronfenbrenner 

 1928, Bayne-Jones and Sandholzer 1933). Not all of these observations have been 

 made under the best modern conditions of microscopy or photomicrography, so 

 that their value, particularly in regard to matters of detail, varies considerably. 

 They differ, also, in certain particulars of the actual happenings that are described, 

 and there can be little doubt that phage lysis may, in fact, occur in several different 

 ways, according to the nature of the phage and bacterium involved. 



Swelling of the bacterial cells is usually seen in the early stages of the process, and 

 is sometimes accompanied by the development of granules. The disruption of the swollen 

 bacterial cell is usually very sudden. The cell may appear to explode, giving place to 

 a cloud of granular debris, or it may lose its outline and disappear, without the Uberation 

 of the granular cloud. 



Merling-Eisenberg (1938) (see also Eisenberg-MerUng 1941) was able to make direct 

 observations on the disruption of invaded bacteria ; he found that 150-200 phage particles 

 were hberated from cells of Bad. coli and 2-4 from cells of Staph, aureus. Delbriick 

 (1940) distinguishes two forms of lysis, lysis from without and lysis from within. Lysis 

 from without occurs almost instantly when phage is adsorbed at the threshold linut. 

 The ceUs swell into sjiherical bodies. Lysis from within occurs in Bad. coli after the 

 adsorption of one or a few particles, but not until the phage has multiphed to the threshold 

 Mmit. The ceUs appear to fade more or less quickly. Electron micrographic studies of 

 Bad. coli undergoing phage lysis (Luria, Delbriick and Anderson 1943) gave precise pictures 

 of the adsorption of phage, lysis of the cell, and hberation of phage particles from the 

 interior of the cell, which were numerically in accord with the results obtained by con- 

 current phage counts of the infected cultures studied. They also revealed that in multiple 

 infection of a cell, the adsorbed particles did not seem to enter the cell, but remained 

 attached to the cell wall. 



The mechanism of the actual disintegration of lysed bacteria is not understood. In 

 some cases it is doubtful whether such disintegration occurs, at any rate at the moment 

 when the turbidity of the culture disappears. With some phages, the absence of micro- 

 scopically apparent disintegration at this moment suggests that the main change is an 

 approximation of the optical character of the bacteria to that of the suspenduag medium. 



The hberation by phage of a lytic enzyme at the time of lysis was postulated by d'Herelle 

 (1926). Sertic (1929, 19376 ; Sertic and Boulgakov 1939) observed a halo round each 

 plaque formed by certain coli phages in an opaque growth of Bact. coli. The halo was 

 due to the induction of variants of the Bad. coli used, by a " lysm " which would pass 

 an ultrafilter ; the variants grew as a transparent film. The addition of the phage -free 

 lysin to a mixture of Bad. coli and a phage to which it was insensitive resulted in lysis. 

 The lysin apparently destroyed certain antigenic constituents of the insensitive Bad. coli, 

 thereby creating a variant of diflFerent sensitivity (see also Schuurman 1936). Evans 

 (1940) was able to enhance the activity of streptococcal phages on resistant strains by 

 addmg a small quantity of a susceptible strain to the mixture ; she explamed the reaction 

 in terms of " nascent " phage. It is probable that she was deahng with a lysin similar 

 to that of Sertic. Camjabell-Renton (1937) observed a related phenomenon when phage 

 filtrates inactivated by ultra-violet Ught were tested on sensitive organisms : there was 

 a diffuse lysis of the bacteria without the formation of plaques. In both these cases the 

 lytic agent appeared to be derived from the bacteriophage. This view was also supported 

 by Gratia (1937). On the other hand, Bronfenbremier and Muckenfuss (1927) obtained 

 a lytic enzyme from both normal and phage-lysed cultures of staphylococci. WoUman 

 (1934a) upheld the bacterial origin of lytic enzymes. Phage action can be enhanced 



