190 BACTERIOPHAGES 



1. Observations on Living Cells 



The lysis of bacteria by phage may be readily observed under 

 the ordinary light microscope. For such studies a culture of the 

 susceptible bacterium in the logarithmic growth phase is exposed 

 to an excess of bacteriophage under conditions in which at least 

 90 per cent of the bacteria are infected within a few minutes. 

 The infected bacteria may then be observed under oil immersion 

 by hanging drop methods or on the surface of agar under a cover 

 glass. Bacterial lysis begins after the usual latent period for the 

 conditions of temperature and bacterial nutrition used. The 

 time course for the number of bacteria lysed corresponds to that 

 for phage liberation as observed in the one-step growth experi- 

 ment, which suggests that the lysis of a bacterium coincides with 

 the liberation of the mature phage particles contained in that 

 bacterium. 



With most of the phage-host cell systems studied, infection in- 

 hibits further cell division. An exception occurs with certain 

 temperate phages studied by Lwoff, Siminovitch, and Kjeld- 

 gaard (1950) in which the infected cell undergoes one or two 

 divisions before lysis. In many cases no gross change in cell 

 morphology occurs up to the moment of lysis, at which time the 

 refractive index of the cell suddenly changes, the cell bursts and 

 becomes invisible. Studies of this event by cinematography have 

 demonstrated that cell disappearance can occur within 3 seconds, 

 the time interval between successive photographs (Bronfen- 

 brenner, Muckenfuss, and Hetler, 1927; Bayne- Jones and Sand- 

 holzer, 1933). 



Many observers have reported morphological changes occur- 

 ring in cells after infection. In some cases the cells swelled 

 markedly and became nearly spherical in shape before bursting. 

 In other cases greatly elongated, filamentous forms developed 

 which might or might not lyse (Burnet, 1925; Bronfenbrenner, 

 1928). This variety of changes in cell morphology gave rise to 

 some controversy in the early literature (see Bronfenbrenner, 

 1 928) and may have been due to differences in phage and host 

 strains or differences in environmental conditions in use in 



