THE INFECTIVE PROCESS 21 



examination of electronmicrographs that the bacterial cell wall 

 ruptures before the end of the latent period and that the phage 

 particles then mature in the extruded cell sap (Wyckoff, 1 949a) . 

 Lysis of the host cell accompanied by release of virus particles 

 is readily demonstrated by direct observation in the dark field 

 microscope. The bacteria can be observed to disintegrate at a 

 time corresponding to the end of the latent period. At the 

 moment of disappearance of each bacterium a cloud of scintil- 

 lating particles is released in numerical agreement with the 

 known burst size (d'Herelle, 1926; Merling-Eisenberg, 1938; 

 Pijper, 1945). Release of phage particles is coincident with 

 lysis of the host cell also in lysogenic bacteria, as demonstrated by 

 micromanipulation techniques (LwofT and Gutmann, 1950). 



6. Lysis in Fluid Media 



We have described the sequence of ev^ents when a few phage 

 particles are added to a growing culture of susceptible bacteria. 

 The phage particles adsorb and in due course the infected 

 bacteria lyse liberating the phage progeny. These then adsorb 

 to unlysed bacteria initiating a second cycle of infection. The 

 infectious cycles follow one another, the phage population in- 

 creasing in each cycle by a factor equal to the burst size, until 

 all susceptible bacteria have lysed. The final phage population 

 is usually of the order of ten to several hundred times the maxi- 

 mum bacterial population achieved. The theoretical phage 

 yield would be the bacterial population multiplied by the aver- 

 age burst size. The actual yield is often less because some of the 

 liberated phage is lost by readsorption to yet unlysed bacteria or 

 to bacterial fragments. The over-all time required for lysis 

 depends principally on the latent period and the number of 

 phage particles inoculated, and to a lesser extent on the ad- 

 sorption rate and the bacterial concentration. 



If the entire bacterial population is susceptible to the phage, 

 the culture after lysing will remain clear indefinitely. However, 

 if the culture contains a few phage-resistant variants, the lysed 

 culture will on further incubation become turbid again due to 



