14 BACTERIOPHAGES 



Similarly, if to a film of susceptible bacteria growing on an agar 

 surface one adds a phage particle, it will adsorb to a bacterium 

 and initiate the first lytic cycle. The phage progeny from the 

 first cycle will infect neighboring bacteria to initiate the second 

 cycle, thus producing a spreading lesion in the bacterial film. 

 Eventually this process will result in a readily visible sterile area 

 in the film of growing bacteria, the bacteria-free area being 

 known as a plaque. Under ideal conditions each infective 

 phage particle will produce a plaque, so that plaque counts give a 

 simple and reproducible measure of the number of phage parti- 

 cles present in a sample inoculated on the assay plate. 



1. The One-Step Growth Experiment 



The over-all aspects of the infective cycle are most conveniently 

 studied by means of the one-step growth experiment of Ellis 

 and Delbriick (1939), which gives the minimum latent period of 

 intracellular virus growth and the average burst size. The latent 

 period is defined as the minimum time between the adsorption of 

 phage to host cell and the lysis of the host cell with release of 

 phage progeny. The burst size is the mean yield of phage par- 

 ticles per infected bacterium. Each of these characteristics can 

 be determined by other techniques, but the one-step growth ex- 

 periment enables both to be determined at once. The tech- 

 nique is as follows: 



7. Phage and bacteria are mixed at concentrations that per- 

 mit rapid adsorption. 



2. After a suitable adsorption period the mixture is diluted 

 into antiphage antibody to stop adsorption and inactivate unad- 

 sorbed phage. 



3. After sufficient time for antibody action the mixture is 

 further diluted in growth medium so that each sample will con- 

 tain about 100 infected bacteria. 



4. The suspension of infected bacteria is sampled at intervals 

 until lysis is completed and each sample is assayed by the plaque 

 count method. 



Each infected bacterium if plated before lysis will produce one 



