480 BACTERIOPHAGES 



is used, the sample volume can be varied from 0.05-0.5 ml. 

 without difficulty. 



Another consideration in choosing the dilution factor in 

 going from F.G.T. to S.G.T. is the sampling error. In the pro- 

 tocol (p. 475) the 0.1 ml. sample from F.G.T. used in preparing 

 S.G.T. contained only 100 infected bacteria. A sample of this 

 size has an expected sampling error of 10 per cent and a pos- 

 sible error of much more so that the burst size determination is 

 subject to at least this error from this cause alone. A larger 

 sample from F.G.T. would decrease this source of error, and the 

 only change involved would be a larger total volume in S.G.T. 

 to keep the dilution factor constant. 



Over-all dilution in going from the adsorption tube to S.G.T. 

 is usually of the order of 10~^-10~'. This is very important in 

 the one-step growth experiment in that it essentially prevents 

 readsorption of liberated phage onto surviving bacteria and so 

 enables one to reach a true stationary period in the curve. 

 However, the surviving bacteria continue to grow, increasing 

 by about a factor of 10 every hour, so that the probability of a 

 phage particle adsorbing to a bacterium is also increasing. 

 If the one-step growth experiment is continued for several 

 hours there will be a steady increase in the phage assays which 

 will finally cease only when all susceptible bacteria are lysed. 

 If this type of experiment is carried out at higher concentrations 

 of bacteria but low concentrations of virus, so that the virus 

 released from the first step is readsorbed, it is possible to get a 

 series of steps which become less pronounced and finally become 

 just a smoothly rising curve of phage activity (Ellis and Del- 

 bruck, 1939). 



An interesting point is demonstrated by the 22 min. sample 

 from S.G.T. This particular plate had 72 plaques, when, 

 judging from F.G.T. samples, it should have had between 1 

 and 5 plaques. The reason for this high plaque count is that 

 the 0.1 ml. sample withdrawn from S.G.T. contained an in- 

 fected bacterium which burst and released some 70 phage 

 particles during the procedure of sampling. These 70 phage 



