452 BACTERIOPHAGES 



cell or an infected host cell which does not liberate phage until 

 near the end of the growth period of the bacteria will not pro- 

 duce a placjuc and will not be counted. 



Some factors known to afTcct markedly the efliciency of plat- 

 ing may be mentioned by way of example. Anderson (1945a) 

 found that the coli phage T4, when plated on a chemically 

 defined medium free of amino acids, produced only 10~^ the 

 number of plaques produced when it was plated on nutrient 

 agar. Addition of 100 jug. of tryptophan/ml. of the chemically 

 defined medium increased efficiency of plating to that obtained 

 in nutrient broth. Tryptophan is essential for adsorption of 

 phage T4 to the host cell. Hershey, Kalmanson, and Bron- 

 fenbrenner (1944) found that the efficiency of plating of a var- 

 iant of coli phage T2 on tryptose agar was markedly a function 

 of salt concentration. Addition of NaCl to a concentration of 

 0.2 M increased efficiency of plating by a factor of 10^ over 

 that of tryptose agar to which no salt was added. Addition of 

 salt in this instance enormously increased the rate of adsorption 

 of phage to bacterium. 



The absolute efficiency of plating cannot be determined unless 

 there is an independent method for determining the total number 

 of infective phage particles in a preparation. Perhaps the best 

 available estimate of the efficiency of plating depends on correla- 

 tion between the size of an infectious unit as estimated by 

 chemical methods and the particle size as determined by means 

 of the electron microscope. 



Hook and co-workers (1946), working with phage T2, found 

 the average dry weight of a plaque-forming particle to be 7 X 

 10~^^ g. and the specific volume of the dried preparation 

 0.66 ml./g., from which the volume of the lytic particle be- 

 comes 5 X 10 ~^^ ml. Assuming the particle to be a sphere, 

 its diameter would be about 100 it\jx. From electron micro- 

 graphs these workers concluded that the head of the phage 

 particle measures about 80 X 100 m^u and the tail about 120 X 

 20 m/i. This close correspondence indicates that the efficiency 

 of plating is certainly not grossly low. 



