SYLLABUS 103 



liams, 1948). This fact allows the determination of the absolute con- 

 centration of particles of latex by simple weighing. Luria and Williams 

 (personal communication) made electron microscope preparations of 

 a mixture of phage with latex suspensions of known concentration. 

 They then counted the number of phage particles relative to the num- 

 ber of latex spheres, thus obtaining the relative titer of the phage 

 suspension. 



The results of the three biological methods usually agree with one 

 another. They also agree with the physical method within a factor of 

 two (T2 gave twice as many particles in the electron microscope as 

 there were infective units measured by biological methods, T4 gave 

 20% less, and T6 30% more particles than infective units). 



Since these four essentially different methods give the same result 

 we may call the titer they give the absolute concentration. 



Sometimes, however, for a strain of bacteria different from the 

 normal host strain the plaque count may be low. We then speak of 

 the strain as having a low efficiency of plating. 



(5) Dark-field microscope visibility. 



The scattering of light by phage particles renders them visible in 

 the dark- field microscope. To see the smaller phages Ti, T3, T7 a 

 much stronger source of light is needed than for the larger ones, T2, 

 T4, T5, T6. It is possible to estimate roughly the concentration of a 

 suspension by looking at it in the microscope. The adsorption of a 

 single phage on a bacterium is difficult to observe because of the violent 

 Brownian motion of the particles, but one can follow the adsorption 

 as a whole by watching the gradual disappearance of the phages when 

 mixed with bacteria under the microscope. At the time of lysis the 

 bacteria burst open liberating several hundred particles having the 

 same scattering power as the infecting phages. (The liberation of 

 particles upon lysis by Ti cannot be seen unless a very strong source 

 of hght is used.) These liberated particles disappear slowly from the 

 field of view by diffusion and adsorption on other bacteria. There is 

 thus no doubt that they are the infective units (Weigle, unpublished). 



4. Morphology. 



(a) Electron micrographs. 



As seen in the electron microscope the phages have the following 

 aspects. 



Ti: Round head, 45 to 50 m\i diameter, of uniform opacity. 



