350 C. E, SCHWERDT 



and those noninfectious due to thermal iiiactivation or incomplete synthesis 

 if their gross physical characteristics are similar. 



1. Bacterial Viruses 



There is little doubt today that plaque assays of bacterial viruses can be 

 ideal under appropriate conditions, thus supporting the concept of infection 

 by a single virus particle. At least this can be said of the T series of coliphages. 

 Luria et al. (1951), in an investigation of the particle-to-plaque relationships 

 of purified T-even phages, found ratios varying approximately between 1 and 

 2 when particle counts were made by the electron microscopic, spray droplet 

 technique using polystyrene latex reference particles. This has been confirmed 

 by Kellenberger and Arber (1957), who did electron microscopic counts of 

 stock Tg, T4, T5, and lambda phages by their agar filtration technique and 

 found a range of particle-to-plaque ratios lying between 1 and 3 with an 

 average value of 1.4. The bacterial virus-bacterium host system has proved 

 most satisfactory m establishing the 1 : 1 relationship between statistical 

 (infectious) and physical particles. 



2. Poxviruses 



Vaccinia was among the earliest of the animal viruses to be studied with 

 respect to particle per infectious unit ratios, no doubt because of its large 

 size and relative ease of purification. At first, elementary body concen- 

 trations were determined indirectly on the basis of estimates of particle mass 

 from physicochemical data. Smadel et al. (1939) divided the dry weight of an 

 aliquot of purified vaccniia virus suspension by the calculated dry particle 

 mass and compared this figure with the number of rabbit IDgo's in an equal 

 volume of the same preparation. They found that values for ratios of 

 elementary bodies to ID50 varied between 2.4 and 9.2 with an average of 4.2. 

 Similarly, Sprunt et al. (1940) counted vaccinia elementary bodies, using 

 nitrogen content, however, as a measure of total purified virus mass per 

 aliquot. They observed an average value of 366 particles per TD^^. This 

 figure exceeds significantly the 4.2 value found by Smadel et al. (1939). It 

 seems unlikely that this 90-fold discrepancy represents great differences in the 

 degree of purity of the preparation employed by the two groups of workers 

 and thus differences in enumeration of elementary bodies. Rather dissimilar- 

 ities in virus virulence and host susceptibility (Sprunt, 1942; Sprunt and 

 McDearman, 1940) as well as in conditions of titration, such as volume of 

 moculum (Sprunt, 1941), may have been responsible. 



With the advent of direct particle counting, by electron microscopy, the 

 particle per infectious miit ratio for vaccinia virus has been reinvestigated. 

 Overman and Tamm (1956b) used a modification of Sharp and Beard's 

 (1952) sedimentation technique for counting particles and estimated the 



