VIRUS PARTICLES AND THEIR FUNCTIONAL ACTIVITY 355 



The potential that particle counting offers to experimental studies concerned 

 with the mechanism of animal virus replication has been realized mostly 

 for influenza virus to date. For example, the agglutinating behavior of 

 "incomplete" and standard influenza virus has been found to be similar in 

 each case (Werner and Schlesinger, 1954; Donald and Isaacs, 1954a) although 

 it has not yet been possible to determine by particle count methods whether 

 "incomplete" virus preparations consist of a mixture of totally noninfective 

 and infective particles or represent a population of virus particles of 

 uniformly low ability to infect (Isaacs, 1957). 



The filamentous forms of mfluenza virus have been compared with the 

 spherical forms with the aid of particle counting techniques and have been 

 found to possess the same infectivity but about seven or eight times the 

 hemagglutinating efficiency of spheres on the basis of filament and particle 

 counts (Donald and Isaacs, 1954b). Ada et al. (1957) have recently confirmed 

 this finding of greater hemagglutinating efficiency of the filamentous forms 

 but noted that they also exceed the spherical particles in specific infectivity. 

 They calculated from their data that two or three filaments are equivalent to 

 one 50 % infective dose. 



The problem of the effect of multiplicity of infection (i.e., the number of 

 physical particles adsorbed per cell) upon "mcomplete" virus production is 

 bemg currently investigated. The quantitative aspects of interference by 

 inactive particles is also under study. One difiiculty in mterpretation of such 

 experiments is the uncertainty with which the total number of available, 

 susceptible entodermal cells Iming the allantoic membrane is estimated. 

 Isaacs (1957) has summarized the results of findings in these areas. It appears 

 that multiplicities rangmg from one to one hundred may produce ' 'incomplete' ' 

 virus, depending upon the strain of virus, while twenty to several hundred 

 particles per cell are necessary to produce interference, again depending 

 upon strain of virus used and conditions of inactivation. 



One can hope, now that more and more viruses are being purified and 

 identified by electron microscopy, that further clarification of their mechan- 

 isms of replication as well as of their chemical and physical nature will also 

 be aided by virus particle-counting techniques. 



References 



Ada, G. L., Perry, B. T., and Edney, M. (1957). Nature 180, 1134. 



Adams, M. H. (1950). Methods in Med. Research 2, 1. 



Andenjon, T. F. (1951). Trans. N.Y. Acad. Sci. 13, 130. 



Armitage, P. (1957). J. Hyg. 55, 564. 



Armitage, P., and Spicer, C. C. (1956). J. Hyg. 54, 401. 



Bachrach, H. L., and Schwerdt, C. E. (1954). J. Immunol. 72, 30. 



Bachrach, H. L., Callis, J. J., Hess, W. R., and Patty, R. E. (1957). Virology 4, 224. 



Backus, R. C, and Williams, R. C. (1950). J. Apjil. Phys. 21, 11. 



