Chapter IV 



Quantitative Relationships between Virus Particles 

 and Their Functional Activity 



Carlton E. Schwerdt 



Department of Medical Microbiology, Stanford University 



School of Medicine, Stanford, California 



I. Introduction 329 



II. Titration of Virus Infectivity 330 



A. Assay Based on All-or-None Response 331 



B. Assay by Local Lesion Count 333 



1. Plant Viruses 333 



2. Bacterial Viruses 334 



3. Animal Viruses 335 



C. Factors Affecting Virus Infectivity Titrations 337 



III. Methods of Determining Total Virus Particle Concentrations 339 



A. Direct Methods: Electron Microscopy 339 



1 . Sedimentation 339 



2. Spraying 340 



3. Agar Filtration 341 



4. Adsorption 342 



B. Indirect Methods 342 



1 . Based on Physical Properties 342 



2. Based on Estimate of Number of Hemagglutinating Particles 344 



IV. Relationships between Total Particle Count and Biological Function 346 



A. Initiation of Infection by a Single Virus Particle 346 



B. Observed Ratios between Virus Particle Count and Infectious Units 349 



1 . Bacterial Viruses 350 



2. Poxviruses 350 



3. Myxoviruses 351 



4. Other Viruses 352 



V. Summary and Conclusion 353 



References 355 



I. Introduction 



All understanding of the nature of viruses progresses only as effective 

 methods of measuring virus concentration are developed. At first, relative 

 virus concentration was expressed in terms of units of infectivity. In time, 

 procedures were devised to enumerate total virus particles by indirect and 

 direct means, thus permitting studies on the quantitative relationship 

 between the physical particles and their biological activities. 



329 



