330 C. E. SCHWERDT 



There are two basic reasons for wanting to know this relationship. An 

 investigation of the physical and particularly the chemical nature of a virus 

 in its extracellular state requires assurance of the identity of the infectious 

 particle and of the purity of its preparation. Confidence in these areas 

 depends in part upon a knowdedge of the ratio of infective to total virus 

 particles. Second, an apj)reciation of this ratio is essential to an understand- 

 ing of the mechanisms of virus replication, beginning with the question of 

 the minimum amount of virus necessary to mitiate infection. 



This chapter is concerned with the method of determining total virus 

 particle count and virus infectivity as well as an interpretation of the ratio of 

 these measurements for certain representative viruses. This field has been 

 most effectively reviewed by Isaacs (1957) for animal viruses. It is inevitable 

 that nmch of this material will be incorporated here, for which this writer 

 offers his grateful acknowledgment. 



II. Titration of Virus Infectivity 



The titration of a virus is essentially a determination of the smallest amount 

 of a virus suspension which wiU produce some manifestation of disease in a 

 susceptible host. Two types of manifestations may be observed, namely, a 

 generalized or systemic infection usually referred to as an all-or-none response, 

 or the production of local lesions. Assays based on the all-or-none response 

 are carried out by inoculating a measured volume of serial (logarithmic) 

 dilutions of the virus suspension into groups of a suitable host, and estimates 

 are made of the minimal volume capable of producing infection. This volume 

 is called an infectious unit, and the titer of the original virus suspension is 

 expressed as the number of infectious units per milliliter. Such titers represent 

 the relative infectivity of virus preparations without indicating the number 

 of virus particles per dose. For the comparison of titers, therefore, it is 

 necessary to define the conditions of titration with care. 



In assay systems where a virus produces local lesions, such as pocks or 

 plaques, serial dilutions are also employed but the titer is expressed as the 

 number of pocks or plaque-forming units per milliliter of original virus 

 suspension. Since each lesion is initiated by a single virus particle (Section 

 IV, A), an estimate can be made of the absolute number of virus particles in 

 the original suspension if the efficiency of lesion production is known. 



Occasionally a virus will fail to produce local lesions or will exhibit a poor 

 correlation between the proportion of positive responses elicited and dose 

 inoculated. End-point titrations are impractical for such a system. The virus 

 may, however, reveal a regidar relationship between the incubation period of 

 the disease produced and log dilution inoculated, thus makmg possible the 

 determination of the relative potency of the virus in question by comparison 

 with a standard or control virus preparation. 



