VIRUS PARTICLES AND THEIR FUNCTIONAL ACTIVITY 353 



identifiable virus particles and infectivity of more and more viruses can be 

 expected. Among the more recent investigations are those of Crocker (1954) 

 on meningopneumonitis virus. Comits of this virus by the spray droplet 

 technique revealed a minimum value of 43 elementary bodies per ID50 when 

 infectivity titrations were carried out by yolk sac inoculations in chick 

 embryos. Poliovirus particles, after purification from mfected cotton rat 

 brains and spinal cords, were identified by electron microscopy and also 

 counted, again by the spray droplet technique (Bachrach and Schwerdt, 

 1954). Approximately 20,000 characteristic physical particles were found 

 equivalent to one ID50 by infectivity assays m cotton rats. In more recent 

 studies, particle counts of tissue culture poliovirus were compared with 

 infectivity assays by the sensitive plaque technique using monolayer cultures 

 of primary human amnion cells. Here an average of 36 particles per plaque was 

 observed for the Mahoney strain of type 1 pohovirus (Schwerdt and Fogh, 1957). 



In those few mstances where tumor-producing viruses have been sufficiently 

 purified to permit identification of the virus particle, the comit per uifective 

 dose has been found to be very high. Bryan and Beard (1940a) estimated the 

 mass of the papilloma virus particle from centrifugation and filtration data. 

 On this basis they calculated the number of virus particles present in the 

 amount of purified virus necessary to cause infection in 50 % of the rabbits 

 inoculated and found it to be approximately 8 X 10''. The virus of avian 

 myeloblastosis exhibited a particle per ID50 ratio in chickens of the same 

 order of magnitude, namely, 2.6 X 10' (Eckert et al., 1955). These high ratios 

 reflect, in part, the enormous range of host susceptibility to tumor viruses. 

 For example, Eckert et al. (1955), upon extrapolating the linear plot of log 

 number of virus particles inoculated against per cent, positive responses on a 

 probit scale, observed that at the 5 % level of incidence approximately 

 6 X 10^ particles infect this proportion of chickens, while 1.45 X 10^^ 

 particles are required to infect 95 % of the hosts. 



Quantitative comparisons between direct particle and lesion counts for 

 plant viruses reveal high ratios, as do the tumor viruses but for different 

 reasons. They are concerned largely with the inefficiency inherent in the 

 inoculation technique. Virus suspensions of known particle concentration 

 are rubbed on susceptible leaves in as reproducible a' manner as possible. The 

 number of lesions produced may be an exceedingly low estimate of viable 

 particles present, however, since the number of suitable entry points per leaf 

 is limited. Steere (1955), taking great precautions to apply his inoculum as 

 effectively as possible, demonstrated, at best, a particle per lesion ratio of 

 50,000 for tobacco mosaic virus. 



V. Summary and Conclusion 

 The various methods of determining virus concentration in terms of 

 infectious units and absolute particle count have been presented. Infectivity 



