284 MANUAL OF MICROBIOLOGICAL METHODS 



of the viruses does not depend on someone's remembering to replenish 

 the dry-ice supply and they may be therefore more reliable. 



Viruses stored at 4°C may retain their infectiousness from a tew days 

 to a few weeks. This may be especially satisfactory because of its con- 

 venience when fresh seed virus may be substituted for the old each week. 

 Storing neurotropic viruses in 50 per cent neutral glycerin at 4°C is 

 usually satisfactory for several months. 



The length of time a virus may remain viable depends not only upon 

 the temperature but upon the menstruum employed. As a rule 10 per 

 cent tissue suspensions store satisfactorily. Fluids of low protein con- 

 tent usually store better if the protein content is increased, such as by 

 mixing them with sterile milk. Embryonic membranes may be stored 

 whole. If the infected animal is small, such as a mouse, certain pneumo- 

 tropic and neurotropic viruses are preserved very well if the whole 

 animal is frozen. When freezing mice it is advisable to use a small 

 airtight container which the mouse or mice will fill sufficiently to exclude 

 practically all the air. 



Some evidence is available that repeated freezing and thawing degrade 

 some viruses (Tyrrell and Horsfall, 1954). It would seem advisable 

 then to put away several smaller containers of virus to avoid repeated 

 freezing and thawing of one large sample. Some viral antigens store well 

 at 4°C but are destroyed at freezing temperatures. It is well to be 

 aware of what activity you wish to store before selecting the storage 

 temperature. 



REFERENCES 



Bernkopf, H. 1950. Study of infectivity and hemagglutination of influenza virus in 



deembryonated eggs. /. Immunol, 65, 571-583. 

 Burnet, F. M., J. F. McCrea, and J. D. Stone. 1946. Modification of human red 



cells by virus action. I. The receptor gradient for virus action in human 



red cells. Brit. J. Exptl Pathol., 27, 228-236. 

 Finney, D. J. 1947. "Probit Analysis." Cambridge University Press, New York. 

 Francis, T. F., Jr. (ed.). 1948. "Diagnostic Procedures for Virus and Rickettsial 



Diseases," pp. 228-233. American Public Health Association, New York. 

 Greiff, D., and H. Pinkerton. 1951. Rickettsiostasis in fertile eggs from use of 



antibiotic residues in poultry feeds. Proc. Soc. Exptl. Biol. Med., 78, 690-692. 

 Hartman, F. W., F. L. Horsfall, Jr., and J. G. Kidd (eds.). 1954. "The Dynamics 



of Virus and Rickettsial Infections," chap. 6. The Blakiston Division, McGraw- 

 Hill Book Company, Inc., New York. 

 Hultin, J. v., and A. P. McKee. 1951. Fixation of "neutralized" influenza virus 



by susceptible cells. J. BacterioL, 63, 437-447. 

 Irwin, J. O., and E. A. Cheeseman. 1939. On an appropriate method of determining 



the median effective dose and its error, in the case of a quantal response. /. Hyg., 



39. 574-58G. 



