VIROLOGICAL METHODS 283 



active virus, while some soluble antigens apparentl}^ are not concentrated 

 by this method. Perfect concentration is probably never achieved, 

 since the law of mass action comes into play, leaving a certain percentage 

 unadsorbed or adsorbed and eluted. In the attempt to determine the 

 amount of concentration achieved, a sample of the original fluid, the 

 fluid from which the Adrus was adsorbed, and the concentrated eluate 

 should be examined. If the sample of original fluid is chilled, heated, 

 and centrifuged along with the specimen being concentrated, some con- 

 clusion may be drawn as to the extent of virus inactivation caused by the 

 procedure itself. The sum of the unadsorbed virus and the adsorbed 

 and eluted virus should equal the original fluid control. Some potential 

 errors arise in attempting to make this determination. The unadsorbed 

 virus in the supernatant fluid would be tested at relatively low dilutions 

 in anticipation of a low titer. The concentrated virus, if started at the 

 same low dilutions, will appear to have a greater titer than possible. 

 This is probably due to excess virus carried as a film on the inside of the 

 pipet which is gradually washed out during successive serial transfers. 

 This can be obviated by starting the concentrated virus off at high dilu- 

 tions. The amount of deviation permitted by the coarseness of the test 

 used to quantitate the virus makes precise appraisals almost impossible. 

 As long as exact titers are not taken too literally, this procedure, as 

 well as those described above, is useful in concentrating virus. 



Chemical methods of precipitation used alone or in combination with 

 centrifugation are sufficiently diverse that original articles should be 

 consulted to learn about them (Warren, 1950; Sharp, 1953). 



STORAGE 



A number of methods are available for storing viruses and rickettsiae. 

 The optimum method varies with the agent, equipment available, and 

 convenience. 



One of the most satisfactory methods involves lyophilization, and 

 this method is most commonly employed for long-term storage, such as 

 periods of over a year. Generally speaking, the method of l3^ophilization 

 used for viruses is the same as that used for bacteria discussed elsewhere. 

 Viruses and particularly rickettsiae may be stored frozen on solid carbon 

 dioxide for variable periods up to a year and beyond. Any insulated 

 container, such as a silvered-glass vacuum jug (thermos), that will delay 

 the dry-ice evaporation is satisfactory. Aside from arranging for a 

 reliable and more or less continuous supply of dry ice this method is 

 quite convenient. Most of the more common viruses store well in the 

 deep-freeze boxes if the temperature is maintained between —20 and 

 — 40°C. Inasmuch as they are electrically controlled, the preservation 



