Preservation of Viral Infectivity

In general, viruses are more sensitive than bacteria or fungi to inactivation by physical and chemical agents. A knowledge of their sensitivity to environmental conditions is therefore important for ensuring the preservation of the infectivity of viruses as reference reagents, and in clinical specimens collected for diagnosis, as well as for their deliberate inactivation for such practical ends as sterilization, disinfection, and the production of inactivated vaccines.


The principal environmental condition thai may adversely affect the infectivity ol viruses is temperature. Surface proteins are denatured within a few minutes at temperatures of 55°-60°C, with the result that the virion is no longer capable of normal cellular attachment and/or uncoating. At ambient temperature the rate of decay of infectivity is slower but significant, especially in the summer or in the tropics. To preserve infectivity, viral preparations must therefore be stored at low temperature; 4°C (wet ice or a refrigerator) is usually satisfactory for a day or so, but longer term preservation requires much lower temperatures. Two convenient temperatures are -70°C, the temperature of frozen C02 ("dry ice") and of some mechanical freezers, or - I96°C, the temperature of liquid nitrogen. As a rule of thumb, the half-life of most viruses can be measured in seconds at 60°C, minutes at 37X1, hours at 20°C, days at 4°C, and years at -70°C or lower. The enveloped viruses are more heat-labile than nonenveloped viruses. Enveloped virions, notably those of respiratory syncytial virus, are also susceptible to repeated freezing and thawing, probably as a result of disruption of the virion by ice crystals. This poses problems in the collection and transportation of clinical specimens. The most practical way of avoiding such problems is to deliver specimens to the laboratory as rapidly as practicable, packed without freezing, on ice-cold gel packs.

In the laboratory, it is often necessary to preserve virus stocks for years

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