Physical Assay of Purified Viruses, Particularly the Small Ones

The assay of virus in water is a matter of present concern to those who seek to preserve a healthy environment. Bioassay, in terms of plaque forming units, can be made more meaningful if supplemented by physical assay of both total number and state of aggregation of the virions in suspension but most methods for preparation of suspended particles for electron microscopy induce aggregation. The drying process is usually responsible for this so it is desirable to make the preparation for microscopy without drying any liquid containing virus upon the preparation to be examined. This can be done by sedimentation methods already in use but these become difficult and cumbersome for very small viruses, particularly those of the enteric group usually found in water. However, it is just these that diffuse most rapidly and the excellent paper of Valentine and Allison (1) demonstrated that the rate of attachment of freely diffusing virions to an aluminized surface is accurately predictable if certain conditions are controlled. Figure 1 shows a simple device for doing this.