Further studies on the use of protein A in immune electron microscopy for detecting virus particles.

The immune electron microscopic technique which involves pre-coating electron microscope grids with protein A before coating them with the specific antiserum, has been found suitable for detecting isometric insect and plant viruses. With the three virus-antibody combinations tested, the optimum antiserum dilution for protein A plus antiserum treatment was found to be 1:100 or less, whereas in the case of grids treated with antiserum alone it ranged from 1:1000 to 1:2000 although the titre of the antisera ranged from 1:512 to 1:4096. The increase in the number of particles on grids treated with protein A plus antiserum over those treated with antiserum alone, at each optimal antiserum dilution, was 25.7-fold (sugarcane mosaic virus), 2.1-fold (tobacco mosaic virus) and 1.6- and 2.4-fold (Erysimum latent virus--two different sap dilutions). Protein A plus antiserum-coated grids can be stored for up to 6 months at 4 degrees C, but not at room temperature or in a desiccator, and still retain about 25% of their activity, sufficient to detect any virus using the electron microscope. Antisera preserved in glycerol can be used successfully for detecting viruses by immune electron microscopy.