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Origins of electron microscopy and viral diagnosis.

The electron microscope (EM) was developed in Germany during the 1930s to visualise objects too small to be resolved clearly by the light microscope'; viruses were among the first objects to be seen with it. Stanley and Anderson2 published pictures of unstained tobacco mosaic virus and other plant viruses in the early 1940s but the contrast was poor and no fine detail was revealed. No further progress in unveiling virus morphology was made until Brenner and Horne3 showed that salts of some heavy metals would form an electron dense amorphous "glass" around viruses, making their fine structure visible by negative contrast (negative stain). The method of preparation was elaborate at first but has since been refined to mixing a stain with a suitably dilute preparation of virus, placing the mixture on a formvar coated EM grid, drawing off the surplus mixture, and allowing it to dry. This extremely simple method has made it possible to visualise most viruses provided that the concentration was higher than about 106 viruses per millilitre.4 In the 10 years or so after Brenner and Horne's paper, micrographs of one virus after another were published and these studies confirmed two important facts. First, although preparation might include ultracentrifugation at 100 000 xg, treatment with various stains, drying, and irradiation in the microscope, the morphology revealed by electron microscopy was very consistent and could be used reliably to identify individual structural groups. Second, morphology was closely linked to other physicochemical properties-for example, all viruses with a herpes morphology contain double stranded DNA of a generally similar size and genomic organisation. Although some scientists maintained that all EM images were artefacts, they were consistent enough possibly to be useful in diagnosing viral infections.5