Abstract
Summary
1) The number of infectious units of measles virus in a suspension were assayed by counting microscopically the number of immunofluorescent foci in infected coverslip tissue cultures, after they were incubated under methyl cellulose overlay. The overlay prevents secondary spread of virus and is removed before the infected cell sheets are treated with antimeasles serum and fluorescein-labeled antiglobulin for detection of virus antigen by the indirect fluorescent antibody procedures. 2) Aliquots of stock virus suspension were serially diluted and assayed separately by this procedure. Results of independent assays agreed well. Ratios between fluorescent focus counts on successive dilutions approximated the reciprocal of dilution factor and there appeared to be a linear relation between amount of virus inoculated and count. In comparison with a virus titration in monolayers of same cell in tubes read for endpoint cytopathogenicity, fluorescent focus count assay of same stock virus had the same advantage as plaque technic. Data were consistent with the hypothesis that one infectious particle of measles virus is sufficient to initiate infection. 3) The method seems useful for investigation of latent infection, incomplete virus multiplication, non-plaque-forming variants of plaque-forming viruses, or other cell-virus systems in which virus antigen is produced but plaques are absent.
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