15 - INTRODUCTION TO QUANTITATIVE ASSAYS

Abstract

This chapter provides an overview of quantitative assays. The quantitative assay quantifies the number of virus particles in an inoculum. A bacterial colony count, in bacteriology, is an example of a quantitative assay. The plaque assay was introduced to animal virology in the 1950's, as a modification of the bacteriophage plaque assay. In this assay, virus is added to a cell monolayer and allowed to adsorb. The infectious center assay is a modification of the plaque assay. This assay measures the number of infected cells present, however, it does not measure the number of infectious virus particles. In an infectious center assay, a population of freshly infected cells is harvested and a single cell suspension is obtained. Transformation assays allow the quantification of infectious, noncytocidal transforming viruses. Certain oncogenic viruses, such as Rous sarcoma virus, do not cause cell death, but rather transform the cells they infect. The direct particle count utilizes the electron microscope. In this case, both infectious and non-infectious virus particles are counted because defective virus particles cannot always be distinguished by morphology alone. It is not surprising that for a given virus, different titers may be obtained for each assay system used. It should be noted that changing conditions within a single assay method can also alter the results obtained. Comparisons under similar conditions within a given assay system, however, are useful and valid.