Viruses Can Be Cloned and Counted in Plaque Assays

The number of infectious viral particles in a sample can be quantified by a plaque assay. This assay is performed by cul-turing a dilute sample of viral particles on a plate covered with host cells and then counting the number of local lesions, called plaques, that develop (Figure 4-39). A plaque develops on the plate wherever a single virion initially infects a single cell. The virus replicates in this initial host cell and then lyses (ruptures) the cell, releasing many progeny virions that infect the neighboring cells on the plate. After a few such cycles of infection, enough cells are lysed to pro

M EXPERIMENTAL FIGURE 4-39 Plaque assay determines the number of infectious particles in a viral suspension.

(a) Each lesion, or plaque, which develops where a single virion initially infected a single cell, constitutes a pure viral clone.

(b) Plate illuminated from behind shows plaques formed by bacteriophage X plated on E. coli. (c) Plate showing plaques produced by poliovirus plated on HeLa cells. [Part (b) courtesy of Barbara Morris; part (c) from S. E. Luria et al., 1978, General Virology, 3d ed., Wiley, p. 26.]

Plaque Assay Plant Cells

duce a visible clear area, or plaque, in the layer of remaining uninfected cells.

Since all the progeny virions in a plaque are derived from a single parental virus, they constitute a virus clone. This type of plaque assay is in standard use for bacterial and animal viruses. Plant viruses can be assayed similarly by counting local lesions on plant leaves inoculated with viruses. Analysis of viral mutants, which are commonly isolated by plaque assays, has contributed extensively to current understanding of molecular cellular processes. The plaque assay also is critical in isolating bacteriophage X clones carrying segments of cellular DNA, as discussed in Chapter 9.

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