Plaques are clear zones formed in a lawn of cells due to lysis by phage. At a low multiplicity of infection (MOI) a cell is infected with a single phage and lysed, releasing progeny phage which can diffuse to neighboring cells and infect them, lysing these cells then infecting the neighboring cells and lysing them, etc, ultimately resulting in a circular area of cell lysis in a turbid lawn of cells.
An example of plaques formed by phage lambda is shown below.
The morphology of the plaque depends upon the phage, the host, and the growth conditions. Usually phage infection is studied in a layer of soft agar (or "top agar") which allows the phage to diffuse rapidly. The size of the plaque is proportional to the efficiency of adsorption, the length of the latent period, and the burst size of the phage. A diversity of plaque sizes can result if the phage infect cells at different times during the bacterial growth phase: phage that adsorb early make larger plaques than those that adsorb later. To avoid this, phage are often preadsorbed to cells under conditions that do not allow phage growth (e.g. low temperature) then shifted to permissive conditions to induce all of the phage to develop at the same time. A clear plaque will be formed if the host is completely susceptible to the phage. (Often a clear plaque will be slightly turbid at the edge because the cells at the edge of the plaque are not yet fully lysed.) If the host is partially resistant to the phage then the plaque may be uniformly turbid (for example, if 10% of the cells survive phage infection).
Why do plaque assays accurately measure the titer of temperate phage? If a phage lysogenized a host cell immediately upon infection, it would never form a plaque. Instead, when temperate phage infects a population of exponentially growing cells, each phage produces a plaque with a "bulls-eye" plaque morphology, a turbid center surrounded by a ring of clearing. This characteristic plaque morphology is due to the role of the MOI and cell physiology on the lysis-lysogeny decision. Lytic growth is favored when cells are growing rapidly and the MOI is low. Lysogeny is favored when cells are growing slowly and the MOI is high. This is why temperate phage typically have plaques with turbid centers. When the cells are initially infected with phage the ratio of phage to cells is usually about 1 phage per 106 cells. Initially the nutrients are plentiful so the bacteria grow rapidly and, since the MOI is low, the phage grow lytically. After several lytic cycles the local MOI increases and most of the cells are lysed, producing a plaque in the lawn of cells. As the cell lawn becomes saturated, the rate of cell growth slows down and, since lysis requires rapid metabolism, the plaque stops increasing in size. However, any lysogens that formed in the center of the plaque are immune to lysis and can continue to grow since they do not have to compete with nearby cells for nutrients. Thus, lysogens begin to grow in the center of the plaque, giving the plaque a turbid, bulls-eye appearence.
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Last modified July 15, 2002