Bacteriophage Ecology Group

Bacteriophage Ecology Group Bacteriophage Ecology Group

Plaque-Forming Unit (PFU)


Entity that can give rise to a single, localized clearing on a bacterial lawn.

A plaque-forming unit, that is, is something that gives rise to a single lawn clearing upon plating, whether a free phage, a phage-infected bacterium, a phage-infected bacterial arrangement, or phage-infected clumping of bacteria.

An infected bacterium can be productively infected upon plating or, instead, can be a lysogen. Clumps of bacteria furthermore also can serve as plaque-forming units whether individually infected by one phage or many, or even whether a single bacterium or many are phage infected (contrast, though, multiple phage infection of such cellular arrangements or microcolonies with multiple infection of one or more individual bacteria making up these clumps). Clumps of phage virions also can serve as individual plaque-forming units. Key is that a plaque-forming unit inherently cannot be distinguished in the course of plating into more than one plaque.

Infective center as well as infectious center serve as synonyms, though those terms tend to be less broadly used than PFU, i.e., are particularly used within the context of keeping track of phage titers during single-step growth experiments. PFU, by contrast, is used as a standard means of phage quantification. See, however, efficiency of plating. See also serial dilution

Plaque-forming units (PFUs) are equivalent in concept to colony-forming units (CFUs) when plating bacteria, that is, where a single bacterium, bacterial arrangement, or clump of bacteria all can give rise to only a single colony upon plating.

From Gill and Young (2011), p. 403, "Analogous to the concept of a colony forming unit (CFU) in bacteriology. As a single virion initiates the formation of a single plaque, enumeration of PFU allows quantification of the number of phages in a given sample. Note that, as is the case with CFU, the number of PFU reflects the number of phages able to form plaques in a sample, not the total number of virions."


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