Detection of Bacteriophages: Phage Plaques

Abstract

Plaques are spatially constrained populations of bacteriophages that become visible to the eye as they locally deplete numbers of susceptible bacterial hosts. Plaques develop within what are known as “lawns” of bacteria, as grown either on or in solid or semi-solid media, media which typically is agar-based. These plaques, by definition, are initiated from an approximation of a point source, that is, usually from a single phage virion or instead from a phage-infected bacterium, what often collectively can be described as plaque-forming units or PFUs. These point sources then spread spherically to form circular “holes” of reduced turbidity, i.e., less cloudiness in the bacterial lawn. Phage plaques are important for at least four reasons. First, the process of their growth can vary in interesting ways, with differences in outcomes that are dependent on differences in phage and bacterial types along with differences in plaquing conditions. Second, phage plaques are the most readily accessible and common circumstance in which spatially structured phage growth is observed in the laboratory. As such, plaques can serve as first-approximation models for phage population growth within naturally occurring spatially structured bacterial populations. Third, phage plaques are a common means by which phage activity can be macroscopically observed for the sake of phage isolation, phage clonal purification, and phage enumeration. Four, phage plaques may be employed to biologically characterize phages such as in terms of their efficiency of plating or host range. Provided here is an overview of the biology of phage plaques and their formation.