Detection of Bacteriophages: Phage Plaques Stephen T. Abedon Contents Introduction ...................................................................................... 508 Plaques ........................................................................................... 509 Initiating Plaques ............................................................................. 509 Bacterial Lawns, Plaques, and Spots ........................................................ 515 Plaque-Forming Units ....................................................................... 516 Too Many or Too Few Plaques .............................................................. 517 Plaques Versus Spots ............................................................................ 519 Plaque Formation ................................................................................ 522 Phage Clumped Dispersion .................................................................. 522 Bacterial Clumped Dispersion ............................................................... 526 Plaque Size ...................................................................................... 528 Plaque-Based Phage Characterization .......................................................... 531 Efficiency of Plating ......................................................................... 531 Efficiency of Center of Infection ............................................................ 532 Mixed-Indicator Technique .................................................................. 534 Conclusions ...................................................................................... 536 References ....................................................................................... 536 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 S. T. Abedon (*) Department of Microbiology, The Ohio State University, Mansfield, OH, USA e-mail: abedon.1@osu.edu © Springer Nature Switzerland AG 2021 D. R. Harper et al. (eds.), Bacteriophages, https://doi.org/10.1007/978-3-319-41986-2_16 507