FIG 2.

Double-overlay plaque assay. A combination of amplified phage and host bacterium in cooled, molten agar supplemented with divalent cations (e.g., CaCl₂, MgSO₄) is poured over solid agar and the medium is incubated overnight. Quantifiable clearings (plaques) in the bacterial lawn indicate the presence of infectious phage. At least two different plaque morphologies are observed in the example photograph, illustrating an experiment involving coculture of Salmonella enterica serotype Typhimurium and phages obtained from municipal sewage. (© Mayo Clinic).