Measure of phage density, typically presented in per-unit-volume units (e.g., phages per ml).
Titers usually are determined via plaque counts, though killing titer assays, for example, represent an alternative approach to titer determination.
Titers along with volumes of phage formulations should be viewed as the default means of expressing phage dosing when reporting phage therapy protocols. Imagine, in contrast, discussing drug dosing but not in terms of the number of units of drug applied! Titers, that is, are simply measures of units of phages per unit volume.
Multiplicity of infection, alternatively, is not an adequate descriptor of phage therapy dosing though nonetheless often serves as one in phage therapy studies. Imagine, however, the insanity of discussing antibiotic dosing in terms of ratios of antibiotic molecules to number of target bacteria! Titers, by contrast, not only represent a pharmacologically defensible dosing measure but also are a better of the phage potential to impact bacteria than do multiplicities of infection.
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