The bacterial types (strains or species) that are susceptible to infection by a given phage.
Host ranges vary as a function of the definition of the term 'infection' which can range from phage adsorbable to non-productive infections (i.e., restrictive, abortive, or even lysogenic) to productive infections possessing varying degrees of infection vigor even among productive infections. Thus, for example, spot formation may be possible without productive infection whereas plaque formation can fail to occur even with productive infection. See, e.g., efficiency of plating and efficiency of center of infection.
The host range of phages typically is relatively narrow, in many cases encompassing just certain strains within a bacterial species though in other cases host range has been found to span multiple genera. Keep in mind, though, that host range breadth can vary depending on how one defines 'infection'. For additional consideration and references, see Hyman and Abedon (2010).
This is the definition from Adams (1959), p. 440, "The range of action of a phage measured in terms of the varieties of bacteria in which it can grow. Host range is often, but not always, determined by success or failure of adsorption."
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