Lytic phage that is unable to display lysogenic cycles.
The terms obligately lytic and professionally lytic can be used similarly to virulent, with "professionally lytic" having perhaps the narrower meaning, that of virulent or obligately lytic but excluding those phages that are directly derived from temperate phages, that is, phages that can be described as intemperate as well as virulent mutants.
Virulent phages along with obligately lytic phages thus, from this perspective, can be described as consisting of two non-overlapping subgroups of phages, virulent mutants on the one hand and professionally lytic phages on the other. This comes with the caveat, however, that the term professionally lytic might not be consistently used with such precision and that the term virulent phage also can be defined differently, in this case with a narrower meaning, i.e., as equivalent to virulent mutant.
As the concept of phage virulence has a somewhat tortured and ambiguous history, it is my preference to limit the use of the term "virulent" as applied to phages. Instead of virulent phage my preference is to use obligately lytic phage or, if appropriate, professionally lytic phage since otherwise it can become confusing to distinguish the potential for phages to destroy bacterial cultures (phage virulence), which is highly relevant to the practice of phage therapy, from the potential for phages to fail to display lysogeny (virulent phage). See also simply virulence.
The definition from Adams (1959) for "virulent phage" (p. 442) is "A phage that lacks the ability to lysogenize. When known to have originated by mutation from a temperate phage, it is called a virulent mutant." In the same monograph, p. 432, note this statement: "A virulent phage could thus be a virulent mutant of a yet unknown temperate phage." The term virulent phage, depending on context, thus can describe both temperate phage mutants, phages that have descended from temperate phages with numerous mutations separating them from an ability to display lysogenic cycles, and phages that effectively are unrelated to temperate phages.
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