Temperate and virulent forms of phage theta attacking Bacillus licheniformis.

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Clear-plaque phage ϑc, attacking bacitracin-producing strains of B. licheniformis, yields spontaneous temperate mutants at high frequency; the temperate mutants fall into several classes phenotypically different in plaque morphology and properties of lysogenised bacteria. The most common phenotype ϑ3 has DNA restriction fragment patterns identical with those of the parent ϑc; some less common temperate forms, i.e. ϑ1 and ϑ2, produce different restriction fragment patterns, sugesting that a part of the original ϑc DNA has been reorganized or replaced by some foreign genetic material. The changed fragment pattern remains stable upon subsequent passaging of the phage or of the lysogenic bacteria. Neither class of temperate phage mutants gives clearplaque revertants at measurable frequency. Lysogenisation of bacteria with any class of temperate phage confers immunity to all temperate forms and to ϑc; virulent mutants ϑvir, which plate with 100% efficiency on lysogens for ϑ1 and ϑ2 but not for ϑ3, occur in stocks of ϑc at a frequency of 10−7. The mutation from ϑc to ϑvir is not accompanied by any change of the restriction fragment patterns of DNA.