Isolation of the Nonviable Cells Produced During Normal Growth of Recombination-Deficient Strains of Escherichia coli K-12

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During normal growth, cultures of recombination-deficient (Rec−) strains contain a population of cells that do not form colonies. Such cells are not present in a culture of an isogenic Rec+ strain. We present a procedure for isolating and studying this defective population of cells. Exposure of a growing Rec− or Rec+ culture to low levels of penicillin causes the dividing cells to elongate. The size of the nonviable cells present in the Rec− cultures is unaffected. The nonviable cells are then separated from the elongated cells by velocity sedimentation. This isolation technique provides a convenient way of analyzing the composition, biosynthetic capacity, and enzymatic function of the nonviable cells before isolation. In this paper we present data showing that before fractionation the nonviable cells in the Rec− culture are defective in their ability to synthesize β-galactosidase, whereas the Rec− viable cells behave like the Rec+ cells in this regard. This observation confirms the existence of at least two classes of cells in liquid cultures of Rec− strains grown under normal conditions. That class of cells which is unable to synthesize β-galactosidase is the same class that cannot form colonies when plated on solid medium.

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