Two Modes of ‘Curing’ Transmissible Bacterial Plasmids

SUMMARY: F'lac was efficiently eliminated by acridine orange from Escherichia coli K12, whether sex pilus synthesis was constitutive or repressed by an fi +R factor. R factors, F-like or I-like, repressed or derepressed. were not eliminated by acridine orange. Sodium dodecyl sulphate (SDS), known to eliminate certain plasmids determining constitutive synthesis of F pili, had no effect on cultures with wild-type (repressed) R factors. Bacteria with derepressed synthesis of pili, either F-like or I-like, but particularly the latter, showed increased sensitivity to SDS. SDS treatment selected from such cultures non-piliated cells, with lost or mutant R factors. Thus acridine orange was a true ‘curing’ agent, specific for F, whilst SDS acted only by selection of spontaneous variants.