Isolation of four phenotypic classes of transfer-proficient, pilus-specific-bacteriophage-resistant mutants of an F-like plasmid of Escherichia coli K12

Summary 173 independent mutants of phenotype Tra⁺ MS2^R were isolated from the derepressed F-like R-plasmid, R192-7, of Escherichia coli K12. Various tests of transfer frequency and phage infectivity of 21 of these mutants grouped them into four clearly distinct phenotypic classes. Class (1): mutants of high transfer frequency proficient in MS2 adsorption but to various extents defective in phage penetration, showing resistance or partial sensitivity to both MS2 and M13 equally. Clss (2): mutants of high transfer frequency which fail to adsorb MS2, resistant to both MS2 and M13. Class (3): mutants of reduced transfer frequency, approximately 10^-2 of that of R192-7, resistant to both MS2 and M13. Class (4): mutants of low transfer frequency, approximately 10^-3 to 10^-4 of that of R192-7 and resembling that of the repressed wild type R192, resistant to both MS2 and M13. Pili of mutants of classes (1) and (2) showed structural abnoramlities in electron microscopy.