Morphological Features of the Pili Associated with Escherichia coli K12 Carrying R Factors or the F Factor

SUMMARY: Strains of Escherichia coli K12 carrying various R factors or the F factor were examined with the electron microscope. The F specific RNA phage MS 2 was added to differentiate the specific F type pili, which adsorbed the phage, from common pili, which did not. Even when phage was not added it was possible to identify F type pili because they were wider and generally longer than common pili. F type pili, but never common pili, often possessed terminal knobs whose presence on attached pili has not been previously described. The F type pili formed by E. coli carrying the different R factors or HfrH could not be morphologically distinguished from each other. Only one morphological type of common pilus was observed.