Characteristics and Function of Thick and Thin Conjugative Pili Determined by Transfer-derepressed Plasmids of Incompatibility Groups I1, I2, I5, B, K and Z

Abstract

Eleven transfer derepressed plasmids from incompatibility groups I1, I5, B, K and Z were constructed using the dnaG3 mutant Escherichia coli strain BW86. All were found to determine thin flexible and thick rigid pili constitutively. Immune EM was used to relate thick and thin pilus serotypes with incompatibility grouping. Mutant plasmids that determined only thick pili constitutively transferred efficiently on an agar surface but not in a liquid. Plasmids with both kinds of pili transferred equally well in both environments. A mutant of the IncI2 plasmid R721 determined thin pili constitutively, and thick pili at a repressed level, as indicated by EM. Apparently, thin pili were not involved directly in conjugation but were only used to stabilize mating aggregates.