Sex pili and common pili in the conjugational transfer of colicin factor Ib bySalmonella typhimurium

The kinetics of spread of colicin factor Ib (colIb) incol− cultures ofS. typhimuriumwas studied. The rate of spread was greater withpil+ strains (those forming common pili) than withpil− strains. The difference reflects inefficient pairing betweenpil− cells, which donate or receivecolIbefficiently only when mated withpil+ cells. The donor function ofcolIbis known to be repressed a few generations after it is acquired by acol− cell. While donor ability is manifest, a new type of pilus (the ‘Ib pilus’) is formed which is morphologically distinct from common pili and other sex pili such as that determined by the F factor. The Ib pilus is presumably involved in the transfer ofcolIbby conjugation.