SUMMARY: The bacterial drug-resistance plasmids Sa, R388 and R7K that comprise the W compatibility group were transferred by conjugation to species in the genera and , chosen for their lack of common pili. On receiving the W plasmids, all strains produced pili, which were similar morphologically, at average frequencies of up to 3·0 pili/cell. The pili determined by the W plasmids were also related serologically, but unrelated to those of other plasmids. R segregant strains which had lost their plasmids showed a simultaneous loss of both pili and drug-resistance characteristics.

W-pili are pointed flexible filaments 10 to 12 nm thick, with an average length of 450 nm. The best host found was


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