Basic characterization of W-pili.

The bacterial drug-resistance plasmids Sa, R388 and R7K that comprise the W compatability group were transferred by conjugation to species in the genera Escherichia, Salmonella, Shigella and Pseudomonas, 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 pila/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 Salmonella typhimurium.