The appendage that connects two bacterial cells together during conjugation.
The conjugation tube is in fact the pilus formed from the donor bacterial cell. It attaches to the surface of the recipient bacterial cell thereby bringing the two cells in contact with each other. With their cell to cell contact, a single strand of DNA from the plasmid of the donor cell is transferred to the recipient cell. Both of them replicate the strand to reform their own plasmid.
Word origin: conjugation + tube.
See also: bacterial conjugation