is it Proven that Pili form the conjugation bridge in bacterial recombination?

Answer 1

Conjugation: transfer of genetic material between 2 cells that are temporarily joined.
a). The cell donating the DNA extends an external appendage called a sex pili.
b). This pili attaches to the cell receiving the DNA.
c). A cytoplasmic bridge forms which the DNA is transferred.

The ability for the sex pili to form is conferred by genes in a plasmid called the F factor. (fertility factor). Cells with the F factor are designated F+ and cells without it are designated F-. During conjugation between an F+ and an F- bacterium both cells finish up as F+ forms. Sometime the the F factor inserts into the circular chromosome. Integrated F factor is still expressed. It is designated Hfr. (high frequency of recombination).

Answer 2

Well, in experimental sciences it is very rare to "prove" things, but what has been observed is that specialized pili which are called "sex pili" form in bacterial conjugation, and so yes.