F pilus

The F pilus is also known as a conjugation bridge. It's a long, narrow structure which protrudes from the surface of the "male" bacterium and connects the "male" to the "female" bacterium during the process of bacterial conjugation, which occurs when genetic material in the form of the F plasmid is swapped between two bacterial cells.

In other words, it's roughly the microbial equivalent of a penis.

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From the science dictionary at http://biotech.icmb.utexas.edu/

Unfortunately, once the F plasmid has been transferred, the recipient becomes "male" as well. This is a bit of a bummer, because bacteria carrying the F plasmid are more susceptible to bacteriophage attack. There's also no homosexuality here - the F plasmid encodes coat proteins that prevent the F pilus from attaching to the carier.

In other words, it's less roughly the microbial equivalent of a penis, except that everyone you have sex with turns into a man afterwards and you're not able to be gay and if you have one you give off a scent that causes dogs to come and attack you.

Thankfully, bacteria aren't intelligent enough to be severely screwed up by this.