Now we come to the third winning question about Microcosm. Kenatiod writes,

Long ago, in bacteriology class, the teacher (an ex-nun at an ex-Catholic college) was telling us about the type “F” pili that are used to pass DNA so coli can have sex. One of the students asked “Why do they call them type F?” The teacher started to answer, but stopped, and then she turned bright red. The class start laughing, and then she did as well, and then someone asked, “What other kinds of pili are there?” She pulled herself together, said “Thank you” and class continued.

I would like to know both the answer to the original question, and also when in evolutionary history these