Pili are important for long-range attachment or binding. Perhaps the most famous pilus is the sex pilus encoded by the F-plasmid. The DNA does not travel through the pilus. Bacterial Pili involved in pathogenesis are generally rigid, rod-like appendages, composed mainly of approximately 1000 copies of a single ca. Most E. coli have what is called common pili whose adherence is blocked by D-mannose.
Adherence is monitored in the laboratory by hemagglutination of red blood cells.
Thus, adherence which is mediated by Type I pili can be examined as mannose-sensitive hemagglutination. Alternatively, there are pili which show mannose resistant hemagglutination. Pili can be classified by what species of erythrocytes they will agglutinate and by which sugars inhibit hemagglutination. This is particularly important to the field of bacterial pathogenesis as most, if not all, effector molecules of pathogenesis are secreted. The only secretion pathway necessary for cell viability.