Flagellum

A flagellum (plural flagella) is a whip-like appendage that is much longer than the cell, but also much thinner. While a Gram stain won't stain the flagella, a silver dye can be used to stain the flagella so that it is visible under the light microscope.
A flagella is used for motility; it can be used to swim throughout the human body.
A prokaryote has a filled flagella, but an eukaryote has a composite flagella, made of 9 microtubule doublets and two central single microtubules. This is referred to as a 9+2 structure.
10% of all bacteria have flagella, but about 50% of E. coli have flagella. Flagella is a trait that makes a bacteria virulent (pathogenic).