Almost all spirillum, half of all rods, and rarely cocci are motile via flagella. You can wave your hands a little to justify why the coccus morphology and motility are mutually exclusive. A spherical shape would probably cause much more drag on the cell than a rod, so if you are going to be motile, be a streamlined rod. Structure Flagella can be thought of as little whips that are free at one end and attached to a cell at the other. Due to their small diameter, flagella cannot be seen in the light microscope unless a special stain is applied.
Bacteria can have one or more flagella arranged in clumps or spread all over the cell.
The number of units, the wavelength and diameter of a single helix of the flagella are determined by the protein subunits. Below is a picture of a common flagella in a Gram negative bacteria. Especially important in disease causing bacteria are the structures that give the bacteria the ability to attach to the indended victim. Fimbriae and Pili, a proteinaceous surface structures on the bacteria, are critical in this process. Fimbriae Fimbriae are shorter and straighter than flagella and are more numerous.