Once assembled, flagella can work in different ways. The motor is bidirectional, and can rotate either clockwise or counterclockwise. Depending on the number and location of flagella on the cell (and the cell’s shape), this can push the cell, pull it, or give rise to even more complicated swimming behavior. Some bacterial species, like the Bdellovibrio bacteriovorus you just saw, are monotrichous (“single haired”), with one flagellum located at one pole to push/pull the cell. Other species, like the Campylobacter jejuni here, have bipolar flagella, one at each end. Still others are lophotrichous (“crest-haired”), with a clump of flagella (⇩).
Many lophotrichous species, like the Hylemonella gracilis you saw in Chapter 3 or this Helicobacter pylori, have a tuft of flagella at their cell pole. In some species, though, the tuft is located elsewhere; for example, a clump of flagella on the concave side of banana-shaped Selenomonas artemidis pushes the cells sideways in a seesawing swimming pattern.
