  Bacteria predate plants and animals by billions of years. Today, they are the
world's smallest cells yet they represent the bulk of the world's biomass, and
the main reservoir of nutrients for higher organisms. Most bacteria can move on
their own, and the majority of motile bacteria are able to swim in viscous
fluids using slender helical appendages called flagella. Low-Reynolds-number
hydrodynamics is at the heart of the ability of flagella to generate propulsion
at the micron scale. In fact, fluid dynamic forces impact many aspects of
bacteriology, ranging from the ability of cells to reorient and search their
surroundings to their interactions within mechanically and chemically-complex
environments. Using hydrodynamics as an organizing framework, we review the
biomechanics of bacterial motility and look ahead to future challenges.
