Most motile bacteria are equipped with multiple helical flagella, slender appendages whose rotation in viscous fluids allow the cells to self-propel. We highlight in this talk three consequences of hydrodynamics for bacteria. First we show how interactions between flagella mediated by the fluid govern the ability of the cells to reorient. We next show how the swimming of bacteria with multiple flagella is enabled by an elastohydrodynamic instability. We finally demonstrate how the flagellar flows induced by bacteria which have differentiated to a swarming state are responsible for large-scale fluid circulation at the scale of the whole swarm.