Magnetotactic bacteria have a built-in compass, in the form of a magnetosome chain made up of magnetic biominerals, that allows them to passively align along terrestrial magnetic field lines. They also sense oxygen gradients and swim using at least one flagellum. Hence, these bacteria are self-propelled active matter capable of displaying flocking behavior. This Colloquium explains the physics behind these various capabilities, as well as their interactions and biological significance.

Artificially engineered mechanical systems, sometimes called metamaterials, offer many promising applications on length scales ranging from macroscopic systems to the nanoscale. A topic of particular interest is the existence of topologically protected phononic edge states in such systems that are analogous to the electronic edge states that give rise to the quantum Hall effect. This Colloquium gives an introduction to topologically protected transport in metamaterials and its applications for controlling acoustic transport.