Sisyphus at the Nanoscale: Bacterial Topotaxis in a Microfabricated Environment*

The ballistic-like motion of self-propelled particles at low-Reynolds number can be exploited to influence their direction of motion. In particular, it has been demonstrated that by using the right topology (in this case a micro-fabricated array of funnel-like asymmetrical barriers), {\it E.coli} bacteria can be ``pumped'' between two adjacent regions (Galajda 2007, Wan 2008). We built upon this idea and developed a micro-habitat array in which chemotaxis and topotaxis --nutrient- and topology-driven motion, respectively-- are in opposition, leading to an inherently unstable environment in which a bacterium is constantly pushed away from the fitness landscape's summit in a Sisyphean fashion. Surprisingly, we find that the bacterial population as a whole is able to overcome the rectifying array. An in-depth microscopic analysis of the swimmer's motion is used to quantify the strategies adopted by the bacteria.