Harnessing the power of bacteria

3 March 2020

Theoretical physicists from Oxford’s Department of Physics have made a step forward in harnessing the power of bacteria.

Bacteria are microscopic engines that convert chemical energy to motion. They form a significant proportion of the total biomass on earth, exceeding that of all plants and animals combined. This raises the possibility of exploiting them to perform useful work: to move particles around and deliver small sized cargoes. The challenge is that in nature most bacterial colonies exhibit chaotic and disorganised patterns of motion, and they need to be organised to move along well defined tracks to be useful as machines. In a paper published today, Oxford’s Kristian Thijssen (pictured above), Amin Doostmohammadi and Julia Yeomans, collaborating with colleagues in the USA, have used ‘living liquid crystals’ to focus bacteria into jets.

Transporting microscopic cargoes

Liquid crystals are rod-like molecules that can be easily organised into complicated patterns: this is the physics underlying liquid crystal displays (LCDs) for televisions, monitors and mobile phones. Now, in the study published in Nature Physics, the team of scientists show that combining living bacteria with a smart structured liquid crystal can cause a bacterial colony to condense into well-defined tracks. To do this, a water-based liquid crystal is put into an experimental cell and made to form a series of fixed adjacent U-shapes by patterning the top and bottom of the cell. Bacteria Bacillus subtilis are then added to the cell. They are quite happy swimming in the liquid crystalline fluid, and they much prefer to swim in the direction of the long axes of the liquid crystal molecules. This means that they are directed away from the base of the U-shapes, towards the points, where they form dense lanes. Strikingly, within these pre-designed lanes bacteria flow like a jet of fluid that pumps itself. Experiments reported in the paper show that microscopic cargoes introduced into the lanes can be transported unidirectionally by the bacterial jets.

This ability of a pre-patterned liquid crystalline medium to streamline the chaotic movements of swimming bacteria into jets that can carry cargo along a predesigned trajectory opens the door for potential applications in cell sorting, microscale delivery and soft micro-robotics.

Polar jets of swimming bacteria condensed by a patterned liquid crystal, Nature Physics, 2 March 2020