Cargo carrying capacity of a single bacterium

In recent times, several groups have demonstrated bio-hybrid micro-robots where bacteria are attached to “cargo” such as polystyrene beads, which are transported using the power generated by bacterial motility. A system consisting of a single bacterium pulling a load is important for a clear understanding of such bacteria powered devices. Previous reports of cargo delivery by single bacteria have relied on attaching the cells to very small particles (~ 1 micron dia.) so that statistically only one cell can bind due to steric restriction. Such an approach is not effective, as dynamics of a single cell carrying a large load, say ~ 10 micron dia., cannot be studied. Systems describing single bacterium carrying such large size cargo have not been described so far to the best of our knowledge. In this talk, we ask a very general question, namely, what is the maximum load of a spherical cargo which can be transported by a flagellated swimmer at a specified speed. We present a technique to attach single loads as large as 12 micron diameter to micron sized single bacteria. Further, results from our experiments and theoretical analysis suggests the crucial role of tuning the flagellar geometry and the torque-speed characteristics of the flagellar motor to maximize cargo carrying capacity.