Before revision: Burgeoning interest in the area of bacteria powered bio-hybrid micro-robotic systems motivates a fundamental question on how one achieves optimal cargo-carrying capacity in such systems. Here we demonstrate a process to attach oil-droplet cargos to cell bodies of flagellated single bacteria which allowed us to probe cargo-carrying capacity of single bacteria over cargo sizes as large as 200 times a single cell volume. The most remarkable finding from our studies is that , for a given cargo size, the swim-speed is maximized when the rotational drag coefficient of the flagella is tuned to the torque-speed  characteristics of the bacterial flagellar motor .
After revision: In a classic paper, Edward Purcell analysed the dynamics of flagellated bacterial swimmers and derived a geometrical relationship which optimizes the propulsion efficiency. Experimental measurements for wild-type bacterial species E. coli have revealed that they closely satisfy this geometric optimality. However, the dependence of the flagellar motor speed on the load and more generally the role of the torque-speed characteristics of the bacterial flagellar motor .