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The bacterial flagellar motor of Escherichia coli is a nanoscale rotary engine essential for bacterial propulsion. Studies on the power output of single motors rely on the measurement of motor torque and rotation under external load. Here, we investigate the use of magnetic tweezers, which in principle allow the application and active control of a calibrated load torque, to study single flagellar motors in Escherichia coli. We manipulate the external load on the motor by adjusting the magnetic field experienced by a magnetic bead linked to the motor, and we probe the motor's response. A simple model describes the average motor speed over the entire range of applied fields. We extract the motor torque at stall and find it to be similar to the motor torque at drag-limited speed. In addition, use of the magnetic tweezers allows us to force motor rotation in both forward and backward directions. We monitor the motor's performance before and after periods of forced rotation and observe no destructive effects on the motor. Our experiments show how magnetic tweezers can provide active and fast control of the external load while also exposing remaining challenges in calibration. Through their non-invasive character and straightforward parallelization, magnetic tweezers provide an attractive platform to study nanoscale rotary motors at the single-motor level.

Includes abstract., Includes bibliographical references (leaves 241-268)., Alginate is a linear, un-branched polysaccharide of (1-4)-linked -D-mannuronate (M) and its C-5 epimer, -L-guluronate acid (G). These uronic acids are arranged in three different block types in the alginate polymer; poly-M, poly-G or poly-MG. Alginate lyases are enzymes that utilize a -elimination reaction to depolymerise the alginate polymer resulting in cleavage of the (1-4)-O-glycosidic linkage between monomers and the formation of an unsaturated uronic acid at the new non-reducing terminus. Alginate lyases have been isolated from a wide range of sources including marine invertebrates and marine bacteria, which often produce more than one alginate lyase enzyme. Haliotis midae is the commercially important abalone species found along the South African coast. Over-fishing and poaching of this species has led to a depletion of the naturally occurring populations and closure of the recreational and commercial fisheries. Abalone farming was initiated and has rapidly increased in response to the increasing demand for this delicacy. However, there are many problems associated with abalone aquaculture, the most significant being disease and the slow growth rates of the animals. The use of probiotics in abalone aquaculture is a potential solution to both of these problems.