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  • Bacterial swimming efficiency measured in optical trap

Nov 2006
Lauren I. Rugani

Escherichia coli swim from site to site in search of food, using several helical flagella (typically a few microns in length and 20 nm in diameter) for propulsion. Although there is significant data on swimming bacteria, including average speeds and torques, other important parameters such as translational and rotational drag coefficients have been difficult to measure for intact cells. Because these parameters are necessary for quantitative understanding of bacterial propulsion, a team of researchers at the University of Pittsburgh and at Pennsylvania State University in Erie employed optical trapping to examine the fundamental swimming properties of intact E. coli cells.

In particular, the team measured the thrust force required for the swimming motion of the bacteria and measured the angular velocity of both the flagellar bundle and the cell body and related them to an applied external flow. “Our method gives direct measurement on intact cells,” researcher Suddhashil Chattopadhyay said. “It provides most of the dynamical quantities related to bacterial swimming.”

A swimming bacterium is held in an optical trap near a surface. In the absence of imposed flow, the thrust force (Fthrust) of the bacterium is balanced by the trap force (Ftrap). The thrust force is generated by rotation of the flagellar bundle (ω). The torque generated is balanced by the counterrotation of the cell body (Ω).

The researchers used a 1064-nm IR laser from Photop Technologies to trap bacteria along the swimming direction. They found that the leading tip of the cell was caught in the trap when the speed of the flow was greater than the swimming speed of the bacterium. When its swimming velocity exceeded the speed of the flow, however, the bacterium was held at the trailing tip of the cell body. Using a two-dimensional position-sensitive detector from Pacific Silicon Sensor of Westlake Village, Calif., the team measured the trapping force from the position of the trapped cell relative to the trap center.

Using video microscopy, the scientists measured the dimensions of the cell body in the trap. In more than 200 bacteria measured, they found that the rotational frequency of the cell body decreased and that the flagellar rotation frequency increased as the cell body got longer. The rotational drag coefficient, dependent on cell body size, was much smaller for the flagella than for the cell body, while the translational drag coefficients of the two were found to be about equal and did not depend on size.

Although these findings were consistent among the bacteria, the researchers found the actual values to vary greatly among individual cells, even those grown from the same colony. Differences in structure and time-dependent changes of the flagella are possible explanations for the variations because the bacteria were in different stages of growth when the measurements were taken. The investigators also inferred incorporation of additional flagella, which is consistent with the increase in power, as the cell body lengthened. The greatest propulsion efficiency, or the ratio of the propulsive output power to the rotary input power, is achieved when the linear drag of the cell body and flagellar bundle balance each other.

Direct force measurements by optical trapping allowed the researchers to determine the contribution of the flagellar bundle to the total drag. They used these measurements to determine the microscopic properties of the flagellar bundle, which they found to be consistent with previous data and theoretical estimations.

“The various dynamical quantities can now be measured accurately and conveniently. From a theoretical perspective, it will now be possible to prove or disprove present theories on propulsion by a helical propeller,” Chattopadhyay remarked.

PNAS, Sept. 12, 2006, pp. 13712-13717.

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