Bacterial swimming efficiency measured in optical trap
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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