Biomedical Engineering Reference
In-Depth Information
6.3.12 Gliding on Small Etched Patterns
To obtain information about the movement of the legs, we made a series of
small steps, ranging from 50 to 1000nm by lithography, which is generally used
to make silicon and DNA chips, and observed the behavior of
M. mobile
cells
at each step [7]. The cells were able to climb over the 50nm step normally, but
the number of cells able to scale the step decreased as steps became higher.
The critical height was around 400nm, which is higher than the cell itself. As
far as stepping down, when the cells came to a cliff edge, they tended to turn
and go along the edge. We also etched a small groove with a tapered end.
M. mobile
glided to the end of the groove and became stuck for a moment,
but did not reverse its direction. If gliding occurs by the repeated movement
of rigid sticky legs, reversing the direction of a cell's movement when gliding
through such a groove might happen by chance when unusual binding of the
structures occurs. Why does
M. mobile
never reverse? As suggested from the
molecular shape of the Gli349 protein, the legs have flexible parts that pull
rather than push the cell body of the
Mycoplasma
. When
Mycoplasma
was
observed gliding on a red blood cell (RBC) by interference microscopy, the
RBC membrane seemed to be pulled to the direction of
Mycoplasma
[60]. The
pulling character of the legs may also be suggested from this observation.
6.3.13 Mechanical Characteristics of Gliding and Binding
Temperature dependence was analyzed for glass binding and gliding speed.
The gliding speed changed linearly as a function of temperature from 4 to
40
◦
C (see Figure 6.12A) [44], as observed for conventional motor proteins
and the bacterial flagella motor [61, 62, 63]. This suggests that the protein
movements depend on the heat wobble of water. Similarly, the number of
Mycoplasma
cells bound to glass decreased linearly with temperature (see
Figure 6.12B). This would also suggest that the stability of the bond between
sialic acid and the leg protein depends on the heat wobble of water.
The mechanical characteristics of motility are a crucial piece of information
about the mechanism. We therefore measured the force velocity relationship
by applying a load to a bead attached to the tail of a
M. mobile
cell, using one
of the two methods shown in Figure 6.13A [44]. The first method employs a
controlled liquid flow. Because
M. mobile
is reoriented upstream when a flow
of liquid is applied, the load applied to
M. mobile
(or rather to the bead) can
be calculated from Stoke's law. Another method, the “optical tweezer” (laser
trap) method, utilizes the fact that a strong beam applied to an object with a
mismatched refractive index traps the object at the center of the beam. The
lateral force generated on the object (the bead carried by a
Mycoplasma
in
this case) can be calculated from the bias between the centers of the laser
and the object. Because the liquid flow washed all gliding cells from the glass
surface, we could not measure the maximal (stall) force using the first method.
Therefore, the optical tweezer method was used to measure the maximal (stall)
