Scientists have used light patterns to control the swimming
speed of bacteria and direct them to form different shapes, according to a new
study in the journal eLife.
Controlling bacteria in this way means it could be possible to
use them as microbricks for building the next generation of microscopic
devices. For example, they could be made to surround a larger object such as a
machine part or a drug carrier, and then used as living propellers to transport
it where it is needed.
Escherichia coli (E. coli) bacteria are known to be fantastic
swimmers. They can move a distance of ten times their length in a second. They
have propellers that are powered by a motor, and they usually recharge this
motor by a process that needs oxygen. Recently, scientists found a protein
(proteorhodopsin) in ocean-dwelling bacteria that allows them to power their
propellers using light. By engineering other types of bacteria to have this
protein, it is possible to place a 'solar panel' on every bacterial cell and
control its swimming speed remotely with light.
"Much like pedestrians who slow down their walking speed
when they encounter a crowd, or cars that are stuck in traffic, swimming
bacteria will spend more time in slower regions than in faster ones,"
explains lead author Giacomo Frangipane, Postdoctoral Scientist at Rome
University, Italy. "We wanted to exploit this phenomenon to see if we
could shape the concentration of bacteria using light."
To do this, Frangipane and his team sent light from a projector
through a microscope lens, shaping the light with high resolution, and explored
how E. coli bacteria alter their speed while swimming through regions with
varying degrees of illumination.
They projected the light uniformly onto a layer of bacterial
cells for five minutes, before exposing them to a more complex light pattern --
a negative image of the Mona Lisa. They found that bacteria started to
concentrate in the dark regions of the image while moving out from the more
illuminated areas. After four minutes, a recognisable bacterial replica of
Leonardo da Vinci's painting could be seen, with brighter areas corresponding
to regions of accumulated bacterial cells.
Although the shape formed by the bacteria was recognisable, the
team found that the engineered E. coli were slow to respond to variations in
light, which led to a blurred formation of the target shape. To remedy this,
they used a feedback control loop where the bacterial shape is compared to the
target image every 20 seconds, and the light pattern is updated accordingly.
This generated an optimal light pattern that shaped cell concentration with
much higher accuracy. The result is a 'photokinetic' bacterial cell layer that
can be turned into an almost perfect replica of a complex black-and-white
target image.
"We have shown how the suspension of swimming bacteria
could lead to a new class of light-controllable active materials whose density
can be shaped accurately, reversibly and quickly using a low-power light
projector," says Roberto Di Leonardo, Associate Professor in the
Department of Physics at Rome University. "With further engineering, the
bacteria could be used to create solid biomechanical structures or novel
microdevices for the transport of small biological cargoes inside miniaturised
laboratories."
See:
Posted by Dr. Tim Sandle, Pharmaceutical Microbiology
