Acid stops bacteria swimming
Microbes' motors are sensitive to
their internal pH.
10 February 2003
KENDALL POWELL
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| Acid test: Low pH slows
Salmonella's motors. |
| © SPL |
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Lowering the pH inside a bacterium stops its motor, shows new
research. The finding could help those trying to learn how to
make microbe-sized machines1.
Spinning hairs called flagella enable microbes to swim
towards nutrients or away from toxins: they turn anticlockwise
for forward motion, and clockwise to change direction.
Researchers are keen to understand such chemically driven
biological motors, which are only millionths of a millimetre
across, as electronics do not work on this scale.
Says Kenji Oosawa, who led the work at Nagoya University in
Japan: "We should pay attention to this type of tiny machine."
The motor runs on the energy of charged hydrogen ions flowing
through the system; exactly how is not known. Oosawa and his
colleagues exposed the bacteria to weak acids, such as potassium
acetate and benzoate, which are neutral until they reach the
inside of the cell where they release hydrogen ions.
Escherichia coli and Salmonella swim fine in
water at a neutral pH of 7.0. And without the weak acids present
to lower their internal pH, they also swim fine in acidic water
at pH 5.0. But, with the weak acids and a lower internal pH - as
the outside water becomes more acidic - they slow and ultimately
stop, reports the Japanese team.
The group also measure rotation of individual flagella motors
by tethering bacteria to microscope slides. This causes the
whole bacteria to spin, making measurements easier. The
researchers show that the acidity only affects the motor's
rotation and does not kill or weaken the bacteria, because when
they are returned to neutral water, they regain mobility.
The researchers think that increasing the number of protons
inside and outside the cell floods the motor, much as too much
petrol floods a car motor. The bacterial motor relies on a
steady flow of protons - if there are too many, it cannot turn.
"This is a motor with quite remarkable properties," says
Robert Macnab of Yale University in New Haven, Connecticut, who
studies the assembly of bacterial motors. "It runs like a
battery, moves like a ship's propeller, has a gear switch so it
can rotate in either direction, and it's under the control of
information from environment. These are biological functions at
their most simplified form, and yet there are 60 different types
of components in this little engine." |
