Acid stops bacteria swimming

Microbes' motors are sensitive to their internal pH.
10 February 2003

KENDALL POWELL

Acid test: Low pH slows Salmonella's motors. © SPL

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."

1. Minamino, T., Imae, Y., Oosawa, F., Kobayashi, Y. & Oosawa, K. Effect of intracellular pH on rotational speed of bacterial flagellar motors. Journal of Bacteriology, 185, 1190 - 1194, (2003). |Article|