Carrying the beads did not slow the cells down

Harvard University experts say, in future, cells could be harnessed to perform micro-scale mechanical work.

The researchers attached a cargo of polystyrene beads to the backs of green algae cells and used light to guide them up and down the chambers.

Details of the work appear in the journal Proceedings of the National Academy of Sciences (PNAS).

We harness their motors to make them perform unconventional tasks

Douglas Weibel, Harvard University

"We have basically developed the system of moving objects with micro-organisms," co-author Douglas Weibel, of Harvard University told the BBC News website.

"We harness their motors to make them perform unconventional tasks."

The team have coined the term "microoxen" for the load-bearing microbes.

Nice motor

The Harvard researchers, led by Professor George Whitesides, used the single-celled photosynthetic algae Chlamydomonas reinhardtii.

The algae are about 10 microns long and propel themselves by beating their two whip-like tails, or flagella, in an action similar to the breaststroke. This action is driven by a type of molecular motor.

The researchers used chemical bonds to attach a cargo load of specially coated polystyrene beads to individual algal cells.

Then they used light of different intensities to guide them up and down the chamber. High-intensity light repels the organisms while low-intensity light attracts them.

Load bearers

Attaching the cargoes seemed to have little or no effect on the speeds at which the cells moved. The loads were unhitched by exposing the algae to ultraviolet light, which broke apart molecules in the coating on the beads.

Dr Weibel said the technique had many potential uses in areas such as molecular medicine.

"You could have a bead that picks up a toxin. So you send them to swim off into a sample of liquid, and when they return, you can carry out analysis on the bead," he said.

There is considerable interest in harnessing biological motors to perform micro-scale mechanical work.

However, most research in this area has focused on isolating the motors within cells and rebuilding them elsewhere, rather than using the living organism to perform the tasks required.