Industry-sponsored and NASA research aimed at growing plants in space has led to a device that attacks and destroys airborne pathogens - like Anthrax.

By Dr Tony Phillips

Unseen and odourless, a cloud of Anthrax spores wafts through an office. People inside are talking, laughing ... breathing. They have no idea something is in the air. One yawn, one gasp, one happy guffaw could be deadly.

That's how bioterrorism works.

But this office has a defence: Bolted to the ceiling is a curious flat box. It's made of metal, about the size of a table-top, and it's humming softly - the sound of fans drawing airborne spores toward it and away from the people. The breeze is gentle but insistent. Eight cubic feet of air per minute flow into the box.

"Spores that pass through the box aren't filtered, they're fried," says John Hayman, whose company, KES Science & Technology, Inc., builds and sells the device called AiroCide TiO2. "That's appealing," he notes, "for people who don't want to change an Anthrax-laden air filter." Tests show that as many as 93% of Anthrax spores that enter AiroCide TiO2 are destroyed. Survivors circulate out of the chamber where they are likely to be sucked back in again for another pass.

Anthrax spores, pictured here in a thin section micrograph, are inactive forms of the bacterium Bacillus anthracis. Such bacteria can survive for decades inside a spore's tough protective coating; they become active when inhaled by humans.

Hayman explains: "[Space faring] astronauts will eventually need to grow some of their own food in greenhouses. But there's a problem: the leaves of growing plants release ethylene (C2H4) - a gas that causes fruits and vegetables to mature." In the close quarters of a spacecraft (or inside an enclosed plant growth chamber), ethylene would build up and ripen greenhouse plants prematurely.

Space greenhouses needed a new technology to remove that ethylene.

In the 1990's, University of Wisconsin professor Marc Anderson and colleagues from the Wisconsin Centre for Space Automation and Robotics (WCSAR) made a crucial discovery: They found that ultra-thin layers of titanium dioxide (TiO2) exposed to ultraviolet light would efficiently convert ethylene into carbon dioxide (CO2) and water (H2O) - substances that are good for plants. Titanium dioxide itself is a harmless, non-toxic colouring agent used in many consumer products. It is a catalyst for the ethylene-destroying reaction; no TiO2 is consumed.

TiO2-based ethylene removers have since flown to space inside a pair of plant growth chambers: ASTROCULTURE™ on board NASA's space shuttle and ADVANCED ASTROCULTURE™ on the International Space Station (ISS). The devices were built by WCSAR - a NASA Commercial Space Centre at the University of Wisconsin. WCSAR is one of 17 such centres around the country sponsored by NASA's Space Product Development program to encourage the commercialisation of space by industry.

A top-down view of the ADVANCED ASTROCULTURE™ plant growth chamber on the ISS, where reddish light illuminates the leafy heads of Arabidopsis plants.

The technology worked so well that the University of Wisconsin collaborators joined forces with KES Science and Technology, Inc., to develop an ethylene scrubber for Earth. The device, called Bio-KES, works wonders in supermarkets where ethylene in the air of produce aisles reduces the shelf life of vegetables. Bio-KES was nominated as Discover Magazine's Product of the Year in 1998, and it's since been shipped across the globe for use by grocers and florists.

Moreover, Bio-KES is the parent of AiroCide TiO2.

"It was a serendipitous discovery," recalls Hayman. Tests showed that Bio-KES not only removed ethylene, but also killed airborne dust mites. Marc Anderson quickly realised why: When ultraviolet (UV) photons strike something coated by TiO2 - like the tubes inside Bio-KES - positive and negative charges appear on its surface. Those charges tear apart nearby water molecules. The OH^- ion, a by-product of the reaction, disrupts organic molecules. It's deadly to dust mites, Anthrax and many other pathogens.

Technicians modified Bio-KES - adding higher-power UV lamps, for example, to give it "an extra kick," says Hayman - and AiroCide TiO2 was born.

In a laboratory at the University of Wisconsin, a plexiglass chamber (left) containing airborne spores covers the entrance to an AiroCide TiO2 unit. Few microbes survive a journey through the machine. The exit port (right) reveals glowing UVC lamps inside.

Dean Tompkins, a colleague of Anderson's at the University of Wisconsin, is in charge of testing AiroCide TiO2. "We don't use real Anthrax," he notes. "That would be too dangerous. Instead, we experiment with one of its non-virulent cousins: Bacillus thurengiensis." During a typical experiment, Tompkins propels a cloud of approximately 1000 spores through the AiroCide chamber. Only 100 or so emerge intact.

Spores that enter AiroCide TiO2 spend at least 5 to 10 seconds in transit through the device. "That's important," adds Hayman, "because pathogens that remain inside longer are more likely to die." To slow the spores, TiO2-coated tubes within the unit are randomly arranged - there's no direct path through the machine. When air moves across the jumbled tubes, the flow becomes turbulent - forcing spores to linger where they can be attacked by OH^- and illuminated by germ-killing ultraviolet light

Such powerful tools against bio-terror indeed seem a far cry from star-trekking greenhouses, but that's how many discoveries are made: You never know what new invention might emerge - like AiroCide TiO2 - or what might be annihilated in the process - like Anthrax!