Rhizobium to the Rescue

...Tiny root bacteria may help clean soils contaminated by chemicals in TNT, plastic and other pollutants, scientists say...

If you think cleaning up your room is a hassle, just imagine having to clean up a hazardous waste site! When it comes to toxic chemicals that have escaped into the environment, clean-up duty can become downright dangerous.

Maybe you’ve tried to bribe your little brother or sister to clean up your messy room. Well, scientists are trying a similar trick. But instead of a sibling, they’re “bribing” tiny plant bacteria into tackling the dirty job of toxic clean-up.

Their bacterial “recruits” belong to a family called Rhizobium, which lives on alfalfa and soybean roots. There, the helpful microbes supply the plants with nitrogen for food and growth. This partnership also helps keep the soil fertile and healthy. That’s something you generally won’t find at contaminated site, like a landfill or mining operation.

To get the bacteria to clean soils polluted by chemicals, scientists first had to give the microbes instructions. These included a special gene that tells the bacteria how to make and secrete an enzyme from its cell walls.

Think of this enzyme as a kind of carpet cleaner your little brother or sister might use in your room. But instead of dirt and grime, the bacteria’s enzyme helps clean soils contaminated by toxic chemicals like TNT or toluene. Toluene is a common ingredient in fuel and dyes.

(Photo: a cross-section of a root nodule--a tiny lump in which Rhizobium bacteria would do their clean-up work. Each nodule is occupied by about a billion of the rod-shaped microbes.)

TNT, you probably know, is the highly flammable stuff that’s found in explosives like dynamite. Once these explosives detonate, their basic chemical building blocks get left behind in soils and sometimes the water. This is common problem at military sites, for example. The chemicals also can escape into the environment from ammunition plants.

But in the lab, scientists found that the bacteria’s enzymes break down these TNT building blocks into harmless substances.

TNT isn’t the only chemical the busy little bacteria will take on. The microbes also break apart DNT. This is a chemical that makes plastic and Styrofoam such a serious waste disposal problem.

“You put them in landfills and they just sit there. They don’t degrade” quickly, says Gail Hollowell, pictured at right. As a student not long ago, she helped ARS and Howard University scientists study the Rhizobium bacteria.

Her mentors, ARS horticulturist David Kuykendall and HU professor Sisir Dutta, came up with the idea of hiring on the Rhizobium bacteria. Their partners include scientists Fawzy Hashem and Bill Gillette, as well the U.S. Army Corps of Engineers.

The Corps is looking for cheap methods of restoring the health of contaminated soils at military sites. But it wants something that won’t cause more environmental harm.

(Photo: Gail Hollowell, posing with her bacterial clean-up crew--contained in test tube and on roots of potted alfalfa plants.)

The scientists imagine coating the bacteria onto alfalfa seeds, and then planting the seeds in polluted soils.

That could cut the cost of trucking the soils off to special treatment plants. It could also mean less need for human clean-up crews, and lower their risk of exposure to dangerous chemicals.

Scientists aren’t sure the bacteria and their plant partners will be the “magic bullet” in the war on wastes.

“It would be nice if this completely decontaminated the fields,” says Hollowell, now a National Institutes of Health researcher. “But even if it doesn’t, and you only get partial degradation, that’s still an improvement.”