Purple Rice Marks the Spot

By Sharon Durham
July 22, 2003

Rice is the latest ally of scientists battling the Colorado potato beetle.

Scientists with the Agricultural Research Service are using rice grains to determine if a newly discovered biocontrol bacterium--now being tested to control the beetle--is in the soil. The beetle feeds on potato, eggplant and tomato plants.

The purple bacterium, Chromobacterium suttsuga, kills beetles in laboratory studies. But before applying the bacterium to the soil, scientists need to know how long it survives, if it changes the soil or if it's harming the environment or other beneficial organisms.

For these experiments, microbiologist Phyllis Martin and her colleagues in the ARS Insect Biocontrol Laboratory in Beltsville, Md., adapted a technique to detect C. suttsuga in the soil. Martin found that by using sterile rice grains, she could detect small amounts of the purple bacteria--fewer than 50 cells in about a teaspoon of soil.

In the experiments, rice grains are placed on moist soil. In about three days, some rice grains turn purple if C. suttsuga is present. The percentage of purple-colored rice can then be used to determine the original amount of bacteria in the soil. According to Martin, the test has proven to be as sensitive as more expensive methods that depend on copying DNA, and it has the advantage of simultaneously detecting, quantifying and determining that the bacteria are alive. The rice method takes a little longer, but detection of bacteria in soil is not as time-critical as detection of bacteria in humans.

This simple technique also works for other bacteria that are pigmented, such as the red bacterium, Serratia marcescens. It also kills Colorado potato beetle, but not as effectively as the purple bacterium does.

ARS is the U.S. Department of Agriculture's chief scientific research agency.