Found throughout the tropics and subtropics, Ralstonia solanacearum causes bacterial wilt in more than 200 plant species. Annual damages to potato total $950 million worldwide. In Hawai‘i this bacterium attacks tomato, pepper, eggplant, and ginger, with culinary ginger crop losses as high as 45 percent. Because cold-tolerant strains can devastate temperate-zone crops, materials potentially infected with them are subject to quarantine by the United States, Canada, and Europe.
DNA-based assays are used to distinguish among R. solanacearum strains. Faster, more efficient identification of individual strains in contaminated water, soil, and plant tissue will help prevent their transport and contain outbreaks. Toward that goal, CTAHR researchers are developing rapid detection methods that employ DNA biosensors.
Winston Su is using a combination of molecular and magnetic interactions to quickly isolate R. solanacearum cells from dilute samples to facilitate subsequent detection. A team lead by Gernot Presting and Anne Alvarez is analyzing the DNA sequences of several hundred R. solanacearum strains collected during the past 30 years by Alvarez to identify regions that can be used to construct probes, DNA molecules that bind selectively to DNA from specific strains.
Construction of the biosensor is taking place in the lab of Daniel Jenkins. Phages—viruses that attack bacteria—release the DNA from R. solanacearum cells. In the future, purified phage enzymes may be used to extract the DNA directly. Next, the DNA is amplified (copied multiple times). This typically requires cycles of near-boiling temperature, but for greater speed, simplicity, and efficiency, amplification processes at lower, constant temperatures are being developed. Detection occurs when a DNA sequence amplified from the bacterium binds specifically to a DNA probe that is linked to a small, disposable electrode. Temperature-control elements on the electrode help sustain optimal conditions for DNA extraction, amplification, and detection. Several of these tools have been successfully demonstrated for the detection of a strain of R. solanacearum that infects ginger, and the technologies can be readily adapted to detect other plant or human pathogens, offering prospects of broad future benefit.